OASIS Virtual I/O Device (VIRTIO) TC

Action Item Subject : Create text version of virtio 0.9.5 document Item Number : #0000 Description As agreed at meeting 2013-07-30, conversion to text format as a basis for discussion / bug tracking. Owner : Mr. Rusty Russell Status : Open Priority : High Due Date : 01 Aug 2013

Rusty Russell <rusty@au1.ibm.com> writes: > ----------------- > Action Item Subject: Create text version of virtio 0.9.5 document OK, I've attached this below. This is exactly the spec as 0.9.5 reformatted into text so we can work it. I'm still waiting for virtio to be added to the OASIS issue tracking system, where I will be prompting you all to open an issue for every change we want to consider. The obvious issues I want to open are: o Major rework to make PCI an appendix, and core bus-independent. o Update spec with changes/fixes since 0.9.5 - This is easy where contributors are already members, trickier for others. In addition here's a brain dump: o Endian for config space - LE everywhere? o Endian for ring - LE as well? o Allow arbitrary descriptor layouts / message framing. o Method to stop activity on a queue? o Size descriptor table independent of ringsize? o Remove VIRTIO_F_NOTIFY_ON_EMPTY? o Remove limit on # indirect descriptors - Some other limit? o Simplify indirect desc - No return to top level on end of desc array o Allow chained indirect desc - indirect bit use to chain? Net: o Remove VIRTIO_NET_F_GSO? Block: o Remove VIRTIO_BLK_F_SCSI? o Revisit flush/barrier semantics? PCI: o New capability layout o Allowing non-zero revision numbers? o Remove 'align' and use explicit addresses for used/avail. Balloon: o Fix endianness o Remove VIRTIO_BALLOON_F_MUST_TELL_HOST? o Remove outgoing page queue? Cheers, Rusty. ==== This document describes the specifications of the “virtio” family of PCI devices. These are devices are found in virtual environments, yet by design they are not all that different from physical PCI devices, and this document treats them as such. This allows the guest to use standard PCI drivers and discovery mechanisms. The purpose of virtio and this specification is that virtual environments and guests should have a straightforward, efficient, standard and extensible mechanism for virtual devices, rather than boutique per-environment or per-OS mechanisms. Straightforward: Virtio PCI devices use normal PCI mechanisms of interrupts and DMA which should be familiar to any device driver author. There is no exotic page-flipping or COW mechanism: it's just a PCI device.[1] Efficient: Virtio PCI devices consist of rings of descriptors for input and output, which are neatly separated to avoid cache effects from both guest and device writing to the same cache lines. Standard: Virtio PCI makes no assumptions about the environment in which it operates, beyond supporting PCI. In fact the virtio devices specified in the appendices do not require PCI at all: they have been implemented on non-PCI buses.[2] Extensible: Virtio PCI devices contain feature bits which are acknowledged by the guest operating system during device setup. This allows forwards and backwards compatibility: the device offers all the features it knows about, and the driver acknowledges those it understands and wishes to use. 1.1 Virtqueues The mechanism for bulk data transport on virtio PCI devices is pretentiously called a virtqueue. Each device can have zero or more virtqueues: for example, the network device has one for transmit and one for receive. Each virtqueue occupies two or more physically-contiguous pages (defined, for the purposes of this specification, as 4096 bytes), and consists of three parts: +-------------------+-----------------------------------+-----------+ Descriptor Table Available Ring (padding) Used Ring +-------------------+-----------------------------------+-----------+ When the driver wants to send a buffer to the device, it fills in a slot in the descriptor table (or chains several together), and writes the descriptor index into the available ring. It then notifies the device. When the device has finished a buffer, it writes the descriptor into the used ring, and sends an interrupt. Specification 2.1 PCI Discovery Any PCI device with Vendor ID 0x1AF4, and Device ID 0x1000 through 0x103F inclusive is a virtio device[3]. The device must also have a Revision ID of 0 to match this specification. The Subsystem Device ID indicates which virtio device is supported by the device. The Subsystem Vendor ID should reflect the PCI Vendor ID of the environment (it's currently only used for informational purposes by the guest). +----------------------+--------------------+---------------+ Subsystem Device ID Virtio Device Specification +----------------------+--------------------+---------------+ +----------------------+--------------------+---------------+ 1 network card Appendix C +----------------------+--------------------+---------------+ 2 block device Appendix D +----------------------+--------------------+---------------+ 3 console Appendix E +----------------------+--------------------+---------------+ 4 entropy source Appendix F +----------------------+--------------------+---------------+ 5 memory ballooning Appendix G +----------------------+--------------------+---------------+ 6 ioMemory - +----------------------+--------------------+---------------+ 7 rpmsg - +----------------------+--------------------+---------------+ 8 SCSI host Appendix I +----------------------+--------------------+---------------+ 9 9P transport - +----------------------+--------------------+---------------+ 10 mac80211 wlan - +----------------------+--------------------+---------------+ 2.2 Device Configuration To configure the device, we use the first I/O region of the PCI device. This contains a virtio header followed by a device-specific region. There may be different widths of accesses to the I/O region; the “natural” access method for each field in the virtio header must be used (i.e. 32-bit accesses for 32-bit fields, etc), but the device-specific region can be accessed using any width accesses, and should obtain the same results. Note that this is possible because while the virtio header is PCI (i.e. little) endian, the device-specific region is encoded in the native endian of the guest (where such distinction is applicable). 2.2.1 Device Initialization Sequence We start with an overview of device initialization, then expand on the details of the device and how each step is preformed. 1. Reset the device. This is not required on initial start up. 2. The ACKNOWLEDGE status bit is set: we have noticed the device. 3. The DRIVER status bit is set: we know how to drive the device. 4. Device-specific setup, including reading the Device Feature Bits, discovery of virtqueues for the device, optional MSI-X setup, and reading and possibly writing the virtio configuration space. 5. The subset of Device Feature Bits understood by the driver is written to the device. 6. The DRIVER_OK status bit is set. 7. The device can now be used (ie. buffers added to the virtqueues)[4] If any of these steps go irrecoverably wrong, the guest should set the FAILED status bit to indicate that it has given up on the device (it can reset the device later to restart if desired). We now cover the fields required for general setup in detail. 2.2.2 Virtio Header The virtio header looks as follows: +------------++---------------------+---------------------+----------+--------+---------+---------+---------+--------+ Bits 32 32 32 16 16 16 8 8 +------------++---------------------+---------------------+----------+--------+---------+---------+---------+--------+ Read/Write R R+W R+W R R+W R+W R+W R +------------++---------------------+---------------------+----------+--------+---------+---------+---------+--------+ Purpose Device Guest Queue Queue Queue Queue Device ISR Features bits 0:31 Features bits 0:31 Address Size Select Notify Status Status +------------++---------------------+---------------------+----------+--------+---------+---------+---------+--------+ If MSI-X is enabled for the device, two additional fields immediately follow this header:[5] +------------++----------------+--------+ Bits 16 16 +----------------+--------+ +------------++----------------+--------+ Read/Write R+W R+W +------------++----------------+--------+ Purpose Configuration Queue (MSI-X) Vector Vector +------------++----------------+--------+ Immediately following these general headers, there may be device-specific headers: +------------++--------------------+ Bits Device Specific +--------------------+ +------------++--------------------+ Read/Write Device Specific +------------++--------------------+ Purpose Device Specific... +------------++--------------------+ 2.2.2.1 Device Status The Device Status field is updated by the guest to indicate its progress. This provides a simple low-level diagnostic: it's most useful to imagine them hooked up to traffic lights on the console indicating the status of each device. The device can be reset by writing a 0 to this field, otherwise at least one bit should be set: ACKNOWLEDGE (1) Indicates that the guest OS has found the device and recognized it as a valid virtio device. DRIVER (2) Indicates that the guest OS knows how to drive the device. Under Linux, drivers can be loadable modules so there may be a significant (or infinite) delay before setting this bit. DRIVER_OK (4) Indicates that the driver is set up and ready to drive the device. FAILED (128) Indicates that something went wrong in the guest, and it has given up on the device. This could be an internal error, or the driver didn't like the device for some reason, or even a fatal error during device operation. The device must be reset before attempting to re-initialize. 2.2.2.2 Feature Bits The first configuration field indicates the features that the device supports. The bits are allocated as follows: 0 to 23 Feature bits for the specific device type 24 to 32 Feature bits reserved for extensions to the queue and feature negotiation mechanisms For example, feature bit 0 for a network device (i.e. Subsystem Device ID 1) indicates that the device supports checksumming of packets. The feature bits are negotiated: the device lists all the features it understands in the Device Features field, and the guest writes the subset that it understands into the Guest Features field. The only way to renegotiate is to reset the device. In particular, new fields in the device configuration header are indicated by offering a feature bit, so the guest can check before accessing that part of the configuration space. This allows for forwards and backwards compatibility: if the device is enhanced with a new feature bit, older guests will not write that feature bit back to the Guest Features field and it can go into backwards compatibility mode. Similarly, if a guest is enhanced with a feature that the device doesn't support, it will not see that feature bit in the Device Features field and can go into backwards compatibility mode (or, for poor implementations, set the FAILED Device Status bit). 2.2.2.3 Configuration/Queue Vectors When MSI-X capability is present and enabled in the device (through standard PCI configuration space) 4 bytes at byte offset 20 are used to map configuration change and queue interrupts to MSI-X vectors. In this case, the ISR Status field is unused, and device specific configuration starts at byte offset 24 in virtio header structure. When MSI-X capability is not enabled, device specific configuration starts at byte offset 20 in virtio header. Writing a valid MSI-X Table entry number, 0 to 0x7FF, to one of Configuration/Queue Vector registers, maps interrupts triggered by the configuration change/selected queue events respectively to the corresponding MSI-X vector. To disable interrupts for a specific event type, unmap it by writing a special NO_VECTOR value: /* Vector value used to disable MSI for queue */ #define VIRTIO_MSI_NO_VECTOR 0xffff Reading these registers returns vector mapped to a given event, or NO_VECTOR if unmapped. All queue and configuration change events are unmapped by default. Note that mapping an event to vector might require allocating internal device resources, and might fail. Devices report such failures by returning the NO_VECTOR value when the relevant Vector field is read. After mapping an event to vector, the driver must verify success by reading the Vector field value: on success, the previously written value is returned, and on failure, NO_VECTOR is returned. If a mapping failure is detected, the driver can retry mapping with fewervectors, or disable MSI-X. 2.3 Virtqueue Configuration As a device can have zero or more virtqueues for bulk data transport (for example, the network driver has two), the driver needs to configure them as part of the device-specific configuration. This is done as follows, for each virtqueue a device has: 1. Write the virtqueue index (first queue is 0) to the Queue Select field. 2. Read the virtqueue size from the Queue Size field, which is always a power of 2. This controls how big the virtqueue is (see below). If this field is 0, the virtqueue does not exist. 3. Allocate and zero virtqueue in contiguous physical memory, on a 4096 byte alignment. Write the physical address, divided by 4096 to the Queue Address field.[6] 4. Optionally, if MSI-X capability is present and enabled on the device, select a vector to use to request interrupts triggered by virtqueue events. Write the MSI-X Table entry number corresponding to this vector in Queue Vector field. Read the Queue Vector field: on success, previously written value is returned; on failure, NO_VECTOR value is returned. The Queue Size field controls the total number of bytes required for the virtqueue according to the following formula: #define ALIGN(x) (((x) + 4095) & ~4095) static inline unsigned vring_size(unsigned int qsz) { return ALIGN(sizeof(struct vring_desc)*qsz + sizeof(u16)*(2 + qsz)) + ALIGN(sizeof(struct vring_used_elem)*qsz); } This currently wastes some space with padding, but also allows future extensions. The virtqueue layout structure looks like this (qsz is the Queue Size field, which is a variable, so this code won't compile): struct vring { /* The actual descriptors (16 bytes each) */ struct vring_desc desc[qsz]; /* A ring of available descriptor heads with free-running index. */ struct vring_avail avail; // Padding to the next 4096 boundary. char pad[]; // A ring of used descriptor heads with free-running index. struct vring_used used; }; 2.3.1 A Note on Virtqueue Endianness Note that the endian of these fields and everything else in the virtqueue is the native endian of the guest, not little-endian as PCI normally is. This makes for simpler guest code, and it is assumed that the host already has to be deeply aware of the guest endian so such an “endian-aware” device is not a significant issue. 2.3.2 Descriptor Table The descriptor table refers to the buffers the guest is using for the device. The addresses are physical addresses, and the buffers can be chained via the next field. Each descriptor describes a buffer which is read-only or write-only, but a chain of descriptors can contain both read-only and write-only buffers. No descriptor chain may be more than 2^32 bytes long in total. struct vring_desc { /* Address (guest-physical). */ u64 addr; /* Length. */ u32 len; /* This marks a buffer as continuing via the next field. */ #define VRING_DESC_F_NEXT 1 /* This marks a buffer as write-only (otherwise read-only). */ #define VRING_DESC_F_WRITE 2 /* This means the buffer contains a list of buffer descriptors. */ #define VRING_DESC_F_INDIRECT 4 /* The flags as indicated above. */ u16 flags; /* Next field if flags & NEXT */ u16 next; }; The number of descriptors in the table is specified by the Queue Size field for this virtqueue. 2.3.3 Indirect Descriptors Some devices benefit by concurrently dispatching a large number of large requests. The VIRTIO_RING_F_INDIRECT_DESC feature can be used to allow this (see Appendix B: Reserved Feature Bits). To increase ring capacity it is possible to store a table of indirect descriptors anywhere in memory, and insert a descriptor in main virtqueue (with flags&INDIRECT on) that refers to memory buffer containing this indirect descriptor table; fields addr and len refer to the indirect table address and length in bytes, respectively. The indirect table layout structure looks like this (len is the length of the descriptor that refers to this table, which is a variable, so this code won't compile): struct indirect_descriptor_table { /* The actual descriptors (16 bytes each) */ struct vring_desc desc[len / 16]; }; The first indirect descriptor is located at start of the indirect descriptor table (index 0), additional indirect descriptors are chained by next field. An indirect descriptor without next field (with flags&NEXT off) signals the end of the indirect descriptor table, and transfers control back to the main virtqueue. An indirect descriptor can not refer to another indirect descriptor table (flags&INDIRECT must be off). A single indirect descriptor table can include both read-only and write-only descriptors; write-only flag (flags&WRITE) in the descriptor that refers to it is ignored. 2.3.4 Available Ring The available ring refers to what descriptors we are offering the device: it refers to the head of a descriptor chain. The “flags” field is currently 0 or 1: 1 indicating that we do not need an interrupt when the device consumes a descriptor from the available ring. Alternatively, the guest can ask the device to delay interrupts until an entry with an index specified by the “ used_event” field is written in the used ring (equivalently, until the idx field in the used ring will reach the value used_event + 1). The method employed by the device is controlled by the VIRTIO_RING_F_EVENT_IDX feature bit (see Appendix B: Reserved Feature Bits). This interrupt suppression is merely an optimization; it may not suppress interrupts entirely. The “idx” field indicates where we would put the next descriptor entry (modulo the ring size). This starts at 0, and increases. struct vring_avail { #define VRING_AVAIL_F_NO_INTERRUPT 1 u16 flags; u16 idx; u16 ring[qsz]; /* qsz is the Queue Size field read from device */ u16 used_event; }; 2.3.5 Used Ring The used ring is where the device returns buffers once it is done with them. The flags field can be used by the device to hint that no notification is necessary when the guest adds to the available ring. Alternatively, the “avail_event” field can be used by the device to hint that no notification is necessary until an entry with an index specified by the “avail_event” is written in the available ring (equivalently, until the idx field in the available ring will reach the value avail_event + 1). The method employed by the device is controlled by the guest through the VIRTIO_RING_F_EVENT_IDX feature bit (see Appendix B: Reserved Feature Bits).[7] Each entry in the ring is a pair: the head entry of the descriptor chain describing the buffer (this matches an entry placed in the available ring by the guest earlier), and the total of bytes written into the buffer. The latter is extremely useful for guests using untrusted buffers: if you do not know exactly how much has been written by the device, you usually have to zero the buffer to ensure no data leakage occurs. /* u32 is used here for ids for padding reasons. */ struct vring_used_elem { /* Index of start of used descriptor chain. */ u32 id; /* Total length of the descriptor chain which was used (written to) */ u32 len; }; struct vring_used { #define VRING_USED_F_NO_NOTIFY 1 u16 flags; u16 idx; struct vring_used_elem ring[qsz]; u16 avail_event; }; 2.3.6 Helpers for Managing Virtqueues The Linux Kernel Source code contains the definitions above and helper routines in a more usable form, in include/linux/virtio_ring.h. This was explicitly licensed by IBM and Red Hat under the (3-clause) BSD license so that it can be freely used by all other projects, and is reproduced (with slight variation to remove Linux assumptions) in Appendix A. 2.4 Device Operation There are two parts to device operation: supplying new buffers to the device, and processing used buffers from the device. As an example, the virtio network device has two virtqueues: the transmit virtqueue and the receive virtqueue. The driver adds outgoing (read-only) packets to the transmit virtqueue, and then frees them after they are used. Similarly, incoming (write-only) buffers are added to the receive virtqueue, and processed after they are used. 2.4.1 Supplying Buffers to The Device Actual transfer of buffers from the guest OS to the device operates as follows: 1. Place the buffer(s) into free descriptor(s). (a) If there are no free descriptors, the guest may choose to notify the device even if notifications are suppressed (to reduce latency).[8] 2. Place the id of the buffer in the next ring entry of the available ring. 3. The steps (1) and (2) may be performed repeatedly if batching is possible. 4. A memory barrier should be executed to ensure the device sees the updated descriptor table and available ring before the next step. 5. The available “idx” field should be increased by the number of entries added to the available ring. 6. A memory barrier should be executed to ensure that we update the idx field before checking for notification suppression. 7. If notifications are not suppressed, the device should be notified of the new buffers. Note that the above code does not take precautions against the available ring buffer wrapping around: this is not possible since the ring buffer is the same size as the descriptor table, so step (1) will prevent such a condition. In addition, the maximum queue size is 32768 (it must be a power of 2 which fits in 16 bits), so the 16-bit “idx” value can always distinguish between a full and empty buffer. Here is a description of each stage in more detail. 2.4.1.1 Placing Buffers Into The Descriptor Table A buffer consists of zero or more read-only physically-contiguous elements followed by zero or more physically-contiguous write-only elements (it must have at least one element). This algorithm maps it into the descriptor table: 1. for each buffer element, b: (a) Get the next free descriptor table entry, d (b) Set d.addr to the physical address of the start of b (c) Set d.len to the length of b. (d) If b is write-only, set d.flags to VRING_DESC_F_WRITE, otherwise 0. (e) If there is a buffer element after this: i. Set d.next to the index of the next free descriptor element. ii. Set the VRING_DESC_F_NEXT bit in d.flags. In practice, the d.next fields are usually used to chain free descriptors, and a separate count kept to check there are enough free descriptors before beginning the mappings. 2.4.1.2 Updating The Available Ring The head of the buffer we mapped is the first d in the algorithm above. A naive implementation would do the following: avail->ring[avail->idx % qsz] = head; However, in general we can add many descriptors before we update the “idx” field (at which point they become visible to the device), so we keep a counter of how many we've added: avail->ring[(avail->idx + added++) % qsz] = head; 2.4.1.3 Updating The Index Field Once the idx field of the virtqueue is updated, the device will be able to access the descriptor entries we've created and the memory they refer to. This is why a memory barrier is generally used before the idx update, to ensure it sees the most up-to-date copy. The idx field always increments, and we let it wrap naturally at 65536: avail->idx += added; 2.4.1.4 Notifying The Device Device notification occurs by writing the 16-bit virtqueue index of this virtqueue to the Queue Notify field of the virtio header in the first I/O region of the PCI device. This can be expensive, however, so the device can suppress such notifications if it doesn't need them. We have to be careful to expose the new idx value before checking the suppression flag: it's OK to notify gratuitously, but not to omit a required notification. So again, we use a memory barrier here before reading the flags or the avail_event field. If the VIRTIO_F_RING_EVENT_IDX feature is not negotiated, and if the VRING_USED_F_NOTIFY flag is not set, we go ahead and write to the PCI configuration space. If the VIRTIO_F_RING_EVENT_IDX feature is negotiated, we read the avail_event field in the available ring structure. If the available index crossed_the avail_event field value since the last notification, we go ahead and write to the PCI configuration space. The avail_event field wraps naturally at 65536 as well: (u16)(new_idx - avail_event - 1) < (u16)(new_idx - old_idx) 2.4.2 Receiving Used Buffers From The Device Once the device has used a buffer (read from or written to it, or parts of both, depending on the nature of the virtqueue and the device), it sends an interrupt, following an algorithm very similar to the algorithm used for the driver to send the device a buffer: 1. Write the head descriptor number to the next field in the used ring. 2. Update the used ring idx. 3. Determine whether an interrupt is necessary: (a) If the VIRTIO_F_RING_EVENT_IDX feature is not negotiated: check if f the VRING_AVAIL_F_NO_INTERRUPT flag is not set in avail->flags (b) If the VIRTIO_F_RING_EVENT_IDX feature is negotiated: check whether the used index crossed the used_event field value since the last update. The used_event field wraps naturally at 65536 as well: (u16)(new_idx - used_event - 1) < (u16)(new_idx - old_idx) 4. If an interrupt is necessary: (a) If MSI-X capability is disabled: i. Set the lower bit of the ISR Status field for the device. ii. Send the appropriate PCI interrupt for the device. (b) If MSI-X capability is enabled: i. Request the appropriate MSI-X interrupt message for the device, Queue Vector field sets the MSI-X Table entry number. ii. If Queue Vector field value is NO_VECTOR, no interrupt message is requested for this event. The guest interrupt handler should: 1. If MSI-X capability is disabled: read the ISR Status field, which will reset it to zero. If the lower bit is zero, the interrupt was not for this device. Otherwise, the guest driver should look through the used rings of each virtqueue for the device, to see if any progress has been made by the device which requires servicing. 2. If MSI-X capability is enabled: look through the used rings of each virtqueue mapped to the specific MSI-X vector for the device, to see if any progress has been made by the device which requires servicing. For each ring, guest should then disable interrupts by writing VRING_AVAIL_F_NO_INTERRUPT flag in avail structure, if required. It can then process used ring entries finally enabling interrupts by clearing the VRING_AVAIL_F_NO_INTERRUPT flag or updating the EVENT_IDX field in the available structure, Guest should then execute a memory barrier, and then recheck the ring empty condition. This is necessary to handle the case where, after the last check and before enabling interrupts, an interrupt has been suppressed by the device: vring_disable_interrupts(vq); for (;;) { if (vq->last_seen_used != vring->used.idx) { vring_enable_interrupts(vq); mb(); if (vq->last_seen_used != vring->used.idx) break; } struct vring_used_elem *e = vring.used->ring[vq->last_seen_used%vsz]; process_buffer(e); vq->last_seen_used++; } 2.4.3 Dealing With Configuration Changes Some virtio PCI devices can change the device configuration state, as reflected in the virtio header in the PCI configuration space. In this case: 1. If MSI-X capability is disabled: an interrupt is delivered and the second highest bit is set in the ISR Status field to indicate that the driver should re-examine the configuration space. Note that a single interrupt can indicate both that one or more virtqueue has been used and that the configuration space has changed: even if the config bit is set, virtqueues must be scanned. 2. If MSI-X capability is enabled: an interrupt message is requested. The Configuration Vector field sets the MSI-X Table entry number to use. If Configuration Vector field value is NO_VECTOR, no interrupt message is requested for this event. Creating New Device Types Various considerations are necessary when creating a new device type: How Many Virtqueues? It is possible that a very simple device will operate entirely through its configuration space, but most will need at least one virtqueue in which it will place requests. A device with both input and output (eg. console and network devices described here) need two queues: one which the driver fills with buffers to receive input, and one which the driver places buffers to transmit output. What Configuration Space Layout? Configuration space is generally used for rarely-changing or initialization-time parameters. But it is a limited resource, so it might be better to use a virtqueue to update configuration information (the network device does this for filtering, otherwise the table in the config space could potentially be very large). Note that this space is generally the guest's native endian, rather than PCI's little-endian. What Device Number? Currently device numbers are assigned quite freely: a simple request mail to the author of this document or the Linux virtualization mailing list[9] will be sufficient to secure a unique one. Meanwhile for experimental drivers, use 65535 and work backwards. How many MSI-X vectors? Using the optional MSI-X capability devices can speed up interrupt processing by removing the need to read ISR Status register by guest driver (which might be an expensive operation), reducing interrupt sharing between devices and queues within the device, and handling interrupts from multiple CPUs. However, some systems impose a limit (which might be as low as 256) on the total number of MSI-X vectors that can be allocated to all devices. Devices and/or device drivers should take this into account, limiting the number of vectors used unless the device is expected to cause a high volume of interrupts. Devices can control the number of vectors used by limiting the MSI-X Table Size or not presenting MSI-X capability in PCI configuration space. Drivers can control this by mapping events to as small number of vectors as possible, or disabling MSI-X capability altogether. Message Framing The descriptors used for a buffer should not effect the semantics of the message, except for the total length of the buffer. For example, a network buffer consists of a 10 byte header followed by the network packet. Whether this is presented in the ring descriptor chain as (say) a 10 byte buffer and a 1514 byte buffer, or a single 1524 byte buffer, or even three buffers, should have no effect. In particular, no implementation should use the descriptor boundaries to determine the size of any header in a request.[10] Device Improvements Any change to configuration space, or new virtqueues, or behavioural changes, should be indicated by negotiation of a new feature bit. This establishes clarity[11] and avoids future expansion problems. Clusters of functionality which are always implemented together can use a single bit, but if one feature makes sense without the others they should not be gratuitously grouped together to conserve feature bits. We can always extend the spec when the first person needs more than 24 feature bits for their device. Appendix A: virtio_ring.h #ifndef VIRTIO_RING_H #define VIRTIO_RING_H /* An interface for efficient virtio implementation. * * This header is BSD licensed so anyone can use the definitions * to implement compatible drivers/servers. * * Copyright 2007, 2009, IBM Corporation * Copyright 2011, Red Hat, Inc * All rights reserved. * * Redistribution and use in source and binary forms, with or without * modification, are permitted provided that the following conditions * are met: * 1. Redistributions of source code must retain the above copyright * notice, this list of conditions and the following disclaimer. * 2. Redistributions in binary form must reproduce the above copyright * notice, this list of conditions and the following disclaimer in the * documentation and/or other materials provided with the distribution. * 3. Neither the name of IBM nor the names of its contributors * may be used to endorse or promote products derived from this software * without specific prior written permission. * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS ``AS IS'' AND * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE * ARE DISCLAIMED. IN NO EVENT SHALL IBM OR CONTRIBUTORS BE LIABLE * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF * SUCH DAMAGE. */ /* This marks a buffer as continuing via the next field. */ #define VRING_DESC_F_NEXT 1 /* This marks a buffer as write-only (otherwise read-only). */ #define VRING_DESC_F_WRITE 2 /* The Host uses this in used->flags to advise the Guest: don't kick me * when you add a buffer. It's unreliable, so it's simply an * optimization. Guest will still kick if it's out of buffers. */ #define VRING_USED_F_NO_NOTIFY 1 /* The Guest uses this in avail->flags to advise the Host: don't * interrupt me when you consume a buffer. It's unreliable, so it's * simply an optimization. */ #define VRING_AVAIL_F_NO_INTERRUPT 1 /* Virtio ring descriptors: 16 bytes. * These can chain together via "next". */ struct vring_desc { /* Address (guest-physical). */ uint64_t addr; /* Length. */ uint32_t len; /* The flags as indicated above. */ uint16_t flags; /* We chain unused descriptors via this, too */ uint16_t next; }; struct vring_avail { uint16_t flags; uint16_t idx; uint16_t ring[]; uint16_t used_event; }; /* u32 is used here for ids for padding reasons. */ struct vring_used_elem { /* Index of start of used descriptor chain. */ uint32_t id; /* Total length of the descriptor chain which was written to. */ uint32_t len; }; struct vring_used { uint16_t flags; uint16_t idx; struct vring_used_elem ring[]; uint16_t avail_event; }; struct vring { unsigned int num; struct vring_desc *desc; struct vring_avail *avail; struct vring_used *used; }; /* The standard layout for the ring is a continuous chunk of memory which * looks like this. We assume num is a power of 2. * * struct vring { * // The actual descriptors (16 bytes each) * struct vring_desc desc[num]; * * // A ring of available descriptor heads with free-running index. * __u16 avail_flags; * __u16 avail_idx; * __u16 available[num]; * * // Padding to the next align boundary. * char pad[]; * * // A ring of used descriptor heads with free-running index. * __u16 used_flags; * __u16 EVENT_IDX; * struct vring_used_elem used[num]; * }; * Note: for virtio PCI, align is 4096. */ static inline void vring_init(struct vring *vr, unsigned int num, void *p, unsigned long align) { vr->num = num; vr->desc = p; vr->avail = p + num*sizeof(struct vring_desc); vr->used = (void *)(((unsigned long)&vr->avail->ring[num] + align-1) & ~(align - 1)); } static inline unsigned vring_size(unsigned int num, unsigned long align) { return ((sizeof(struct vring_desc)*num + sizeof(uint16_t)*(2+num) + align - 1) & ~(align - 1)) + sizeof(uint16_t)*3 + sizeof(struct vring_used_elem)*num; } static inline int vring_need_event(uint16_t event_idx, uint16_t new_idx, uint16_t old_idx) { return (uint16_t)(new_idx - event_idx - 1) < (uint16_t)(new_idx - old_idx); } #endif /* VIRTIO_RING_H */ Appendix B: Reserved Feature Bits Currently there are five device-independent feature bits defined: VIRTIO_F_NOTIFY_ON_EMPTY (24) Negotiating this feature indicates that the driver wants an interrupt if the device runs out of available descriptors on a virtqueue, even though interrupts are suppressed using the VRING_AVAIL_F_NO_INTERRUPT flag or the used_event field. An example of this is the networking driver: it doesn't need to know every time a packet is transmitted, but it does need to free the transmitted packets a finite time after they are transmitted. It can avoid using a timer if the device interrupts it when all the packets are transmitted. VIRTIO_F_RING_INDIRECT_DESC (28) Negotiating this feature indicates that the driver can use descriptors with the VRING_DESC_F_INDIRECT flag set, as described in 2.3.3 Indirect Descriptors. VIRTIO_F_RING_EVENT_IDX(29) This feature enables the used_event and the avail_event fields. If set, it indicates that the device should ignore the flags field in the available ring structure. Instead, the used_event field in this structure is used by guest to suppress device interrupts. Further, the driver should ignore the flags field in the used ring structure. Instead, the avail_event field in this structure is used by the device to suppress notifications. If unset, the driver should ignore the used_event field; the device should ignore the avail_event field; the flags field is used Appendix C: Network Device The virtio network device is a virtual ethernet card, and is the most complex of the devices supported so far by virtio. It has enhanced rapidly and demonstrates clearly how support for new features should be added to an existing device. Empty buffers are placed in one virtqueue for receiving packets, and outgoing packets are enqueued into another for transmission in that order. A third command queue is used to control advanced filtering features. Configuration Subsystem Device ID 1 Virtqueues 0:receiveq. 1:transmitq. 2:controlq[12] Feature bits VIRTIO_NET_F_CSUM (0) Device handles packets with partial checksum VIRTIO_NET_F_GUEST_CSUM (1) Guest handles packets with partial checksum VIRTIO_NET_F_MAC (5) Device has given MAC address. VIRTIO_NET_F_GSO (6) (Deprecated) device handles packets with any GSO type.[13] VIRTIO_NET_F_GUEST_TSO4 (7) Guest can receive TSOv4. VIRTIO_NET_F_GUEST_TSO6 (8) Guest can receive TSOv6. VIRTIO_NET_F_GUEST_ECN (9) Guest can receive TSO with ECN. VIRTIO_NET_F_GUEST_UFO (10) Guest can receive UFO. VIRTIO_NET_F_HOST_TSO4 (11) Device can receive TSOv4. VIRTIO_NET_F_HOST_TSO6 (12) Device can receive TSOv6. VIRTIO_NET_F_HOST_ECN (13) Device can receive TSO with ECN. VIRTIO_NET_F_HOST_UFO (14) Device can receive UFO. VIRTIO_NET_F_MRG_RXBUF (15) Guest can merge receive buffers. VIRTIO_NET_F_STATUS (16) Configuration status field is available. VIRTIO_NET_F_CTRL_VQ (17) Control channel is available. VIRTIO_NET_F_CTRL_RX (18) Control channel RX mode support. VIRTIO_NET_F_CTRL_VLAN (19) Control channel VLAN filtering. VIRTIO_NET_F_GUEST_ANNOUNCE(21) Guest can send gratuitous packets. Device configuration layout Two configuration fields are currently defined. The mac address field always exists (though is only valid if VIRTIO_NET_F_MAC is set), and the status field only exists if VIRTIO_NET_F_STATUS is set. Two read-only bits are currently defined for the status field: VIRTIO_NET_S_LINK_UP and VIRTIO_NET_S_ANNOUNCE. #define VIRTIO_NET_S_LINK_UP 1 #define VIRTIO_NET_S_ANNOUNCE 2 struct virtio_net_config { u8 mac[6]; u16 status; }; Device Initialization 1. The initialization routine should identify the receive and transmission virtqueues. 2. If the VIRTIO_NET_F_MAC feature bit is set, the configuration space “mac” entry indicates the “physical” address of the the network card, otherwise a private MAC address should be assigned. All guests are expected to negotiate this feature if it is set. 3. If the VIRTIO_NET_F_CTRL_VQ feature bit is negotiated, identify the control virtqueue. 4. If the VIRTIO_NET_F_STATUS feature bit is negotiated, the link status can be read from the bottom bit of the “status” config field. Otherwise, the link should be assumed active. 5. The receive virtqueue should be filled with receive buffers. This is described in detail below in “Setting Up Receive Buffers”. 6. A driver can indicate that it will generate checksumless packets by negotating the VIRTIO_NET_F_CSUM feature. This “ checksum offload” is a common feature on modern network cards. 7. If that feature is negotiated[14], a driver can use TCP or UDP segmentation offload by negotiating the VIRTIO_NET_F_HOST_TSO4 (IPv4 TCP), VIRTIO_NET_F_HOST_TSO6 (IPv6 TCP) and VIRTIO_NET_F_HOST_UFO (UDP fragmentation) features. It should not send TCP packets requiring segmentation offload which have the Explicit Congestion Notification bit set, unless the VIRTIO_NET_F_HOST_ECN feature is negotiated.[15] 8. The converse features are also available: a driver can save the virtual device some work by negotiating these features.[16] The VIRTIO_NET_F_GUEST_CSUM feature indicates that partially checksummed packets can be received, and if it can do that then the VIRTIO_NET_F_GUEST_TSO4, VIRTIO_NET_F_GUEST_TSO6, VIRTIO_NET_F_GUEST_UFO and VIRTIO_NET_F_GUEST_ECN are the input equivalents of the features described above. See “Receiving Packets” below. Device Operation Packets are transmitted by placing them in the transmitq, and buffers for incoming packets are placed in the receiveq. In each case, the packet itself is preceeded by a header: struct virtio_net_hdr { #define VIRTIO_NET_HDR_F_NEEDS_CSUM 1 u8 flags; #define VIRTIO_NET_HDR_GSO_NONE 0 #define VIRTIO_NET_HDR_GSO_TCPV4 1 #define VIRTIO_NET_HDR_GSO_UDP 3 #define VIRTIO_NET_HDR_GSO_TCPV6 4 #define VIRTIO_NET_HDR_GSO_ECN 0x80 u8 gso_type; u16 hdr_len; u16 gso_size; u16 csum_start; u16 csum_offset; /* Only if VIRTIO_NET_F_MRG_RXBUF: */ u16 num_buffers }; The controlq is used to control device features such as filtering. Packet Transmission Transmitting a single packet is simple, but varies depending on the different features the driver negotiated. 1. If the driver negotiated VIRTIO_NET_F_CSUM, and the packet has not been fully checksummed, then the virtio_net_hdr's fields are set as follows. Otherwise, the packet must be fully checksummed, and flags is zero. • flags has the VIRTIO_NET_HDR_F_NEEDS_CSUM set, • csum_start is set to the offset within the packet to begin checksumming, and • csum_offset indicates how many bytes after the csum_start the new (16 bit ones' complement) checksum should be placed.[17] 2. If the driver negotiated VIRTIO_NET_F_HOST_TSO4, TSO6 or UFO, and the packet requires TCP segmentation or UDP fragmentation, then the “gso_type” field is set to VIRTIO_NET_HDR_GSO_TCPV4, TCPV6 or UDP. (Otherwise, it is set to VIRTIO_NET_HDR_GSO_NONE). In this case, packets larger than 1514 bytes can be transmitted: the metadata indicates how to replicate the packet header to cut it into smaller packets. The other gso fields are set: • hdr_len is a hint to the device as to how much of the header needs to be kept to copy into each packet, usually set to the length of the headers, including the transport header.[18] • gso_size is the maximum size of each packet beyond that header (ie. MSS). • If the driver negotiated the VIRTIO_NET_F_HOST_ECN feature, the VIRTIO_NET_HDR_GSO_ECN bit may be set in “gso_type” as well, indicating that the TCP packet has the ECN bit set.[19] 3. If the driver negotiated the VIRTIO_NET_F_MRG_RXBUF feature, the num_buffers field is set to zero. 4. The header and packet are added as one output buffer to the transmitq, and the device is notified of the new entry (see 2.4.1.4 Notifying The Device).[20] Packet Transmission Interrupt Often a driver will suppress transmission interrupts using the VRING_AVAIL_F_NO_INTERRUPT flag (see 2.4.2 Receiving Used Buffers From The Device) and check for used packets in the transmit path of following packets. However, it will still receive interrupts if the VIRTIO_F_NOTIFY_ON_EMPTY feature is negotiated, indicating that the transmission queue is completely emptied. The normal behavior in this interrupt handler is to retrieve and new descriptors from the used ring and free the corresponding headers and packets. Setting Up Receive Buffers It is generally a good idea to keep the receive virtqueue as fully populated as possible: if it runs out, network performance will suffer. If the VIRTIO_NET_F_GUEST_TSO4, VIRTIO_NET_F_GUEST_TSO6 or VIRTIO_NET_F_GUEST_UFO features are used, the Guest will need to accept packets of up to 65550 bytes long (the maximum size of a TCP or UDP packet, plus the 14 byte ethernet header), otherwise 1514 bytes. So unless VIRTIO_NET_F_MRG_RXBUF is negotiated, every buffer in the receive queue needs to be at least this length [20a] If VIRTIO_NET_F_MRG_RXBUF is negotiated, each buffer must be at least the size of the struct virtio_net_hdr. Packet Receive Interrupt When a packet is copied into a buffer in the receiveq, the optimal path is to disable further interrupts for the receiveq (see [sub:Receiving-Used-Buffers]) and process packets until no more are found, then re-enable them. Processing packet involves: 1. If the driver negotiated the VIRTIO_NET_F_MRG_RXBUF feature, then the “num_buffers” field indicates how many descriptors this packet is spread over (including this one). This allows receipt of large packets without having to allocate large buffers. In this case, there will be at least “num_buffers” in the used ring, and they should be chained together to form a single packet. The other buffers will not begin with a struct virtio_net_hdr. 2. If the VIRTIO_NET_F_MRG_RXBUF feature was not negotiated, or the “num_buffers” field is one, then the entire packet will be contained within this buffer, immediately following the struct virtio_net_hdr. 3. If the VIRTIO_NET_F_GUEST_CSUM feature was negotiated, the VIRTIO_NET_HDR_F_NEEDS_CSUM bit in the “flags” field may be set: if so, the checksum on the packet is incomplete and the “ csum_start” and “csum_offset” fields indicate how to calculate it (see Packet Transmission point 1). 4. If the VIRTIO_NET_F_GUEST_TSO4, TSO6 or UFO options were negotiated, then the “gso_type” may be something other than VIRTIO_NET_HDR_GSO_NONE, and the “gso_size” field indicates the desired MSS (see Packet Transmission point 2). Control Virtqueue The driver uses the control virtqueue (if VIRTIO_NET_F_VTRL_VQ is negotiated) to send commands to manipulate various features of the device which would not easily map into the configuration space. All commands are of the following form: struct virtio_net_ctrl { u8 class; u8 command; u8 command-specific-data[]; u8 ack; }; /* ack values */ #define VIRTIO_NET_OK 0 #define VIRTIO_NET_ERR 1 The class, command and command-specific-data are set by the driver, and the device sets the ack byte. There is little it can do except issue a diagnostic if the ack byte is not VIRTIO_NET_OK. Packet Receive Filtering If the VIRTIO_NET_F_CTRL_RX feature is negotiated, the driver can send control commands for promiscuous mode, multicast receiving, and filtering of MAC addresses. Note that in general, these commands are best-effort: unwanted packets may still arrive. Setting Promiscuous Mode #define VIRTIO_NET_CTRL_RX 0 #define VIRTIO_NET_CTRL_RX_PROMISC 0 #define VIRTIO_NET_CTRL_RX_ALLMULTI 1 The class VIRTIO_NET_CTRL_RX has two commands: VIRTIO_NET_CTRL_RX_PROMISC turns promiscuous mode on and off, and VIRTIO_NET_CTRL_RX_ALLMULTI turns all-multicast receive on and off. The command-specific-data is one byte containing 0 (off) or 1 (on). Setting MAC Address Filtering struct virtio_net_ctrl_mac { u32 entries; u8 macs[entries][ETH_ALEN]; }; #define VIRTIO_NET_CTRL_MAC 1 #define VIRTIO_NET_CTRL_MAC_TABLE_SET 0 The device can filter incoming packets by any number of destination MAC addresses.[21] This table is set using the class VIRTIO_NET_CTRL_MAC and the command VIRTIO_NET_CTRL_MAC_TABLE_SET. The command-specific-data is two variable length tables of 6-byte MAC addresses. The first table contains unicast addresses, and the second contains multicast addresses. VLAN Filtering If the driver negotiates the VIRTION_NET_F_CTRL_VLAN feature, it can control a VLAN filter table in the device. #define VIRTIO_NET_CTRL_VLAN 2 #define VIRTIO_NET_CTRL_VLAN_ADD 0 #define VIRTIO_NET_CTRL_VLAN_DEL 1 Both the VIRTIO_NET_CTRL_VLAN_ADD and VIRTIO_NET_CTRL_VLAN_DEL command take a 16-bit VLAN id as the command-specific-data. Gratuitous Packet Sending If the driver negotiates the VIRTIO_NET_F_GUEST_ANNOUNCE (depends on VIRTIO_NET_F_CTRL_VQ), it can ask the guest to send gratuitous packets; this is usually done after the guest has been physically migrated, and needs to announce its presence on the new network links. (As hypervisor does not have the knowledge of guest network configuration (eg. tagged vlan) it is simplest to prod the guest in this way). #define VIRTIO_NET_CTRL_ANNOUNCE 3 #define VIRTIO_NET_CTRL_ANNOUNCE_ACK 0 The Guest needs to check VIRTIO_NET_S_ANNOUNCE bit in status field when it notices the changes of device configuration. The command VIRTIO_NET_CTRL_ANNOUNCE_ACK is used to indicate that driver has recevied the notification and device would clear the VIRTIO_NET_S_ANNOUNCE bit in the status filed after it received this command. Processing this notification involves: 1. Sending the gratuitous packets or marking there are pending gratuitous packets to be sent and letting deferred routine to send them. 2. Sending VIRTIO_NET_CTRL_ANNOUNCE_ACK command through control vq. 3. . Appendix D: Block Device The virtio block device is a simple virtual block device (ie. disk). Read and write requests (and other exotic requests) are placed in the queue, and serviced (probably out of order) by the device except where noted. Configuration Subsystem Device ID 2 Virtqueues 0:requestq. Feature bits VIRTIO_BLK_F_BARRIER (0) Host supports request barriers. VIRTIO_BLK_F_SIZE_MAX (1) Maximum size of any single segment is in “size_max”. VIRTIO_BLK_F_SEG_MAX (2) Maximum number of segments in a request is in “seg_max”. VIRTIO_BLK_F_GEOMETRY (4) Disk-style geometry specified in “ geometry”. VIRTIO_BLK_F_RO (5) Device is read-only. VIRTIO_BLK_F_BLK_SIZE (6) Block size of disk is in “blk_size”. VIRTIO_BLK_F_SCSI (7) Device supports scsi packet commands. VIRTIO_BLK_F_FLUSH (9) Cache flush command support. Device configuration layout The capacity of the device (expressed in 512-byte sectors) is always present. The availability of the others all depend on various feature bits as indicated above. struct virtio_blk_config { u64 capacity; u32 size_max; u32 seg_max; struct virtio_blk_geometry { u16 cylinders; u8 heads; u8 sectors; } geometry; u32 blk_size; }; Device Initialization 1. The device size should be read from the “capacity” configuration field. No requests should be submitted which goes beyond this limit. 2. If the VIRTIO_BLK_F_BLK_SIZE feature is negotiated, the blk_size field can be read to determine the optimal sector size for the driver to use. This does not effect the units used in the protocol (always 512 bytes), but awareness of the correct value can effect performance. 3. If the VIRTIO_BLK_F_RO feature is set by the device, any write requests will fail. Device Operation The driver queues requests to the virtqueue, and they are used by the device (not necessarily in order). Each request is of form: struct virtio_blk_req { u32 type; u32 ioprio; u64 sector; char data[][512]; u8 status; }; If the device has VIRTIO_BLK_F_SCSI feature, it can also support scsi packet command requests, each of these requests is of form: struct virtio_scsi_pc_req { u32 type; u32 ioprio; u64 sector; char cmd[]; char data[][512]; #define SCSI_SENSE_BUFFERSIZE 96 u8 sense[SCSI_SENSE_BUFFERSIZE]; u32 errors; u32 data_len; u32 sense_len; u32 residual; u8 status; }; The type of the request is either a read (VIRTIO_BLK_T_IN), a write (VIRTIO_BLK_T_OUT), a scsi packet command (VIRTIO_BLK_T_SCSI_CMD or VIRTIO_BLK_T_SCSI_CMD_OUT[22]) or a flush (VIRTIO_BLK_T_FLUSH or VIRTIO_BLK_T_FLUSH_OUT[23]). If the device has VIRTIO_BLK_F_BARRIER feature the high bit (VIRTIO_BLK_T_BARRIER) indicates that this request acts as a barrier and that all preceeding requests must be complete before this one, and all following requests must not be started until this is complete. Note that a barrier does not flush caches in the underlying backend device in host, and thus does not serve as data consistency guarantee. Driver must use FLUSH request to flush the host cache. #define VIRTIO_BLK_T_IN 0 #define VIRTIO_BLK_T_OUT 1 #define VIRTIO_BLK_T_SCSI_CMD 2 #define VIRTIO_BLK_T_SCSI_CMD_OUT 3 #define VIRTIO_BLK_T_FLUSH 4 #define VIRTIO_BLK_T_FLUSH_OUT 5 #define VIRTIO_BLK_T_BARRIER 0x80000000 The ioprio field is a hint about the relative priorities of requests to the device: higher numbers indicate more important requests. The sector number indicates the offset (multiplied by 512) where the read or write is to occur. This field is unused and set to 0 for scsi packet commands and for flush commands. The cmd field is only present for scsi packet command requests, and indicates the command to perform. This field must reside in a single, separate read-only buffer; command length can be derived from the length of this buffer. Note that these first three (four for scsi packet commands) fields are always read-only: the data field is either read-only or write-only, depending on the request. The size of the read or write can be derived from the total size of the request buffers. The sense field is only present for scsi packet command requests, and indicates the buffer for scsi sense data. The data_len field is only present for scsi packet command requests, this field is deprecated, and should be ignored by the driver. Historically, devices copied data length there. The sense_len field is only present for scsi packet command requests and indicates the number of bytes actually written to the sense buffer. The residual field is only present for scsi packet command requests and indicates the residual size, calculated as data length - number of bytes actually transferred. The final status byte is written by the device: either VIRTIO_BLK_S_OK for success, VIRTIO_BLK_S_IOERR for host or guest error or VIRTIO_BLK_S_UNSUPP for a request unsupported by host: #define VIRTIO_BLK_S_OK 0 #define VIRTIO_BLK_S_IOERR 1 #define VIRTIO_BLK_S_UNSUPP 2 Historically, devices assumed that the fields type, ioprio and sector reside in a single, separate read-only buffer; the fields errors, data_len, sense_len and residual reside in a single, separate write-only buffer; the sense field in a separate write-only buffer of size 96 bytes, by itself; the fields errors, data_len, sense_len and residual in a single write-only buffer; and the status field is a separate read-only buffer of size 1 byte, by itself. Appendix E: Console Device The virtio console device is a simple device for data input and output. A device may have one or more ports. Each port has a pair of input and output virtqueues. Moreover, a device has a pair of control IO virtqueues. The control virtqueues are used to communicate information between the device and the driver about ports being opened and closed on either side of the connection, indication from the host about whether a particular port is a console port, adding new ports, port hot-plug/unplug, etc., and indication from the guest about whether a port or a device was successfully added, port open/close, etc.. For data IO, one or more empty buffers are placed in the receive queue for incoming data and outgoing characters are placed in the transmit queue. Configuration Subsystem Device ID 3 Virtqueues 0:receiveq(port0). 1:transmitq(port0), 2:control receiveq[24], 3:control transmitq, 4:receiveq(port1), 5:transmitq(port1), ... Feature bits VIRTIO_CONSOLE_F_SIZE (0) Configuration cols and rows fields are valid. VIRTIO_CONSOLE_F_MULTIPORT(1) Device has support for multiple ports; configuration fields nr_ports and max_nr_ports are valid and control virtqueues will be used. Device configuration layout The size of the console is supplied in the configuration space if the VIRTIO_CONSOLE_F_SIZE feature is set. Furthermore, if the VIRTIO_CONSOLE_F_MULTIPORT feature is set, the maximum number of ports supported by the device can be fetched. struct virtio_console_config { u16 cols; u16 rows; u32 max_nr_ports; }; Device Initialization 1. If the VIRTIO_CONSOLE_F_SIZE feature is negotiated, the driver can read the console dimensions from the configuration fields. 2. If the VIRTIO_CONSOLE_F_MULTIPORT feature is negotiated, the driver can spawn multiple ports, not all of which may be attached to a console. Some could be generic ports. In this case, the control virtqueues are enabled and according to the max_nr_ports configuration-space value, the appropriate number of virtqueues are created. A control message indicating the driver is ready is sent to the host. The host can then send control messages for adding new ports to the device. After creating and initializing each port, a VIRTIO_CONSOLE_PORT_READY control message is sent to the host for that port so the host can let us know of any additional configuration options set for that port. 3. The receiveq for each port is populated with one or more receive buffers. Device Operation 1. For output, a buffer containing the characters is placed in the port's transmitq.[25] 2. When a buffer is used in the receiveq (signalled by an interrupt), the contents is the input to the port associated with the virtqueue for which the notification was received. 3. If the driver negotiated the VIRTIO_CONSOLE_F_SIZE feature, a configuration change interrupt may occur. The updated size can be read from the configuration fields. 4. If the driver negotiated the VIRTIO_CONSOLE_F_MULTIPORT feature, active ports are announced by the host using the VIRTIO_CONSOLE_PORT_ADD control message. The same message is used for port hot-plug as well. 5. If the host specified a port `name', a sysfs attribute is created with the name filled in, so that udev rules can be written that can create a symlink from the port's name to the char device for port discovery by applications in the guest. 6. Changes to ports' state are effected by control messages. Appropriate action is taken on the port indicated in the control message. The layout of the structure of the control buffer and the events associated are: struct virtio_console_control { uint32_t id; /* Port number */ uint16_t event; /* The kind of control event */ uint16_t value; /* Extra information for the event */ }; /* Some events for the internal messages (control packets) */ #define VIRTIO_CONSOLE_DEVICE_READY 0 #define VIRTIO_CONSOLE_PORT_ADD 1 #define VIRTIO_CONSOLE_PORT_REMOVE 2 #define VIRTIO_CONSOLE_PORT_READY 3 #define VIRTIO_CONSOLE_CONSOLE_PORT 4 #define VIRTIO_CONSOLE_RESIZE 5 #define VIRTIO_CONSOLE_PORT_OPEN 6 #define VIRTIO_CONSOLE_PORT_NAME 7 Appendix F: Entropy Device The virtio entropy device supplies high-quality randomness for guest use. Configuration Subsystem Device ID 4 Virtqueues 0:requestq. Feature bits None currently defined Device configuration layout None currently defined. Device Initialization 1. The virtqueue is initialized Device Operation When the driver requires random bytes, it places the descriptor of one or more buffers in the queue. It will be completely filled by random data by the device. Appendix G: Memory Balloon Device The virtio memory balloon device is a primitive device for managing guest memory: the device asks for a certain amount of memory, and the guest supplies it (or withdraws it, if the device has more than it asks for). This allows the guest to adapt to changes in allowance of underlying physical memory. If the feature is negotiated, the device can also be used to communicate guest memory statistics to the host. Configuration Subsystem Device ID 5 Virtqueues 0:inflateq. 1:deflateq. 2:statsq.[26] Feature bits VIRTIO_BALLOON_F_MUST_TELL_HOST (0) Host must be told before pages from the balloon are used. VIRTIO_BALLOON_F_STATS_VQ (1) A virtqueue for reporting guest memory statistics is present. Device configuration layout Both fields of this configuration are always available. Note that they are little endian, despite convention that device fields are guest endian: struct virtio_balloon_config { u32 num_pages; u32 actual; }; Device Initialization 1. The inflate and deflate virtqueues are identified. 2. If the VIRTIO_BALLOON_F_STATS_VQ feature bit is negotiated: (a) Identify the stats virtqueue. (b) Add one empty buffer to the stats virtqueue and notify the host. Device operation begins immediately. Device Operation Memory Ballooning The device is driven by the receipt of a configuration change interrupt. 1. The “num_pages” configuration field is examined. If this is greater than the “actual” number of pages, memory must be given to the balloon. If it is less than the “actual” number of pages, memory may be taken back from the balloon for general use. 2. To supply memory to the balloon (aka. inflate): (a) The driver constructs an array of addresses of unused memory pages. These addresses are divided by 4096[27] and

On Wed, Jul 31, 2013 at 01:33:39PM +0930, Rusty Russell wrote: > Rusty Russell <rusty@au1.ibm.com> writes: > The obvious issues I want to open are: > > o Major rework to make PCI an appendix, and core bus-independent. > o Update spec with changes/fixes since 0.9.5 > - This is easy where contributors are already members, > trickier for others. Include a complete vring reference implementation with tests? This way more code snippets in the specification can be runnable and tested. > Block: > o Remove VIRTIO_BLK_F_SCSI? > o Revisit flush/barrier semantics? Barrier is ignored in modern QEMU virtio-blk, instead guests send the flush command. We could deprecate or drop the barrier from the spec so that only flush commands are used.

On Wed, 2013-07-31 at 05:03 +0100, Rusty Russell wrote: > o Endian for config space > - LE everywhere? +1 My agenda: o Change addressing scheme in virtio-mmio > Appendix X: virtio-mmio > • 0x028 W GuestPageSize > • 0x040 RW QueuePFN Replace these two guys with normal 64-bit wide physical address (in two Hi/Lo registers). o NOP/null/dummy device Reserve ID 0 for devices with no function and specify drivers behaviour for them. Usecase: qemu folk want to be able to create a pool of memory mapped devices and assign functions to them later, although before the OS is actually initialized. Pawel

On Wed, Jul 31, 2013 at 10:06:31AM +0100, Pawel Moll wrote: > On Wed, 2013-07-31 at 05:03 +0100, Rusty Russell wrote: > > o Endian for config space > > - LE everywhere? > > +1 > > My agenda: > > o Change addressing scheme in virtio-mmio > > > Appendix X: virtio-mmio > > • 0x028 W GuestPageSize > > • 0x040 RW QueuePFN > > Replace these two guys with normal 64-bit wide physical address (in two > Hi/Lo registers). > > o NOP/null/dummy device > > Reserve ID 0 for devices with no function and specify drivers behaviour > for them. Usecase: qemu folk want to be able to create a pool of memory > mapped devices and assign functions to them later, although before the > OS is actually initialized. > > Pawel Sounds a bit crazy, probably won't work for PCI (spec violation). Link to a discussion? > > > --------------------------------------------------------------------- > To unsubscribe from this mail list, you must leave the OASIS TC that > generates this mail. Follow this link to all your TCs in OASIS at: > https://www.oasis-open.org/apps/org/workgroup/portal/my_workgroups.php

On Wed, 2013-07-31 at 10:46 +0100, Michael S. Tsirkin wrote: > > o NOP/null/dummy device > > > > Reserve ID 0 for devices with no function and specify drivers behaviour > > for them. Usecase: qemu folk want to be able to create a pool of memory > > mapped devices and assign functions to them later, although before the > > OS is actually initialized. > > Sounds a bit crazy, Thank you :-) > probably won't work for PCI (spec violation). Em, which one of specs? What I'm talking about is reserving a "Subsystem Device ID" (p.2.1 of the current spec) value 0 to a "device that does nothing" (is not a network card, nor block device, nor console, nor etc. nor anything else) and specifying how is the guest supposed to behave with such device (do nothing). In the current speech it would be "Appendix I: Doing Nothing Device", following "Appendix I: SCSI Host Device". How does this break anything? > Link to a discussion? http://thread.gmane.org/gmane.linux.kernel.virtualization/20144 Pawel

On Wed, Jul 31, 2013 at 10:58:21AM +0100, Pawel Moll wrote: > On Wed, 2013-07-31 at 10:46 +0100, Michael S. Tsirkin wrote: > > > o NOP/null/dummy device > > > > > > Reserve ID 0 for devices with no function and specify drivers behaviour > > > for them. Usecase: qemu folk want to be able to create a pool of memory > > > mapped devices and assign functions to them later, although before the > > > OS is actually initialized. > > > > Sounds a bit crazy, > > Thank you :-) > > > probably won't work for PCI (spec violation). > > Em, which one of specs? What I'm talking about is reserving a "Subsystem > Device ID" (p.2.1 of the current spec) value 0 to a "device that does > nothing" (is not a network card, nor block device, nor console, nor etc. > nor anything else) and specifying how is the guest supposed to behave > with such device (do nothing). In the current speech it would be > "Appendix I: Doing Nothing Device", following "Appendix I: SCSI Host > Device". How does this break anything? It doesn't but then it will have to keep doing nothing :) The current spec actually doesn't work either - I was going to open an item for this - is the system open already? Basically OS-es expect each PCI device to declare the function using device ID, not subsystem device ID. Linux is more flexible bug e.g. windows isn't, so windows drivers require device id to match the intended function. > > Link to a discussion? > > http://thread.gmane.org/gmane.linux.kernel.virtualization/20144 > > Pawel > > > > --------------------------------------------------------------------- > To unsubscribe from this mail list, you must leave the OASIS TC that > generates this mail. Follow this link to all your TCs in OASIS at: > https://www.oasis-open.org/apps/org/workgroup/portal/my_workgroups.php

On Wed, 2013-07-31 at 11:29 +0100, Michael S. Tsirkin wrote: > Em, which one of specs? What I'm talking about is reserving a "Subsystem > > Device ID" (p.2.1 of the current spec) value 0 to a "device that does > > nothing" (is not a network card, nor block device, nor console, nor etc. > > nor anything else) and specifying how is the guest supposed to behave > > with such device (do nothing). In the current speech it would be > > "Appendix I: Doing Nothing Device", following "Appendix I: SCSI Host > > Device". How does this break anything? > > It doesn't but then it will have to keep doing nothing :) And that's the general plan. I had a vague idea of "dynamic devices" (ie. changing their "identity" in runtime), but this is just one step too far. On Wed, 2013-07-31 at 11:34 +0100, Michael S. Tsirkin wrote: > Reading through this, you want a way to say "there's no device there" > for virtio-mmio? And maybe virtio-ccw. And virtio-smoke-signals is anyone ever implements it :-) But seriously speaking, I'm just trying to formalize the current behaviour of the core. Today it does nothing if there is no driver for a particular VIRTIO_ID_*, and there is no (and will never be) driver with 0 in id_table. > > I think this is a bus-specific problem. PCI has a different, standard > way to do this (report 0xfffffffff as both device and vendor ID). > And I want to provide a bus-agnostic solution. Of course the PCI backends may never instantiate such device if they wish (and it probably wouldn't make any sense whatsoever). Pawel

On Wed, Jul 31, 2013 at 12:05:07PM +0100, Pawel Moll wrote: > On Wed, 2013-07-31 at 11:29 +0100, Michael S. Tsirkin wrote: > > Em, which one of specs? What I'm talking about is reserving a "Subsystem > > > Device ID" (p.2.1 of the current spec) value 0 to a "device that does > > > nothing" (is not a network card, nor block device, nor console, nor etc. > > > nor anything else) and specifying how is the guest supposed to behave > > > with such device (do nothing). In the current speech it would be > > > "Appendix I: Doing Nothing Device", following "Appendix I: SCSI Host > > > Device". How does this break anything? > > > > It doesn't but then it will have to keep doing nothing :) > > And that's the general plan. Confused. You wrote: usecase: create a pool of memory mapped devices and assign functions to them later Sounds like you really want a way to say "there's no device there" not "there's a device here but it does nothing". > I had a vague idea of "dynamic > devices" (ie. changing their "identity" in runtime), but this is just > one step too far. > > On Wed, 2013-07-31 at 11:34 +0100, Michael S. Tsirkin wrote: > > Reading through this, you want a way to say "there's no device there" > > for virtio-mmio? > > And maybe virtio-ccw. And virtio-smoke-signals is anyone ever implements > it :-) Yay, virtio-isa :). Many buses have device presence detection built-in. Many have hotplug support. If a bus doesn't have these you have to design something device specific but I don't think it makes sense to add this overhead for buses that do. > But seriously speaking, I'm just trying to formalize the current > behaviour of the core. Today it does nothing if there is no driver for a > particular VIRTIO_ID_*, and there is no (and will never be) driver with > 0 in id_table. The core might not do anything, but the OS does (e.g. prompt for a driver). Basically spec describes what Linux virtio does, but it's not possible to support what spec describes (match all device IDs, use subsystem ID to select the driver) on all OS-es. So we need to fix this part of spec, not build upon this bug to add more features that not all guests will be able to support. > > > > I think this is a bus-specific problem. PCI has a different, standard > > way to do this (report 0xfffffffff as both device and vendor ID). > > > And I want to provide a bus-agnostic solution. Of course the PCI > backends may never instantiate such device if they wish (and it probably > wouldn't make any sense whatsoever). > > Pawel > There's no way to implement this in a bus-agnostic way - it conflicts with existing bus-specific mechanisms. So just put this in the virtio-mmio appendix and don't try to anticipate the future. > > --------------------------------------------------------------------- > To unsubscribe from this mail list, you must leave the OASIS TC that > generates this mail. Follow this link to all your TCs in OASIS at: > https://www.oasis-open.org/apps/org/workgroup/portal/my_workgroups.php

On Wed, 2013-07-31 at 12:43 +0100, Michael S. Tsirkin wrote: > On Wed, Jul 31, 2013 at 12:05:07PM +0100, Pawel Moll wrote: > > On Wed, 2013-07-31 at 11:29 +0100, Michael S. Tsirkin wrote: > > > Em, which one of specs? What I'm talking about is reserving a "Subsystem > > > > Device ID" (p.2.1 of the current spec) value 0 to a "device that does > > > > nothing" (is not a network card, nor block device, nor console, nor etc. > > > > nor anything else) and specifying how is the guest supposed to behave > > > > with such device (do nothing). In the current speech it would be > > > > "Appendix I: Doing Nothing Device", following "Appendix I: SCSI Host > > > > Device". How does this break anything? > > > > > > It doesn't but then it will have to keep doing nothing :) > > > > And that's the general plan. > > Confused. You wrote: usecase: > create a pool of memory > mapped devices and assign functions to them later > Sounds like you really want a way to say > "there's no device there" not "there's a device here but it does nothing". Hm. Qemu people can't say "there's no device there", so they wanted to define a set of "devices doing nothing" as a default (at well known addresses, so the guest OS knows exactly where to look for virtio devices) and sometimes make some of them do something depending on the command line options. All this *before* the OS is actually launched, so when this happens the landscape is already set, eg. either 4 do-nothing devices, or 3 do-nothings and one block device, or 2 do-nothings, one block, one network device, etc. Of course you can argue that they should go back and redesign Qemu (this problem pretty much boils down to the way they generate Device Trees). Maybe they should, but I really really don't see anything wrong with providing them this little courtesy. > Many buses have device presence detection built-in. > Many have hotplug support. If a bus doesn't have these you > have to design something device specific but I don't think > it makes sense to add this overhead for buses that do. What kind of overhead do you mean? I see completely no overhead for anyone. Any example where would it have an impact on the PCI implementation? > > But seriously speaking, I'm just trying to formalize the current > > behaviour of the core. Today it does nothing if there is no driver for a > > particular VIRTIO_ID_*, and there is no (and will never be) driver with > > 0 in id_table. > > The core might not do anything, but the OS does (e.g. prompt for > a driver). > > Basically spec describes what Linux virtio does, but it's > not possible to support what spec describes (match all > device IDs, use subsystem ID to select the driver) on all OS-es. > So we need to fix this part of spec, not build upon this bug > to add more features that not all guests will be able to support. Now it's me who is confused. At some point the bus-specific implementation (mmio, PCI, ccw) does ...(struct virtio_device).id.device = <value> or (equivalent in other OS) and than the virtio-core (or equivalent in other OS) makes some further decision based on the <value>. I want to make sure that if the <value> == 0, the core will not even try to look for a driver for such device. I think any OS can support such "feature". Or maybe I'm missing some fundamental problem (what would it be) or we are simply talking about different things? Pawel

On Wed, Jul 31, 2013 at 01:25:41PM +0100, Pawel Moll wrote: > On Wed, 2013-07-31 at 12:43 +0100, Michael S. Tsirkin wrote: > > On Wed, Jul 31, 2013 at 12:05:07PM +0100, Pawel Moll wrote: > > > On Wed, 2013-07-31 at 11:29 +0100, Michael S. Tsirkin wrote: > > > > Em, which one of specs? What I'm talking about is reserving a "Subsystem > > > > > Device ID" (p.2.1 of the current spec) value 0 to a "device that does > > > > > nothing" (is not a network card, nor block device, nor console, nor etc. > > > > > nor anything else) and specifying how is the guest supposed to behave > > > > > with such device (do nothing). In the current speech it would be > > > > > "Appendix I: Doing Nothing Device", following "Appendix I: SCSI Host > > > > > Device". How does this break anything? > > > > > > > > It doesn't but then it will have to keep doing nothing :) > > > > > > And that's the general plan. > > > > Confused. You wrote: usecase: > > create a pool of memory > > mapped devices and assign functions to them later > > Sounds like you really want a way to say > > "there's no device there" not "there's a device here but it does nothing". > > Hm. Qemu people can't say "there's no device there", so they wanted to > define a set of "devices doing nothing" as a default (at well known > addresses, so the guest OS knows exactly where to look for virtio > devices) and sometimes make some of them do something depending on the > command line options. All this *before* the OS is actually launched, so > when this happens the landscape is already set, eg. either 4 do-nothing > devices, or 3 do-nothings and one block device, or 2 do-nothings, one > block, one network device, etc. > > Of course you can argue that they should go back and redesign Qemu (this > problem pretty much boils down to the way they generate Device Trees). > Maybe they should, but I really really don't see anything wrong with > providing them this little courtesy. So is this an intermediate configuration guest never sees? In that case why is anything at all required in the spec? > > Many buses have device presence detection built-in. > > Many have hotplug support. If a bus doesn't have these you > > have to design something device specific but I don't think > > it makes sense to add this overhead for buses that do. > > What kind of overhead do you mean? I see completely no overhead for > anyone. Any example where would it have an impact on the PCI > implementation? Hard to say since you didn't send a patch. Basically are you adding a new device with ID 0 for all buses? In practice in PCI we use same device ID and subsystem device ID. Here we'll have to come up with some special ID just for that. Not good, better to use ID from a standard range. > > > But seriously speaking, I'm just trying to formalize the current > > > behaviour of the core. Today it does nothing if there is no driver for a > > > particular VIRTIO_ID_*, and there is no (and will never be) driver with > > > 0 in id_table. > > > > The core might not do anything, but the OS does (e.g. prompt for > > a driver). > > > > Basically spec describes what Linux virtio does, but it's > > not possible to support what spec describes (match all > > device IDs, use subsystem ID to select the driver) on all OS-es. > > So we need to fix this part of spec, not build upon this bug > > to add more features that not all guests will be able to support. > > Now it's me who is confused. At some point the bus-specific > implementation (mmio, PCI, ccw) does > > ...(struct virtio_device).id.device = <value> > > or > (equivalent in other OS) > > and than the virtio-core (or equivalent in other OS) makes some further > decision based on the <value>. > > I want to make sure that if the <value> == 0, the core will not even try > to look for a driver for such device. I think any OS can support such > "feature". > > Or maybe I'm missing some fundamental problem (what would it be) or we > are simply talking about different things? > > Pawel I'm confused by you discussing "core" here. We are talking about driver/device interface right? This is what the spec describes. So some buses don't allow ID 0, and some OSes switch to strange failure modes if you specify device without a driver. Some OSes and buses have standard ways to detect device presence and report it to OS. So we can insert text that requires handling ID 0 in virtio-mmio appendix which is bus specifix and appears to be somewhat linux specific. > > > --------------------------------------------------------------------- > To unsubscribe from this mail list, you must leave the OASIS TC that > generates this mail. Follow this link to all your TCs in OASIS at: > https://www.oasis-open.org/apps/org/workgroup/portal/my_workgroups.php

On Wed, 2013-07-31 at 13:57 +0100, Michael S. Tsirkin wrote: > > Hm. Qemu people can't say "there's no device there", so they wanted to > > define a set of "devices doing nothing" as a default (at well known > > addresses, so the guest OS knows exactly where to look for virtio > > devices) and sometimes make some of them do something depending on the > > command line options. All this *before* the OS is actually launched, so > > when this happens the landscape is already set, eg. either 4 do-nothing > > devices, or 3 do-nothings and one block device, or 2 do-nothings, one > > block, one network device, etc. > > > > Of course you can argue that they should go back and redesign Qemu (this > > problem pretty much boils down to the way they generate Device Trees). > > Maybe they should, but I really really don't see anything wrong with > > providing them this little courtesy. > > So is this an intermediate configuration guest never sees? > In that case why is anything at all required in the spec? Because eventually there will be always 4 devices seen by the guest. And how do you make all of them do nothing in a universal way? > > What kind of overhead do you mean? I see completely no overhead for > > anyone. Any example where would it have an impact on the PCI > > implementation? > > Hard to say since you didn't send a patch. > Basically are you adding a new device with ID 0 > for all buses? > > In practice in PCI we use same device ID and > subsystem device ID. Here we'll have to come up > with some special ID just for that. Not good, better > to use ID from a standard range. Ok, are you saying that pci_dev->subsystem_device must no be 0? If so, would #define VIRTIO_ID_NULL 42 /* or _NOP or _DUMMY */ (the value has been chosen semi-randomly ;-) be better? I'm talking about 0 because I never expected it to have any side-effects. > I'm confused by you discussing "core" here. We are talking > about driver/device interface right? This is what > the spec describes. Yes, but to my mind the virtio devices (block, network etc.) constitute their own bus with driver/device interface, which is independent of the underlying physical bus (being PCI, mmio etc.). And what I propose is to make one ID special > So some buses don't allow ID 0, and some OSes switch to > strange failure modes if you specify device without > a driver. Are you saying that the existence of (let's just pick an example) VIRTIO_ID_RPROC_SERIAL ID is a problem, because not all OSes will implement it? > Some OSes and buses have standard ways to detect > device presence and report it to OS. Ok, so I treat VIRTIO_ID_* values (include/uapi/linux/virtio_ids.h) as bus-independent. If you're saying that 0 will be a problem for PCI, I'm perfectly happy to use 13 (currently the next available number) or any other value, really. 0 seemed the most natural choice, but nothing more than this. > So we can insert text that requires handling ID 0 > in virtio-mmio appendix which is bus specifix and > appears to be somewhat linux specific. The list of IDs is not in the mmio appendix. And I hope that the OASIS spec will not make PCI implementation in anyway privileged over mmio, thus both will use the same abstract list of *virtio device* IDs. Which is maybe what you meant by "fixing the spec" in the first place? Pawel

On Wed, Jul 31, 2013 at 02:33:51PM +0100, Pawel Moll wrote: > On Wed, 2013-07-31 at 13:57 +0100, Michael S. Tsirkin wrote: > > > Hm. Qemu people can't say "there's no device there", so they wanted to > > > define a set of "devices doing nothing" as a default (at well known > > > addresses, so the guest OS knows exactly where to look for virtio > > > devices) and sometimes make some of them do something depending on the > > > command line options. All this *before* the OS is actually launched, so > > > when this happens the landscape is already set, eg. either 4 do-nothing > > > devices, or 3 do-nothings and one block device, or 2 do-nothings, one > > > block, one network device, etc. > > > > > > Of course you can argue that they should go back and redesign Qemu (this > > > problem pretty much boils down to the way they generate Device Trees). > > > Maybe they should, but I really really don't see anything wrong with > > > providing them this little courtesy. > > > > So is this an intermediate configuration guest never sees? > > In that case why is anything at all required in the spec? > > Because eventually there will be always 4 devices seen by the guest. And > how do you make all of them do nothing in a universal way? okay so that's something very virtio-mmio specific, isn't it? > > > What kind of overhead do you mean? I see completely no overhead for > > > anyone. Any example where would it have an impact on the PCI > > > implementation? > > > > Hard to say since you didn't send a patch. > > Basically are you adding a new device with ID 0 > > for all buses? > > > > In practice in PCI we use same device ID and > > subsystem device ID. Here we'll have to come up > > with some special ID just for that. Not good, better > > to use ID from a standard range. > > Ok, are you saying that pci_dev->subsystem_device must no be 0? If so, > would > > #define VIRTIO_ID_NULL 42 /* or _NOP or _DUMMY */ If we need this device, just use the next free number. 13 isn't it? But see below. > (the value has been chosen semi-randomly ;-) be better? I'm talking > about 0 because I never expected it to have any side-effects. > > I'm confused by you discussing "core" here. We are talking > > about driver/device interface right? This is what > > the spec describes. > > Yes, but to my mind the virtio devices (block, network etc.) constitute > their own bus with driver/device interface, which is independent of the > underlying physical bus (being PCI, mmio etc.). And what I propose is to > make one ID special That's hardly universal. At least from the spec and guest/host interface perspective, there's no virtio bus. Linux drivers have their own bus mostly because it's so easy in Linux, but it's messier in windows so they don't do this. > > So some buses don't allow ID 0, and some OSes switch to > > strange failure modes if you specify device without > > a driver. > > Are you saying that the existence of (let's just pick an example) > VIRTIO_ID_RPROC_SERIAL ID is a problem, because not all OSes will > implement it? OS-es that trigger errors/warnings on unknown devices will have to at least add a stub driver to avoid these errors. So there's cost associated with adding more device IDs for all buses, there should be some benefit. > > Some OSes and buses have standard ways to detect > > device presence and report it to OS. > > Ok, so I treat VIRTIO_ID_* values (include/uapi/linux/virtio_ids.h) as > bus-independent. If you're saying that 0 will be a problem for PCI, I'm > perfectly happy to use 13 (currently the next available number) or any > other value, really. 0 seemed the most natural choice, but nothing more > than this. > > > So we can insert text that requires handling ID 0 > > in virtio-mmio appendix which is bus specifix and > > appears to be somewhat linux specific. > > The list of IDs is not in the mmio appendix. And I hope that the OASIS > spec will not make PCI implementation in anyway privileged over mmio, For now we are using PCI IDs for enumeration. So we better make sure they are legal PCI IDs. virtio PCI is used by many non linux guests. This makes us worry more about compatibility and legacy with respect to virtio PCI. It's different, I'm not sure I would call it "priveledged" :) > thus both will use the same abstract list of *virtio device* IDs. Which > is maybe what you meant by "fixing the spec" in the first place? > > Pawel I meant using standard bus specific things where we are describing bus specific behaviour. In this case, I think you really want a "no device" flag for virtio-mmio which originally lacked device presence detection. I think for this purpose, it is best to to: 1. declare device ID 0 (or e.g. FFFF) as reserved and illegal for devices, explicitly in core spec 2. in the mmio appendix, say that if ID 0 (or FFFF) is detected, this means device not present I think this is the cleaner approach than saying there's a dummy "null device" in particular because 1. there won't be dummy devices on the bus, in sysfs etc 2. down the road you will be able to support hotplug. -- MST

Pawel Moll <pawel.moll@arm.com> writes: > On Wed, 2013-07-31 at 12:43 +0100, Michael S. Tsirkin wrote: >> On Wed, Jul 31, 2013 at 12:05:07PM +0100, Pawel Moll wrote: >> > On Wed, 2013-07-31 at 11:29 +0100, Michael S. Tsirkin wrote: >> > > Em, which one of specs? What I'm talking about is reserving a "Subsystem >> > > > Device ID" (p.2.1 of the current spec) value 0 to a "device that does >> > > > nothing" (is not a network card, nor block device, nor console, nor etc. >> > > > nor anything else) and specifying how is the guest supposed to behave >> > > > with such device (do nothing). In the current speech it would be >> > > > "Appendix I: Doing Nothing Device", following "Appendix I: SCSI Host >> > > > Device". How does this break anything? >> > > >> > > It doesn't but then it will have to keep doing nothing :) >> > >> > And that's the general plan. >> >> Confused. You wrote: usecase: >> create a pool of memory >> mapped devices and assign functions to them later >> Sounds like you really want a way to say >> "there's no device there" not "there's a device here but it does nothing". > > Hm. Qemu people can't say "there's no device there", so they wanted to > define a set of "devices doing nothing" as a default (at well known > addresses, so the guest OS knows exactly where to look for virtio > devices) I'm not sure where this discussion took place, but there is nothing in QEMU that requires a dummy device to be used. The analogy would be a PCI slot that doesn't have a device plugged into it. Per the PCI spec, if a device is absent, any reads to the slot will return a well defined value (all ones). Those values are invalid for things like a device id. > and sometimes make some of them do something depending on the > command line options. All this *before* the OS is actually launched, so > when this happens the landscape is already set, eg. either 4 do-nothing > devices, or 3 do-nothings and one block device, or 2 do-nothings, one > block, one network device, etc. I think you're confusing things a bit. The way virtio-mmio is initialized in QEMU means that the QEMU machine has to explicitly create the transport devices. They must have fixed MMIO addresses, etc. virtio-mmio is a essentially a virtio host controller that can support at most one device attached to it. I really dislike the design of virtio-mmio. I don't understand why it limits itself to a single device. But that's a different discussion. virtio-blk is a separate device that can be connected to the transport. And yes, there should be a way to specify "this transport is not connected to anything". But that's distinctly different than a device that does nothing. We do not model a dummy device in QEMU. Regards, Anthony Liguori

On Wed, 2013-07-31 at 14:25 +0100, Anthony Liguori wrote: > I'm not sure where this discussion took place, but there is nothing in > QEMU that requires a dummy device to be used. http://thread.gmane.org/gmane.linux.kernel.virtualization/20144 > The analogy would be a PCI slot that doesn't have a device plugged into > it. Per the PCI spec, if a device is absent, any reads to the slot > will return a well defined value (all ones). Those values are invalid > for things like a device id. It is very unsafe to "probe" memory mapped peripherals. > I really dislike the design of virtio-mmio. I don't understand why it > limits itself to a single device. It is modelled after simple memory mapped peripherals, so it is simple itself. And if I was doing it today from scratch again, I would still focus on doing one thing only. After all there are discoverable buses out there, I don't see any reason to try to invent my own discovery mechanism. > But that's a different discussion. It is indeed, and I am open to all suggestions how to make it more likeable :-) > virtio-blk is a separate device that can be connected to the transport. As in: block device of zero size? This has also been suggested in the thread linked above. > And yes, there should be a way to specify "this transport is not > connected to anything". In a "virtio generic" way or mmio-specific one? Maybe instead of defining zero as a do-nothing-device we should simply make this value reserved or illegal? This would make PCI situation clear (no device will be ever allowed to have it) and mmio driver could "overload" its meaning as "ignore". Pawel

On Wed, Jul 31, 2013 at 03:37:01PM +0100, Pawel Moll wrote: > On Wed, 2013-07-31 at 14:25 +0100, Anthony Liguori wrote: > > I'm not sure where this discussion took place, but there is nothing in > > QEMU that requires a dummy device to be used. > > http://thread.gmane.org/gmane.linux.kernel.virtualization/20144 > > > The analogy would be a PCI slot that doesn't have a device plugged into > > it. Per the PCI spec, if a device is absent, any reads to the slot > > will return a well defined value (all ones). Those values are invalid > > for things like a device id. > > It is very unsafe to "probe" memory mapped peripherals. > > > I really dislike the design of virtio-mmio. I don't understand why it > > limits itself to a single device. > > It is modelled after simple memory mapped peripherals, so it is simple > itself. And if I was doing it today from scratch again, I would still > focus on doing one thing only. After all there are discoverable buses > out there, I don't see any reason to try to invent my own discovery > mechanism. > > > But that's a different discussion. > > It is indeed, and I am open to all suggestions how to make it more > likeable :-) > > > virtio-blk is a separate device that can be connected to the transport. > > As in: block device of zero size? This has also been suggested in the > thread linked above. > > > And yes, there should be a way to specify "this transport is not > > connected to anything". > > In a "virtio generic" way or mmio-specific one? Maybe instead of > defining zero as a do-nothing-device we should simply make this value > reserved or illegal? This would make PCI situation clear (no device will > be ever allowed to have it) and mmio driver could "overload" its meaning > as "ignore". > > Pawel This last option is what I was suggesting, I think it'll work well.

On Wed, 2013-07-31 at 15:43 +0100, Michael S. Tsirkin wrote: > > In a "virtio generic" way or mmio-specific one? Maybe instead of > > defining zero as a do-nothing-device we should simply make this value > > reserved or illegal? This would make PCI situation clear (no device will > > be ever allowed to have it) and mmio driver could "overload" its meaning > > as "ignore". > > This last option is what I was suggesting, I think it'll work well. As long as it is reserved (made illegal) at the "top level" of the spec (equivalent of the section 2.1 of the current spec), it's fine with me. Pawel

Pawel Moll <pawel.moll@arm.com> writes: > On Wed, 2013-07-31 at 14:25 +0100, Anthony Liguori wrote: >> I'm not sure where this discussion took place, but there is nothing in >> QEMU that requires a dummy device to be used. > > http://thread.gmane.org/gmane.linux.kernel.virtualization/20144 N.B. If you aren't discussing something on qemu-devel, then you aren't getting the full input of the community. >> The analogy would be a PCI slot that doesn't have a device plugged into >> it. Per the PCI spec, if a device is absent, any reads to the slot >> will return a well defined value (all ones). Those values are invalid >> for things like a device id. > > It is very unsafe to "probe" memory mapped peripherals. It simply depends on the devices. Prior to PnP, probing was common practice on the PC. Yes, sometimes badness ensued. >> I really dislike the design of virtio-mmio. I don't understand why it >> limits itself to a single device. > > It is modelled after simple memory mapped peripherals, so it is simple > itself. And if I was doing it today from scratch again, I would still > focus on doing one thing only. After all there are discoverable buses > out there, I don't see any reason to try to invent my own discovery > mechanism. Right, I've already argued with Peter that ya'll should simply use PCI for mach-virt... We don't need to repeat that here though. >> virtio-blk is a separate device that can be connected to the transport. > > As in: block device of zero size? This has also been suggested in the > thread linked above. That's irrelevant here. >> And yes, there should be a way to specify "this transport is not >> connected to anything". > > In a "virtio generic" way or mmio-specific one? If we can do it in a backwards compatible fashion, I think it would be a very good idea to reserve a device ID to indicate that there is not a valid device connected to the transport. > Maybe instead of > defining zero as a do-nothing-device we should simply make this value > reserved or illegal? Yes, I think that's a good idea. > This would make PCI situation clear (no device will > be ever allowed to have it) and mmio driver could "overload" its meaning > as "ignore". Agreed. Regards, Anthony Liguori > > Pawel

On Wed, 2013-07-31 at 15:51 +0100, Anthony Liguori wrote: > > On Wed, 2013-07-31 at 14:25 +0100, Anthony Liguori wrote: > >> I'm not sure where this discussion took place, but there is nothing in > >> QEMU that requires a dummy device to be used. > > > > http://thread.gmane.org/gmane.linux.kernel.virtualization/20144 > > N.B. If you aren't discussing something on qemu-devel, then you aren't > getting the full input of the community. The discussion was more about virtio, not qemu. Nevertheless I blame Peter :-) > > It is very unsafe to "probe" memory mapped peripherals. > It simply depends on the devices. It depends on the architecture and interconnect more than on the device. I'm not talking about potential dangerous side effects of reading a device's register. I'm talking about generating a read transaction on a address not known to the interconnect or - even worse - belonging to a powered-down device. In such cases you can *completely* lock down the system. It's not really relevant here - let's just agree that blind reads are unsafe. > > Maybe instead of > > defining zero as a do-nothing-device we should simply make this value > > reserved or illegal? > > Yes, I think that's a good idea. > > > This would make PCI situation clear (no device will > > be ever allowed to have it) and mmio driver could "overload" its meaning > > as "ignore". > > Agreed. Cool, I'll proceed in this direction then and once we have a base to work on will submit a patch adding the relevant position to the IDs list. Then we can argue on the value and naming again :-) Pawel

On Wed, Jul 31, 2013 at 10:58:21AM +0100, Pawel Moll wrote: > > Link to a discussion? > > http://thread.gmane.org/gmane.linux.kernel.virtualization/20144 > > Pawel Reading through this, you want a way to say "there's no device there" for virtio-mmio? I think this is a bus-specific problem. PCI has a different, standard way to do this (report 0xfffffffff as both device and vendor ID). If you want to use subsystem ID for this in virtio-mmio, that's fine. But I think that we don't need a "do nothing device" for this - just reserve some IDs (declare them invalid) and bus will then be able to define behaviour as it sees fit.

On Wed, Jul 31, 2013 at 01:33:39PM +0930, Rusty Russell wrote: > Rusty Russell <rusty@au1.ibm.com> writes: > > ----------------- > > Action Item Subject: Create text version of virtio 0.9.5 document > > OK, I've attached this below. This is exactly the spec as 0.9.5 > reformatted into text so we can work it. I copied virtio-comment which is an open-list. Hope that's OK with everyone. People aren't auto-subscribed there but if everyone is willing to subscribe to virtio-comment, we could reserve the virtio@ members-only list to administrative issues only? > > I'm still waiting for virtio to be added to the OASIS issue tracking > system, where I will be prompting you all to open an issue for every > change we want to consider. > > The obvious issues I want to open are: > > o Major rework to make PCI an appendix, and core bus-independent. > o Update spec with changes/fixes since 0.9.5 > - This is easy where contributors are already members, > trickier for others. > > In addition here's a brain dump: > > o Endian for config space > - LE everywhere? > o Endian for ring > - LE as well? > o Allow arbitrary descriptor layouts / message framing. > o Method to stop activity on a queue? > o Size descriptor table independent of ringsize? > o Remove VIRTIO_F_NOTIFY_ON_EMPTY? > o Remove limit on # indirect descriptors > - Some other limit? > o Simplify indirect desc > - No return to top level on end of desc array > o Allow chained indirect desc > - indirect bit use to chain? > > Net: > o Remove VIRTIO_NET_F_GSO? > > Block: > o Remove VIRTIO_BLK_F_SCSI? > o Revisit flush/barrier semantics? > > PCI: > o New capability layout > o Allowing non-zero revision numbers? > o Remove 'align' and use explicit addresses for used/avail. > > Balloon: > o Fix endianness > o Remove VIRTIO_BALLOON_F_MUST_TELL_HOST? > o Remove outgoing page queue? > > Cheers, > Rusty. > > ==== > This document describes the specifications of the “virtio” family > of PCI devices. These are devices > are found in virtual environments, > yet by design they are not all that different from physical PCI > devices, and this document treats them as such. This allows the > guest to use standard PCI drivers and discovery mechanisms. > > The purpose of virtio and this specification is that virtual > environments and guests should have a straightforward, efficient, > standard and extensible mechanism for virtual devices, rather > than boutique per-environment or per-OS mechanisms. > > Straightforward: Virtio PCI devices use normal PCI mechanisms > of interrupts and DMA which should be familiar to any device > driver author. There is no exotic page-flipping or COW > mechanism: it's just a PCI device.[1] > > Efficient: Virtio PCI devices consist of rings of descriptors > for input and output, which are neatly separated to avoid cache > effects from both guest and device writing to the same cache > lines. > > Standard: Virtio PCI makes no assumptions about the environment > in which it operates, beyond supporting PCI. In fact the virtio > devices specified in the appendices do not require PCI at all: > they have been implemented on non-PCI buses.[2] > > Extensible: Virtio PCI devices contain feature bits which are > acknowledged by the guest operating system during device setup. > This allows forwards and backwards compatibility: the device > offers all the features it knows about, and the driver > acknowledges those it understands and wishes to use. > > 1.1 Virtqueues > > The mechanism for bulk data transport on virtio PCI devices is > pretentiously called a virtqueue. Each device can have zero or > more virtqueues: for example, the network device has one for > transmit and one for receive. > > Each virtqueue occupies two or more physically-contiguous pages > (defined, for the purposes of this specification, as 4096 bytes), > and consists of three parts: > > > +-------------------+-----------------------------------+-----------+ > Descriptor Table Available Ring (padding) Used Ring > +-------------------+-----------------------------------+-----------+ > > > When the driver wants to send a buffer to the device, it fills in > a slot in the descriptor table (or chains several together), and > writes the descriptor index into the available ring. It then > notifies the device. When the device has finished a buffer, it > writes the descriptor into the used ring, and sends an interrupt. > > Specification > > 2.1 PCI Discovery > > Any PCI device with Vendor ID 0x1AF4, and Device ID 0x1000 through > 0x103F inclusive is a virtio device[3]. The device must also have a > Revision ID of 0 to match this specification. > > The Subsystem Device ID indicates which virtio device is > supported by the device. The Subsystem Vendor ID should reflect > the PCI Vendor ID of the environment (it's currently only used > for informational purposes by the guest). > > > +----------------------+--------------------+---------------+ > Subsystem Device ID Virtio Device Specification > +----------------------+--------------------+---------------+ > +----------------------+--------------------+---------------+ > 1 network card Appendix C > +----------------------+--------------------+---------------+ > 2 block device Appendix D > +----------------------+--------------------+---------------+ > 3 console Appendix E > +----------------------+--------------------+---------------+ > 4 entropy source Appendix F > +----------------------+--------------------+---------------+ > 5 memory ballooning Appendix G > +----------------------+--------------------+---------------+ > 6 ioMemory - > +----------------------+--------------------+---------------+ > 7 rpmsg - > +----------------------+--------------------+---------------+ > 8 SCSI host Appendix I > +----------------------+--------------------+---------------+ > 9 9P transport - > +----------------------+--------------------+---------------+ > 10 mac80211 wlan - > +----------------------+--------------------+---------------+ > > > 2.2 Device Configuration > > To configure the device, we use the first I/O region of the PCI > device. This contains a virtio header followed by a > device-specific region. > > There may be different widths of accesses to the I/O region; the > “natural” access method for each field in the virtio header must be > used (i.e. 32-bit accesses for 32-bit fields, etc), but the > device-specific region can be accessed using any width accesses, and > should obtain the same results. > > Note that this is possible because while the virtio header is PCI > (i.e. little) endian, the device-specific region is encoded in > the native endian of the guest (where such distinction is > applicable). > > 2.2.1 Device Initialization Sequence > > We start with an overview of device initialization, then expand > on the details of the device and how each step is preformed. > > 1. Reset the device. This is not required on initial start up. > > 2. The ACKNOWLEDGE status bit is set: we have noticed the device. > > 3. The DRIVER status bit is set: we know how to drive the device. > > 4. Device-specific setup, including reading the Device Feature > Bits, discovery of virtqueues for the device, optional MSI-X > setup, and reading and possibly writing the virtio > configuration space. > > 5. The subset of Device Feature Bits understood by the driver is > written to the device. > > 6. The DRIVER_OK status bit is set. > > 7. The device can now be used (ie. buffers added to the > virtqueues)[4] > > If any of these steps go irrecoverably wrong, the guest should > set the FAILED status bit to indicate that it has given up on the > device (it can reset the device later to restart if desired). > > We now cover the fields required for general setup in detail. > > 2.2.2 Virtio Header > > The virtio header looks as follows: > > > +------------++---------------------+---------------------+----------+--------+---------+---------+---------+--------+ > Bits 32 32 32 16 16 16 8 8 > +------------++---------------------+---------------------+----------+--------+---------+---------+---------+--------+ > Read/Write R R+W R+W R R+W R+W R+W R > +------------++---------------------+---------------------+----------+--------+---------+---------+---------+--------+ > Purpose Device Guest Queue Queue Queue Queue Device ISR > Features bits 0:31 Features bits 0:31 Address Size Select Notify Status Status > +------------++---------------------+---------------------+----------+--------+---------+---------+---------+--------+ > > > If MSI-X is enabled for the device, two additional fields > immediately follow this header:[5] > > > +------------++----------------+--------+ > Bits 16 16 > +----------------+--------+ > +------------++----------------+--------+ > Read/Write R+W R+W > +------------++----------------+--------+ > Purpose Configuration Queue > (MSI-X) Vector Vector > +------------++----------------+--------+ > > > Immediately following these general headers, there may be > device-specific headers: > > > +------------++--------------------+ > Bits Device Specific > +--------------------+ > +------------++--------------------+ > Read/Write Device Specific > +------------++--------------------+ > Purpose Device Specific... > > +------------++--------------------+ > > > 2.2.2.1 Device Status > > The Device Status field is updated by the guest to indicate its > progress. This provides a simple low-level diagnostic: it's most > useful to imagine them hooked up to traffic lights on the console > indicating the status of each device. > > The device can be reset by writing a 0 to this field, otherwise > at least one bit should be set: > > ACKNOWLEDGE (1) Indicates that the guest OS has found the > device and recognized it as a valid virtio device. > > DRIVER (2) Indicates that the guest OS knows how to drive the > device. Under Linux, drivers can be loadable modules so there > may be a significant (or infinite) delay before setting this > bit. > > DRIVER_OK (4) Indicates that the driver is set up and ready to > drive the device. > > FAILED (128) Indicates that something went wrong in the guest, > and it has given up on the device. This could be an internal > error, or the driver didn't like the device for some reason, or > even a fatal error during device operation. The device must be > reset before attempting to re-initialize. > > 2.2.2.2 Feature Bits > > The first configuration field indicates the features that the > device supports. The bits are allocated as follows: > > 0 to 23 Feature bits for the specific device type > > 24 to 32 Feature bits reserved for extensions to the queue and > feature negotiation mechanisms > > For example, feature bit 0 for a network device (i.e. Subsystem > Device ID 1) indicates that the device supports checksumming of > packets. > > The feature bits are negotiated: the device lists all the > features it understands in the Device Features field, and the > guest writes the subset that it understands into the Guest > Features field. The only way to renegotiate is to reset the > device. > > In particular, new fields in the device configuration header are > indicated by offering a feature bit, so the guest can check > before accessing that part of the configuration space. > > This allows for forwards and backwards compatibility: if the > device is enhanced with a new feature bit, older guests will not > write that feature bit back to the Guest Features field and it > can go into backwards compatibility mode. Similarly, if a guest > is enhanced with a feature that the device doesn't support, it > will not see that feature bit in the Device Features field and > can go into backwards compatibility mode (or, for poor > implementations, set the FAILED Device Status bit). > > 2.2.2.3 Configuration/Queue Vectors > > When MSI-X capability is present and enabled in the device > (through standard PCI configuration space) 4 bytes at byte offset > 20 are used to map configuration change and queue interrupts to > MSI-X vectors. In this case, the ISR Status field is unused, and > device specific configuration starts at byte offset 24 in virtio > header structure. When MSI-X capability is not enabled, device > specific configuration starts at byte offset 20 in virtio header. > > Writing a valid MSI-X Table entry number, 0 to 0x7FF, to one of > Configuration/Queue Vector registers, maps interrupts triggered > by the configuration change/selected queue events respectively to > the corresponding MSI-X vector. To disable interrupts for a > specific event type, unmap it by writing a special NO_VECTOR > value: > > /* Vector value used to disable MSI for queue */ > > #define VIRTIO_MSI_NO_VECTOR 0xffff > > Reading these registers returns vector mapped to a given event, > or NO_VECTOR if unmapped. All queue and configuration change > events are unmapped by default. > > Note that mapping an event to vector might require allocating > internal device resources, and might fail. Devices report such > failures by returning the NO_VECTOR value when the relevant > Vector field is read. After mapping an event to vector, the > driver must verify success by reading the Vector field value: on > success, the previously written value is returned, and on > failure, NO_VECTOR is returned. If a mapping failure is detected, > the driver can retry mapping with fewervectors, or disable MSI-X. > > 2.3 Virtqueue Configuration > > As a device can have zero or more virtqueues for bulk data > transport (for example, the network driver has two), the driver > needs to configure them as part of the device-specific > configuration. > > This is done as follows, for each virtqueue a device has: > > 1. Write the virtqueue index (first queue is 0) to the Queue > Select field. > > 2. Read the virtqueue size from the Queue Size field, which is > always a power of 2. This controls how big the virtqueue is > (see below). If this field is 0, the virtqueue does not exist. > > 3. Allocate and zero virtqueue in contiguous physical memory, on > a 4096 byte alignment. Write the physical address, divided by > 4096 to the Queue Address field.[6] > > 4. Optionally, if MSI-X capability is present and enabled on the > device, select a vector to use to request interrupts triggered > by virtqueue events. Write the MSI-X Table entry number > corresponding to this vector in Queue Vector field. Read the > Queue Vector field: on success, previously written value is > returned; on failure, NO_VECTOR value is returned. > > The Queue Size field controls the total number of bytes required > for the virtqueue according to the following formula: > > #define ALIGN(x) (((x) + 4095) & ~4095) > > static inline unsigned vring_size(unsigned int qsz) > { > return ALIGN(sizeof(struct vring_desc)*qsz + sizeof(u16)*(2 + qsz)) > + ALIGN(sizeof(struct vring_used_elem)*qsz); > } > > This currently wastes some space with padding, but also allows > future extensions. The virtqueue layout structure looks like this > (qsz is the Queue Size field, which is a variable, so this code > won't compile): > > struct vring { > /* The actual descriptors (16 bytes each) */ > struct vring_desc desc[qsz]; > > /* A ring of available descriptor heads with free-running index. */ > struct vring_avail avail; > > // Padding to the next 4096 boundary. > char pad[]; > > // A ring of used descriptor heads with free-running index. > struct vring_used used; > }; > > 2.3.1 A Note on Virtqueue Endianness > > Note that the endian of these fields and everything else in the > virtqueue is the native endian of the guest, not little-endian as > PCI normally is. This makes for simpler guest code, and it is > assumed that the host already has to be deeply aware of the guest > endian so such an “endian-aware” device is not a significant > issue. > > 2.3.2 Descriptor Table > > The descriptor table refers to the buffers the guest is using for > the device. The addresses are physical addresses, and the buffers > can be chained via the next field. Each descriptor describes a > buffer which is read-only or write-only, but a chain of > descriptors can contain both read-only and write-only buffers. > > No descriptor chain may be more than 2^32 bytes long in total. > > struct vring_desc { > /* Address (guest-physical). */ > u64 addr; > /* Length. */ > u32 len; > > /* This marks a buffer as continuing via the next field. */ > #define VRING_DESC_F_NEXT 1 > /* This marks a buffer as write-only (otherwise read-only). */ > #define VRING_DESC_F_WRITE 2 > /* This means the buffer contains a list of buffer descriptors. */ > #define VRING_DESC_F_INDIRECT 4 > /* The flags as indicated above. */ > u16 flags; > /* Next field if flags & NEXT */ > u16 next; > }; > > The number of descriptors in the table is specified by the Queue > Size field for this virtqueue. > > 2.3.3 Indirect Descriptors > > Some devices benefit by concurrently dispatching a large number > of large requests. The VIRTIO_RING_F_INDIRECT_DESC feature can be > used to allow this (see Appendix B: Reserved Feature Bits). To increase > ring capacity it is possible to store a table of indirect > descriptors anywhere in memory, and insert a descriptor in main > virtqueue (with flags&INDIRECT on) that refers to memory buffer > containing this indirect descriptor table; fields addr and len > refer to the indirect table address and length in bytes, > respectively. The indirect table layout structure looks like this > (len is the length of the descriptor that refers to this table, > which is a variable, so this code won't compile): > > struct indirect_descriptor_table { > /* The actual descriptors (16 bytes each) */ > struct vring_desc desc[len / 16]; > }; > > The first indirect descriptor is located at start of the indirect > descriptor table (index 0), additional indirect descriptors are > chained by next field. An indirect descriptor without next field > (with flags&NEXT off) signals the end of the indirect descriptor > table, and transfers control back to the main virtqueue. An > indirect descriptor can not refer to another indirect descriptor > table (flags&INDIRECT must be off). A single indirect descriptor > table can include both read-only and write-only descriptors; > write-only flag (flags&WRITE) in the descriptor that refers to it > is ignored. > > 2.3.4 Available Ring > > The available ring refers to what descriptors we are offering the > device: it refers to the head of a descriptor chain. The “flags” field > is currently 0 or 1: 1 indicating that we do not need an interrupt > when the device consumes a descriptor from the available > ring. Alternatively, the guest can ask the device to delay interrupts > until an entry with an index specified by the “ used_event” field is > written in the used ring (equivalently, until the idx field in the > used ring will reach the value used_event + 1). The method employed by > the device is controlled by the VIRTIO_RING_F_EVENT_IDX feature bit > (see Appendix B: Reserved Feature Bits). This interrupt suppression is > merely an optimization; it may not suppress interrupts entirely. > > The “idx” field indicates where we would put the next descriptor > entry (modulo the ring size). This starts at 0, and increases. > > struct vring_avail { > #define VRING_AVAIL_F_NO_INTERRUPT 1 > u16 flags; > u16 idx; > u16 ring[qsz]; /* qsz is the Queue Size field read from device */ > u16 used_event; > }; > > 2.3.5 Used Ring > > The used ring is where the device returns buffers once it is done > with them. The flags field can be used by the device to hint that > no notification is necessary when the guest adds to the available > ring. Alternatively, the “avail_event” field can be used by the > device to hint that no notification is necessary until an entry > with an index specified by the “avail_event” is written in the > available ring (equivalently, until the idx field in the > available ring will reach the value avail_event + 1). The method > employed by the device is controlled by the guest through the > VIRTIO_RING_F_EVENT_IDX feature bit (see Appendix B: Reserved > Feature Bits).[7] > > Each entry in the ring is a pair: the head entry of the > descriptor chain describing the buffer (this matches an entry > placed in the available ring by the guest earlier), and the total > of bytes written into the buffer. The latter is extremely useful > for guests using untrusted buffers: if you do not know exactly > how much has been written by the device, you usually have to zero > the buffer to ensure no data leakage occurs. > > /* u32 is used here for ids for padding reasons. */ > struct vring_used_elem { > /* Index of start of used descriptor chain. */ > u32 id; > /* Total length of the descriptor chain which was used (written to) */ > u32 len; > }; > > struct vring_used { > #define VRING_USED_F_NO_NOTIFY 1 > u16 flags; > u16 idx; > struct vring_used_elem ring[qsz]; > u16 avail_event; > }; > > 2.3.6 Helpers for Managing Virtqueues > > The Linux Kernel Source code contains the definitions above and > helper routines in a more usable form, in > include/linux/virtio_ring.h. This was explicitly licensed by IBM > and Red Hat under the (3-clause) BSD license so that it can be > freely used by all other projects, and is reproduced (with slight > variation to remove Linux assumptions) in Appendix A. > > 2.4 Device Operation > > There are two parts to device operation: supplying new buffers to > the device, and processing used buffers from the device. As an > example, the virtio network device has two virtqueues: the > transmit virtqueue and the receive virtqueue. The driver adds > outgoing (read-only) packets to the transmit virtqueue, and then > frees them after they are used. Similarly, incoming (write-only) > buffers are added to the receive virtqueue, and processed after > they are used. > > 2.4.1 Supplying Buffers to The Device > > Actual transfer of buffers from the guest OS to the device > operates as follows: > > 1. Place the buffer(s) into free descriptor(s). > > (a) If there are no free descriptors, the guest may choose to > notify the device even if notifications are suppressed (to > reduce latency).[8] > > 2. Place the id of the buffer in the next ring entry of the > available ring. > > 3. The steps (1) and (2) may be performed repeatedly if batching > is possible. > > 4. A memory barrier should be executed to ensure the device sees > the updated descriptor table and available ring before the next > step. > > 5. The available “idx” field should be increased by the number of > entries added to the available ring. > > 6. A memory barrier should be executed to ensure that we update > the idx field before checking for notification suppression. > > 7. If notifications are not suppressed, the device should be > notified of the new buffers. > > Note that the above code does not take precautions against the > available ring buffer wrapping around: this is not possible since > the ring buffer is the same size as the descriptor table, so step > (1) will prevent such a condition. > > In addition, the maximum queue size is 32768 (it must be a power > of 2 which fits in 16 bits), so the 16-bit “idx” value can always > distinguish between a full and empty buffer. > > Here is a description of each stage in more detail. > > 2.4.1.1 Placing Buffers Into The Descriptor Table > > A buffer consists of zero or more read-only physically-contiguous > elements followed by zero or more physically-contiguous > write-only elements (it must have at least one element). This > algorithm maps it into the descriptor table: > > 1. for each buffer element, b: > > (a) Get the next free descriptor table entry, d > > (b) Set d.addr to the physical address of the start of b > > (c) Set d.len to the length of b. > > (d) If b is write-only, set d.flags to VRING_DESC_F_WRITE, > otherwise 0. > > (e) If there is a buffer element after this: > > i. Set d.next to the index of the next free descriptor > element. > > ii. Set the VRING_DESC_F_NEXT bit in d.flags. > > In practice, the d.next fields are usually used to chain free > descriptors, and a separate count kept to check there are enough > free descriptors before beginning the mappings. > > 2.4.1.2 Updating The Available Ring > > The head of the buffer we mapped is the first d in the algorithm > above. A naive implementation would do the following: > > avail->ring[avail->idx % qsz] = head; > > However, in general we can add many descriptors before we update > the “idx” field (at which point they become visible to the > device), so we keep a counter of how many we've added: > > avail->ring[(avail->idx + added++) % qsz] = head; > > 2.4.1.3 Updating The Index Field > > Once the idx field of the virtqueue is updated, the device will > be able to access the descriptor entries we've created and the > memory they refer to. This is why a memory barrier is generally > used before the idx update, to ensure it sees the most up-to-date > copy. > > The idx field always increments, and we let it wrap naturally at > 65536: > > avail->idx += added; > > 2.4.1.4 Notifying The Device > > Device notification occurs by writing the 16-bit virtqueue index > of this virtqueue to the Queue Notify field of the virtio header > in the first I/O region of the PCI device. This can be expensive, > however, so the device can suppress such notifications if it > doesn't need them. We have to be careful to expose the new idx > value before checking the suppression flag: it's OK to notify > gratuitously, but not to omit a required notification. So again, > we use a memory barrier here before reading the flags or the > avail_event field. > > If the VIRTIO_F_RING_EVENT_IDX feature is not negotiated, and if > the VRING_USED_F_NOTIFY flag is not set, we go ahead and write to > the PCI configuration space. > > If the VIRTIO_F_RING_EVENT_IDX feature is negotiated, we read the > avail_event field in the available ring structure. If the > available index crossed_the avail_event field value since the > last notification, we go ahead and write to the PCI configuration > space. The avail_event field wraps naturally at 65536 as well: > > (u16)(new_idx - avail_event - 1) < (u16)(new_idx - old_idx) > > 2.4.2 Receiving Used Buffers From The Device > > Once the device has used a buffer (read from or written to it, or > parts of both, depending on the nature of the virtqueue and the > device), it sends an interrupt, following an algorithm very > similar to the algorithm used for the driver to send the device a > buffer: > > 1. Write the head descriptor number to the next field in the used > ring. > > 2. Update the used ring idx. > > 3. Determine whether an interrupt is necessary: > > (a) If the VIRTIO_F_RING_EVENT_IDX feature is not negotiated: > check if f the VRING_AVAIL_F_NO_INTERRUPT flag is not set in > avail->flags > > (b) If the VIRTIO_F_RING_EVENT_IDX feature is negotiated: check > whether the used index crossed the used_event field value > since the last update. The used_event field wraps naturally > at 65536 as well: > (u16)(new_idx - used_event - 1) < (u16)(new_idx - old_idx) > > 4. If an interrupt is necessary: > > (a) If MSI-X capability is disabled: > > i. Set the lower bit of the ISR Status field for the device. > > ii. Send the appropriate PCI interrupt for the device. > > (b) If MSI-X capability is enabled: > > i. Request the appropriate MSI-X interrupt message for the > device, Queue Vector field sets the MSI-X Table entry > number. > > ii. If Queue Vector field value is NO_VECTOR, no interrupt > message is requested for this event. > > The guest interrupt handler should: > > 1. If MSI-X capability is disabled: read the ISR Status field, > which will reset it to zero. If the lower bit is zero, the > interrupt was not for this device. Otherwise, the guest driver > should look through the used rings of each virtqueue for the > device, to see if any progress has been made by the device > which requires servicing. > > 2. If MSI-X capability is enabled: look through the used rings of > each virtqueue mapped to the specific MSI-X vector for the > device, to see if any progress has been made by the device > which requires servicing. > > For each ring, guest should then disable interrupts by writing > VRING_AVAIL_F_NO_INTERRUPT flag in avail structure, if required. > It can then process used ring entries finally enabling interrupts > by clearing the VRING_AVAIL_F_NO_INTERRUPT flag or updating the > EVENT_IDX field in the available structure, Guest should then > execute a memory barrier, and then recheck the ring empty > condition. This is necessary to handle the case where, after the > last check and before enabling interrupts, an interrupt has been > suppressed by the device: > > vring_disable_interrupts(vq); > > for (;;) { > if (vq->last_seen_used != vring->used.idx) { > vring_enable_interrupts(vq); > mb(); > > if (vq->last_seen_used != vring->used.idx) > break; > } > > struct vring_used_elem *e = vring.used->ring[vq->last_seen_used%vsz]; > process_buffer(e); > vq->last_seen_used++; > } > > 2.4.3 Dealing With Configuration Changes > > Some virtio PCI devices can change the device configuration > state, as reflected in the virtio header in the PCI configuration > space. In this case: > > 1. If MSI-X capability is disabled: an interrupt is delivered and > the second highest bit is set in the ISR Status field to > indicate that the driver should re-examine the configuration > space. Note that a single interrupt can indicate both that one > or more virtqueue has been used and that the configuration > space has changed: even if the config bit is set, virtqueues > must be scanned. > > 2. If MSI-X capability is enabled: an interrupt message is > requested. The Configuration Vector field sets the MSI-X Table > entry number to use. If Configuration Vector field value is > NO_VECTOR, no interrupt message is requested for this event. > > > Creating New Device Types > > Various considerations are necessary when creating a new device > type: > > How Many Virtqueues? > > It is possible that a very simple device will operate entirely > through its configuration space, but most will need at least one > virtqueue in which it will place requests. A device with both > input and output (eg. console and network devices described here) > need two queues: one which the driver fills with buffers to > receive input, and one which the driver places buffers to > transmit output. > > What Configuration Space Layout? > > Configuration space is generally used for rarely-changing or > initialization-time parameters. But it is a limited resource, so > it might be better to use a virtqueue to update configuration > information (the network device does this for filtering, > otherwise the table in the config space could potentially be very > large). > > Note that this space is generally the guest's native endian, > rather than PCI's little-endian. > > What Device Number? > > Currently device numbers are assigned quite freely: a simple > request mail to the author of this document or the Linux > virtualization mailing list[9] will be sufficient to secure a unique one. > > Meanwhile for experimental drivers, use 65535 and work backwards. > > How many MSI-X vectors? > > Using the optional MSI-X capability devices can speed up > interrupt processing by removing the need to read ISR Status > register by guest driver (which might be an expensive operation), > reducing interrupt sharing between devices and queues within the > device, and handling interrupts from multiple CPUs. However, some > systems impose a limit (which might be as low as 256) on the > total number of MSI-X vectors that can be allocated to all > devices. Devices and/or device drivers should take this into > account, limiting the number of vectors used unless the device is > expected to cause a high volume of interrupts. Devices can > control the number of vectors used by limiting the MSI-X Table > Size or not presenting MSI-X capability in PCI configuration > space. Drivers can control this by mapping events to as small > number of vectors as possible, or disabling MSI-X capability > altogether. > > Message Framing > > The descriptors used for a buffer should not effect the semantics > of the message, except for the total length of the buffer. For > example, a network buffer consists of a 10 byte header followed > by the network packet. Whether this is presented in the ring > descriptor chain as (say) a 10 byte buffer and a 1514 byte > buffer, or a single 1524 byte buffer, or even three buffers, > should have no effect. > > In particular, no implementation should use the descriptor > boundaries to determine the size of any header in a request.[10] > > Device Improvements > > Any change to configuration space, or new virtqueues, or > behavioural changes, should be indicated by negotiation of a new > feature bit. This establishes clarity[11] and avoids future expansion problems. > > Clusters of functionality which are always implemented together > can use a single bit, but if one feature makes sense without the > others they should not be gratuitously grouped together to > conserve feature bits. We can always extend the spec when the > first person needs more than 24 feature bits for their device. > > > > > Appendix A: virtio_ring.h > > #ifndef VIRTIO_RING_H > #define VIRTIO_RING_H > /* An interface for efficient virtio implementation. > * > * This header is BSD licensed so anyone can use the definitions > * to implement compatible drivers/servers. > * > * Copyright 2007, 2009, IBM Corporation > * Copyright 2011, Red Hat, Inc > * All rights reserved. > * > * Redistribution and use in source and binary forms, with or without > * modification, are permitted provided that the following conditions > * are met: > * 1. Redistributions of source code must retain the above copyright > * notice, this list of conditions and the following disclaimer. > * 2. Redistributions in binary form must reproduce the above copyright > * notice, this list of conditions and the following disclaimer in the > * documentation and/or other materials provided with the distribution. > * 3. Neither the name of IBM nor the names of its contributors > * may be used to endorse or promote products derived from this software > * without specific prior written permission. > * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS ``AS IS'' AND > * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE > * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE > * ARE DISCLAIMED. IN NO EVENT SHALL IBM OR CONTRIBUTORS BE LIABLE > * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL > * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS > * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) > * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT > * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY > * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF > * SUCH DAMAGE. > */ > > /* This marks a buffer as continuing via the next field. */ > #define VRING_DESC_F_NEXT 1 > /* This marks a buffer as write-only (otherwise read-only). */ > #define VRING_DESC_F_WRITE 2 > > /* The Host uses this in used->flags to advise the Guest: don't kick me > * when you add a buffer. It's unreliable, so it's simply an > * optimization. Guest will still kick if it's out of buffers. */ > #define VRING_USED_F_NO_NOTIFY 1 > /* The Guest uses this in avail->flags to advise the Host: don't > * interrupt me when you consume a buffer. It's unreliable, so it's > * simply an optimization. */ > #define VRING_AVAIL_F_NO_INTERRUPT 1 > > /* Virtio ring descriptors: 16 bytes. > * These can chain together via "next". */ > struct vring_desc { > /* Address (guest-physical). */ > uint64_t addr; > /* Length. */ > uint32_t len; > /* The flags as indicated above. */ > uint16_t flags; > /* We chain unused descriptors via this, too */ > uint16_t next; > }; > > struct vring_avail { > uint16_t flags; > uint16_t idx; > uint16_t ring[]; > uint16_t used_event; > }; > > /* u32 is used here for ids for padding reasons. */ > struct vring_used_elem { > /* Index of start of used descriptor chain. */ > uint32_t id; > /* Total length of the descriptor chain which was written to. */ > uint32_t len; > }; > > struct vring_used { > uint16_t flags; > uint16_t idx; > struct vring_used_elem ring[]; > uint16_t avail_event; > }; > > struct vring { > unsigned int num; > > struct vring_desc *desc; > struct vring_avail *avail; > struct vring_used *used; > }; > > /* The standard layout for the ring is a continuous chunk of memory which > * looks like this. We assume num is a power of 2. > * > * struct vring { > * // The actual descriptors (16 bytes each) > * struct vring_desc desc[num]; > * > * // A ring of available descriptor heads with free-running index. > * __u16 avail_flags; > * __u16 avail_idx; > * __u16 available[num]; > * > * // Padding to the next align boundary. > * char pad[]; > * > * // A ring of used descriptor heads with free-running index. > * __u16 used_flags; > * __u16 EVENT_IDX; > * struct vring_used_elem used[num]; > * }; > * Note: for virtio PCI, align is 4096. > */ > static inline void vring_init(struct vring *vr, unsigned int num, void *p, > unsigned long align) > { > vr->num = num; > vr->desc = p; > vr->avail = p + num*sizeof(struct vring_desc); > vr->used = (void *)(((unsigned long)&vr->avail->ring[num] > + align-1) > & ~(align - 1)); > } > > static inline unsigned vring_size(unsigned int num, unsigned long align) > { > return ((sizeof(struct vring_desc)*num + sizeof(uint16_t)*(2+num) > + align - 1) & ~(align - 1)) > + sizeof(uint16_t)*3 + sizeof(struct vring_used_elem)*num; > } > > static inline int vring_need_event(uint16_t event_idx, uint16_t new_idx, uint16_t old_idx) > { > return (uint16_t)(new_idx - event_idx - 1) < (uint16_t)(new_idx - old_idx); > } > #endif /* VIRTIO_RING_H */ > > > Appendix B: Reserved Feature Bits > > Currently there are five device-independent feature bits defined: > > VIRTIO_F_NOTIFY_ON_EMPTY (24) Negotiating this feature > indicates that the driver wants an interrupt if the device runs > out of available descriptors on a virtqueue, even though > interrupts are suppressed using the VRING_AVAIL_F_NO_INTERRUPT > flag or the used_event field. An example of this is the > networking driver: it doesn't need to know every time a packet > is transmitted, but it does need to free the transmitted > packets a finite time after they are transmitted. It can avoid > using a timer if the device interrupts it when all the packets > are transmitted. > > VIRTIO_F_RING_INDIRECT_DESC (28) Negotiating this feature indicates > that the driver can use descriptors with the VRING_DESC_F_INDIRECT > flag set, as described in 2.3.3 Indirect Descriptors. > > VIRTIO_F_RING_EVENT_IDX(29) This feature enables the used_event > and the avail_event fields. If set, it indicates that the > device should ignore the flags field in the available ring > structure. Instead, the used_event field in this structure is > used by guest to suppress device interrupts. Further, the > driver should ignore the flags field in the used ring > structure. Instead, the avail_event field in this structure is > used by the device to suppress notifications. If unset, the > driver should ignore the used_event field; the device should > ignore the avail_event field; the flags field is used > > Appendix C: Network Device > > The virtio network device is a virtual ethernet card, and is the > most complex of the devices supported so far by virtio. It has > enhanced rapidly and demonstrates clearly how support for new > features should be added to an existing device. Empty buffers are > placed in one virtqueue for receiving packets, and outgoing > packets are enqueued into another for transmission in that order. > A third command queue is used to control advanced filtering > features. > > Configuration > > Subsystem Device ID 1 > > Virtqueues 0:receiveq. 1:transmitq. 2:controlq[12] > > Feature bits > > VIRTIO_NET_F_CSUM (0) Device handles packets with partial checksum > > VIRTIO_NET_F_GUEST_CSUM (1) Guest handles packets with partial checksum > > VIRTIO_NET_F_MAC (5) Device has given MAC address. > > VIRTIO_NET_F_GSO (6) (Deprecated) device handles packets with > any GSO type.[13] > > VIRTIO_NET_F_GUEST_TSO4 (7) Guest can receive TSOv4. > > VIRTIO_NET_F_GUEST_TSO6 (8) Guest can receive TSOv6. > > VIRTIO_NET_F_GUEST_ECN (9) Guest can receive TSO with ECN. > > VIRTIO_NET_F_GUEST_UFO (10) Guest can receive UFO. > > VIRTIO_NET_F_HOST_TSO4 (11) Device can receive TSOv4. > > VIRTIO_NET_F_HOST_TSO6 (12) Device can receive TSOv6. > > VIRTIO_NET_F_HOST_ECN (13) Device can receive TSO with ECN. > > VIRTIO_NET_F_HOST_UFO (14) Device can receive UFO. > > VIRTIO_NET_F_MRG_RXBUF (15) Guest can merge receive buffers. > > VIRTIO_NET_F_STATUS (16) Configuration status field is > available. > > VIRTIO_NET_F_CTRL_VQ (17) Control channel is available. > > VIRTIO_NET_F_CTRL_RX (18) Control channel RX mode support. > > VIRTIO_NET_F_CTRL_VLAN (19) Control channel VLAN filtering. > > VIRTIO_NET_F_GUEST_ANNOUNCE(21) Guest can send gratuitous > packets. > > Device configuration layout Two configuration fields are > currently defined. The mac address field always exists (though > is only valid if VIRTIO_NET_F_MAC is set), and the status field > only exists if VIRTIO_NET_F_STATUS is set. Two read-only bits > are currently defined for the status field: > VIRTIO_NET_S_LINK_UP and VIRTIO_NET_S_ANNOUNCE. > > #define VIRTIO_NET_S_LINK_UP 1 > #define VIRTIO_NET_S_ANNOUNCE 2 > > struct virtio_net_config { > u8 mac[6]; > u16 status; > }; > > Device Initialization > > 1. The initialization routine should identify the receive and > transmission virtqueues. > > 2. If the VIRTIO_NET_F_MAC feature bit is set, the configuration > space “mac” entry indicates the “physical” address of the the > network card, otherwise a private MAC address should be > assigned. All guests are expected to negotiate this feature if > it is set. > > 3. If the VIRTIO_NET_F_CTRL_VQ feature bit is negotiated, > identify the control virtqueue. > > 4. If the VIRTIO_NET_F_STATUS feature bit is negotiated, the link > status can be read from the bottom bit of the “status” config > field. Otherwise, the link should be assumed active. > > 5. The receive virtqueue should be filled with receive buffers. > This is described in detail below in “Setting Up Receive > Buffers”. > > 6. A driver can indicate that it will generate checksumless > packets by negotating the VIRTIO_NET_F_CSUM feature. This “ > checksum offload” is a common feature on modern network cards. > > 7. If that feature is negotiated[14], a driver can use TCP or UDP > segmentation offload by negotiating the VIRTIO_NET_F_HOST_TSO4 (IPv4 > TCP), VIRTIO_NET_F_HOST_TSO6 (IPv6 TCP) and VIRTIO_NET_F_HOST_UFO > (UDP fragmentation) features. It should not send TCP packets > requiring segmentation offload which have the Explicit Congestion > Notification bit set, unless the VIRTIO_NET_F_HOST_ECN feature is > negotiated.[15] > > 8. The converse features are also available: a driver can save > the virtual device some work by negotiating these features.[16] > The VIRTIO_NET_F_GUEST_CSUM feature indicates that partially > checksummed packets can be received, and if it can do that then > the VIRTIO_NET_F_GUEST_TSO4, VIRTIO_NET_F_GUEST_TSO6, > VIRTIO_NET_F_GUEST_UFO and VIRTIO_NET_F_GUEST_ECN are the input > equivalents of the features described above. See “Receiving > Packets” below. > > Device Operation > > Packets are transmitted by placing them in the transmitq, and > buffers for incoming packets are placed in the receiveq. In each > case, the packet itself is preceeded by a header: > > struct virtio_net_hdr { > #define VIRTIO_NET_HDR_F_NEEDS_CSUM 1 > u8 flags; > #define VIRTIO_NET_HDR_GSO_NONE 0 > #define VIRTIO_NET_HDR_GSO_TCPV4 1 > #define VIRTIO_NET_HDR_GSO_UDP 3 > #define VIRTIO_NET_HDR_GSO_TCPV6 4 > #define VIRTIO_NET_HDR_GSO_ECN 0x80 > u8 gso_type; > u16 hdr_len; > u16 gso_size; > u16 csum_start; > u16 csum_offset; > /* Only if VIRTIO_NET_F_MRG_RXBUF: */ > u16 num_buffers > }; > > The controlq is used to control device features such as > filtering. > > Packet Transmission > > Transmitting a single packet is simple, but varies depending on > the different features the driver negotiated. > > 1. If the driver negotiated VIRTIO_NET_F_CSUM, and the packet has > not been fully checksummed, then the virtio_net_hdr's fields > are set as follows. Otherwise, the packet must be fully > checksummed, and flags is zero. > > • flags has the VIRTIO_NET_HDR_F_NEEDS_CSUM set, > > • csum_start is set to the offset within the packet to begin checksumming, > and > > • csum_offset indicates how many bytes after the csum_start the > new (16 bit ones' complement) checksum should be placed.[17] > > 2. If the driver negotiated > VIRTIO_NET_F_HOST_TSO4, TSO6 or UFO, and the packet requires > TCP segmentation or UDP fragmentation, then the “gso_type” > field is set to VIRTIO_NET_HDR_GSO_TCPV4, TCPV6 or UDP. > (Otherwise, it is set to VIRTIO_NET_HDR_GSO_NONE). In this > case, packets larger than 1514 bytes can be transmitted: the > metadata indicates how to replicate the packet header to cut it > into smaller packets. The other gso fields are set: > > • hdr_len is a hint to the device as to how much of the header > needs to be kept to copy into each packet, usually set to the > length of the headers, including the transport header.[18] > > • gso_size is the maximum size of each packet beyond that > header (ie. MSS). > > • If the driver negotiated the VIRTIO_NET_F_HOST_ECN feature, > the VIRTIO_NET_HDR_GSO_ECN bit may be set in “gso_type” as > well, indicating that the TCP packet has the ECN bit set.[19] > > 3. If the driver negotiated the VIRTIO_NET_F_MRG_RXBUF feature, > the num_buffers field is set to zero. > > 4. The header and packet are added as one output buffer to the > transmitq, and the device is notified of the new entry (see 2.4.1.4 > Notifying The Device).[20] > > Packet Transmission Interrupt > > Often a driver will suppress transmission interrupts using the > VRING_AVAIL_F_NO_INTERRUPT flag (see 2.4.2 Receiving Used Buffers From > The Device) and check for used packets in the transmit path of following > packets. However, it will still receive interrupts if the > VIRTIO_F_NOTIFY_ON_EMPTY feature is negotiated, indicating that > the transmission queue is completely emptied. > > The normal behavior in this interrupt handler is to retrieve and > new descriptors from the used ring and free the corresponding > headers and packets. > > Setting Up Receive Buffers > > It is generally a good idea to keep the receive virtqueue as > fully populated as possible: if it runs out, network performance > will suffer. > > If the VIRTIO_NET_F_GUEST_TSO4, VIRTIO_NET_F_GUEST_TSO6 or > VIRTIO_NET_F_GUEST_UFO features are used, the Guest will need to > accept packets of up to 65550 bytes long (the maximum size of a > TCP or UDP packet, plus the 14 byte ethernet header), otherwise > 1514 bytes. So unless VIRTIO_NET_F_MRG_RXBUF is negotiated, every > buffer in the receive queue needs to be at least this length [20a] > > If VIRTIO_NET_F_MRG_RXBUF is negotiated, each buffer must be at > least the size of the struct virtio_net_hdr. > > Packet Receive Interrupt > > When a packet is copied into a buffer in the receiveq, the > optimal path is to disable further interrupts for the receiveq > (see [sub:Receiving-Used-Buffers]) and process packets until no > more are found, then re-enable them. > > Processing packet involves: > > 1. If the driver negotiated the VIRTIO_NET_F_MRG_RXBUF feature, > then the “num_buffers” field indicates how many descriptors > this packet is spread over (including this one). This allows > receipt of large packets without having to allocate large > buffers. In this case, there will be at least “num_buffers” in > the used ring, and they should be chained together to form a > single packet. The other buffers will not begin with a struct > virtio_net_hdr. > > 2. If the VIRTIO_NET_F_MRG_RXBUF feature was not negotiated, or > the “num_buffers” field is one, then the entire packet will be > contained within this buffer, immediately following the struct > virtio_net_hdr. > > 3. If the VIRTIO_NET_F_GUEST_CSUM feature was negotiated, the > VIRTIO_NET_HDR_F_NEEDS_CSUM bit in the “flags” field may be > set: if so, the checksum on the packet is incomplete and the “ > csum_start” and “csum_offset” fields indicate how to calculate > it (see Packet Transmission point 1). > > 4. If the VIRTIO_NET_F_GUEST_TSO4, TSO6 or UFO options were > negotiated, then the “gso_type” may be something other than > VIRTIO_NET_HDR_GSO_NONE, and the “gso_size” field indicates the > desired MSS (see Packet Transmission point 2). > > Control Virtqueue > > The driver uses the control virtqueue (if VIRTIO_NET_F_VTRL_VQ is > negotiated) to send commands to manipulate various features of > the device which would not easily map into the configuration > space. > > All commands are of the following form: > > struct virtio_net_ctrl { > u8 class; > u8 command; > u8 command-specific-data[]; > u8 ack; > }; > > /* ack values */ > #define VIRTIO_NET_OK 0 > #define VIRTIO_NET_ERR 1 > > The class, command and command-specific-data are set by the > driver, and the device sets the ack byte. There is little it can > do except issue a diagnostic if the ack byte is not > VIRTIO_NET_OK. > > Packet Receive Filtering > > If the VIRTIO_NET_F_CTRL_RX feature is negotiated, the driver can > send control commands for promiscuous mode, multicast receiving, > and filtering of MAC addresses. > > Note that in general, these commands are best-effort: unwanted > packets may still arrive. > > Setting Promiscuous Mode > > #define VIRTIO_NET_CTRL_RX 0 > #define VIRTIO_NET_CTRL_RX_PROMISC 0 > #define VIRTIO_NET_CTRL_RX_ALLMULTI 1 > > The class VIRTIO_NET_CTRL_RX has two commands: > VIRTIO_NET_CTRL_RX_PROMISC turns promiscuous mode on and off, and > VIRTIO_NET_CTRL_RX_ALLMULTI turns all-multicast receive on and > off. The command-specific-data is one byte containing 0 (off) or > 1 (on). > > Setting MAC Address Filtering > > struct virtio_net_ctrl_mac { > u32 entries; > u8 macs[entries][ETH_ALEN]; > }; > > #define VIRTIO_NET_CTRL_MAC 1 > #define VIRTIO_NET_CTRL_MAC_TABLE_SET 0 > > The device can filter incoming packets by any number of destination > MAC addresses.[21] This table is set using the class > VIRTIO_NET_CTRL_MAC and the command VIRTIO_NET_CTRL_MAC_TABLE_SET. The > command-specific-data is two variable length tables of 6-byte MAC > addresses. The first table contains unicast addresses, and the second > contains multicast addresses. > > VLAN Filtering > > If the driver negotiates the VIRTION_NET_F_CTRL_VLAN feature, it > can control a VLAN filter table in the device. > > #define VIRTIO_NET_CTRL_VLAN 2 > #define VIRTIO_NET_CTRL_VLAN_ADD 0 > #define VIRTIO_NET_CTRL_VLAN_DEL 1 > > Both the VIRTIO_NET_CTRL_VLAN_ADD and VIRTIO_NET_CTRL_VLAN_DEL > command take a 16-bit VLAN id as the command-specific-data. > > Gratuitous Packet Sending > > If the driver negotiates the VIRTIO_NET_F_GUEST_ANNOUNCE (depends > on VIRTIO_NET_F_CTRL_VQ), it can ask the guest to send gratuitous > packets; this is usually done after the guest has been physically > migrated, and needs to announce its presence on the new network > links. (As hypervisor does not have the knowledge of guest > network configuration (eg. tagged vlan) it is simplest to prod > the guest in this way). > > #define VIRTIO_NET_CTRL_ANNOUNCE 3 > #define VIRTIO_NET_CTRL_ANNOUNCE_ACK 0 > > The Guest needs to check VIRTIO_NET_S_ANNOUNCE bit in status > field when it notices the changes of device configuration. The > command VIRTIO_NET_CTRL_ANNOUNCE_ACK is used to indicate that > driver has recevied the notification and device would clear the > VIRTIO_NET_S_ANNOUNCE bit in the status filed after it received > this command. > > Processing this notification involves: > > 1. Sending the gratuitous packets or marking there are pending > gratuitous packets to be sent and letting deferred routine to > send them. > > 2. Sending VIRTIO_NET_CTRL_ANNOUNCE_ACK command through control > vq. > > 3. . > > Appendix D: Block Device > > The virtio block device is a simple virtual block device (ie. > disk). Read and write requests (and other exotic requests) are > placed in the queue, and serviced (probably out of order) by the > device except where noted. > > Configuration > > Subsystem Device ID 2 > > Virtqueues 0:requestq. > > Feature bits > > VIRTIO_BLK_F_BARRIER (0) Host supports request barriers. > > VIRTIO_BLK_F_SIZE_MAX (1) Maximum size of any single segment is > in “size_max”. > > VIRTIO_BLK_F_SEG_MAX (2) Maximum number of segments in a > request is in “seg_max”. > > VIRTIO_BLK_F_GEOMETRY (4) Disk-style geometry specified in “ > geometry”. > > VIRTIO_BLK_F_RO (5) Device is read-only. > > VIRTIO_BLK_F_BLK_SIZE (6) Block size of disk is in “blk_size”. > > VIRTIO_BLK_F_SCSI (7) Device supports scsi packet commands. > > VIRTIO_BLK_F_FLUSH (9) Cache flush command support. > > Device configuration layout The capacity of the device > (expressed in 512-byte sectors) is always present. The > availability of the others all depend on various feature bits > as indicated above. > > struct virtio_blk_config { > u64 capacity; > u32 size_max; > u32 seg_max; > struct virtio_blk_geometry { > u16 cylinders; > u8 heads; > u8 sectors; > } geometry; > u32 blk_size; > }; > > Device Initialization > > 1. The device size should be read from the “capacity” > configuration field. No requests should be submitted which goes > beyond this limit. > > 2. If the VIRTIO_BLK_F_BLK_SIZE feature is negotiated, the > blk_size field can be read to determine the optimal sector size > for the driver to use. This does not effect the units used in > the protocol (always 512 bytes), but awareness of the correct > value can effect performance. > > 3. If the VIRTIO_BLK_F_RO feature is set by the device, any write > requests will fail. > > Device Operation > > The driver queues requests to the virtqueue, and they are used by > the device (not necessarily in order). Each request is of form: > > struct virtio_blk_req { > u32 type; > u32 ioprio; > u64 sector; > char data[][512]; > u8 status; > }; > > If the device has VIRTIO_BLK_F_SCSI feature, it can also support > scsi packet command requests, each of these requests is of form: > > struct virtio_scsi_pc_req { > u32 type; > u32 ioprio; > u64 sector; > char cmd[]; > char data[][512]; > #define SCSI_SENSE_BUFFERSIZE 96 > u8 sense[SCSI_SENSE_BUFFERSIZE]; > u32 errors; > u32 data_len; > u32 sense_len; > u32 residual; > u8 status; > }; > > The type of the request is either a read (VIRTIO_BLK_T_IN), a write > (VIRTIO_BLK_T_OUT), a scsi packet command (VIRTIO_BLK_T_SCSI_CMD or > VIRTIO_BLK_T_SCSI_CMD_OUT[22]) or a flush (VIRTIO_BLK_T_FLUSH or > VIRTIO_BLK_T_FLUSH_OUT[23]). If the device has VIRTIO_BLK_F_BARRIER > feature the high bit (VIRTIO_BLK_T_BARRIER) indicates that this > request acts as a barrier and that all preceeding requests must be > complete before this one, and all following requests must not be > started until this is complete. Note that a barrier does not flush > caches in the underlying backend device in host, and thus does not > serve as data consistency guarantee. Driver must use FLUSH request to > flush the host cache. > > #define VIRTIO_BLK_T_IN 0 > #define VIRTIO_BLK_T_OUT 1 > #define VIRTIO_BLK_T_SCSI_CMD 2 > #define VIRTIO_BLK_T_SCSI_CMD_OUT 3 > #define VIRTIO_BLK_T_FLUSH 4 > #define VIRTIO_BLK_T_FLUSH_OUT 5 > #define VIRTIO_BLK_T_BARRIER 0x80000000 > > The ioprio field is a hint about the relative priorities of > requests to the device: higher numbers indicate more important > requests. > > The sector number indicates the offset (multiplied by 512) where > the read or write is to occur. This field is unused and set to 0 > for scsi packet commands and for flush commands. > > The cmd field is only present for scsi packet command requests, > and indicates the command to perform. This field must reside in a > single, separate read-only buffer; command length can be derived > from the length of this buffer. > > Note that these first three (four for scsi packet commands) > fields are always read-only: the data field is either read-only > or write-only, depending on the request. The size of the read or > write can be derived from the total size of the request buffers. > > The sense field is only present for scsi packet command requests, > and indicates the buffer for scsi sense data. > > The data_len field is only present for scsi packet command > requests, this field is depr

On Wed, Jul 31, 2013 at 01:33:39PM +0930, Rusty Russell wrote:
> Rusty Russell <rusty@au1.ibm.com> writes:
> > -----------------
> > Action Item Subject: Create text version of virtio 0.9.5 document
>
> OK, I've attached this below. This is exactly the spec as 0.9.5
> reformatted into text so we can work it.

I copied virtio-comment which is an open-list.

Hope that's OK with everyone.
People aren't auto-subscribed there but if everyone is
willing to subscribe to virtio-comment, we could
reserve the virtio@ members-only list to administrative
issues only?


>
> I'm still waiting for virtio to be added to the OASIS issue tracking
> system, where I will be prompting you all to open an issue for every
> change we want to consider.
>
> The obvious issues I want to open are:
>
> o Major rework to make PCI an appendix, and core bus-independent.
> o Update spec with changes/fixes since 0.9.5
> - This is easy where contributors are already members,
> trickier for others.
>
> In addition here's a brain dump:
>
> o Endian for config space
> - LE everywhere?
> o Endian for ring
> - LE as well?
> o Allow arbitrary descriptor layouts / message framing.
> o Method to stop activity on a queue?
> o Size descriptor table independent of ringsize?
> o Remove VIRTIO_F_NOTIFY_ON_EMPTY?
> o Remove limit on # indirect descriptors
> - Some other limit?
> o Simplify indirect desc
> - No return to top level on end of desc array
> o Allow chained indirect desc
> - indirect bit use to chain?
>
> Net:
> o Remove VIRTIO_NET_F_GSO?
>
> Block:
> o Remove VIRTIO_BLK_F_SCSI?
> o Revisit flush/barrier semantics?
>
> PCI:
> o New capability layout
> o Allowing non-zero revision numbers?
> o Remove 'align' and use explicit addresses for used/avail.
>
> Balloon:
> o Fix endianness
> o Remove VIRTIO_BALLOON_F_MUST_TELL_HOST?
> o Remove outgoing page queue?
>
> Cheers,
> Rusty.
>
> ====
> This document describes the specifications of the “virtio” family
> of PCI devices. These are devices
> are found in virtual environments,
> yet by design they are not all that different from physical PCI
> devices, and this document treats them as such. This allows the
> guest to use standard PCI drivers and discovery mechanisms.
>
> The purpose of virtio and this specification is that virtual
> environments and guests should have a straightforward, efficient,
> standard and extensible mechanism for virtual devices, rather
> than boutique per-environment or per-OS mechanisms.
>
> Straightforward: Virtio PCI devices use normal PCI mechanisms
> of interrupts and DMA which should be familiar to any device
> driver author. There is no exotic page-flipping or COW
> mechanism: it's just a PCI device.[1]
>
> Efficient: Virtio PCI devices consist of rings of descriptors
> for input and output, which are neatly separated to avoid cache
> effects from both guest and device writing to the same cache
> lines.
>
> Standard: Virtio PCI makes no assumptions about the environment
> in which it operates, beyond supporting PCI. In fact the virtio
> devices specified in the appendices do not require PCI at all:
> they have been implemented on non-PCI buses.[2]
>
> Extensible: Virtio PCI devices contain feature bits which are
> acknowledged by the guest operating system during device setup.
> This allows forwards and backwards compatibility: the device
> offers all the features it knows about, and the driver
> acknowledges those it understands and wishes to use.
>
> 1.1 Virtqueues
>
> The mechanism for bulk data transport on virtio PCI devices is
> pretentiously called a virtqueue. Each device can have zero or
> more virtqueues: for example, the network device has one for
> transmit and one for receive.
>
> Each virtqueue occupies two or more physically-contiguous pages
> (defined, for the purposes of this specification, as 4096 bytes),
> and consists of three parts:
>
>
> +-------------------+-----------------------------------+-----------+
> | Descriptor Table | Available Ring (padding) | Used Ring |
> +-------------------+-----------------------------------+-----------+
>
>
> When the driver wants to send a buffer to the device, it fills in
> a slot in the descriptor table (or chains several together), and
> writes the descriptor index into the available ring. It then
> notifies the device. When the device has finished a buffer, it
> writes the descriptor into the used ring, and sends an interrupt.
>
> Specification
>
> 2.1 PCI Discovery
>
> Any PCI device with Vendor ID 0x1AF4, and Device ID 0x1000 through
> 0x103F inclusive is a virtio device[3]. The device must also have a
> Revision ID of 0 to match this specification.
>
> The Subsystem Device ID indicates which virtio device is
> supported by the device. The Subsystem Vendor ID should reflect
> the PCI Vendor ID of the environment (it's currently only used
> for informational purposes by the guest).
>
>
> +----------------------+--------------------+---------------+
> | Subsystem Device ID | Virtio Device | Specification |
> +----------------------+--------------------+---------------+
> +----------------------+--------------------+---------------+
> | 1 | network card | Appendix C |
> +----------------------+--------------------+---------------+
> | 2 | block device | Appendix D |
> +----------------------+--------------------+---------------+
> | 3 | console | Appendix E |
> +----------------------+--------------------+---------------+
> | 4 | entropy source | Appendix F |
> +----------------------+--------------------+---------------+
> | 5 | memory ballooning | Appendix G |
> +----------------------+--------------------+---------------+
> | 6 | ioMemory | - |
> +----------------------+--------------------+---------------+
> | 7 | rpmsg | - |
> +----------------------+--------------------+---------------+
> | 8 | SCSI host | Appendix I |
> +----------------------+--------------------+---------------+
> | 9 | 9P transport | - |
> +----------------------+--------------------+---------------+
> | 10 | mac80211 wlan | - |
> +----------------------+--------------------+---------------+
>
>
> 2.2 Device Configuration
>
> To configure the device, we use the first I/O region of the PCI
> device. This contains a virtio header followed by a
> device-specific region.
>
> There may be different widths of accesses to the I/O region; the
> “natural” access method for each field in the virtio header must be
> used (i.e. 32-bit accesses for 32-bit fields, etc), but the
> device-specific region can be accessed using any width accesses, and
> should obtain the same results.
>
> Note that this is possible because while the virtio header is PCI
> (i.e. little) endian, the device-specific region is encoded in
> the native endian of the guest (where such distinction is
> applicable).
>
> 2.2.1 Device Initialization Sequence
>
> We start with an overview of device initialization, then expand
> on the details of the device and how each step is preformed.
>
> 1. Reset the device. This is not required on initial start up.
>
> 2. The ACKNOWLEDGE status bit is set: we have noticed the device.
>
> 3. The DRIVER status bit is set: we know how to drive the device.
>
> 4. Device-specific setup, including reading the Device Feature
> Bits, discovery of virtqueues for the device, optional MSI-X
> setup, and reading and possibly writing the virtio
> configuration space.
>
> 5. The subset of Device Feature Bits understood by the driver is
> written to the device.
>
> 6. The DRIVER_OK status bit is set.
>
> 7. The device can now be used (ie. buffers added to the
> virtqueues)[4]
>
> If any of these steps go irrecoverably wrong, the guest should
> set the FAILED status bit to indicate that it has given up on the
> device (it can reset the device later to restart if desired).
>
> We now cover the fields required for general setup in detail.
>
> 2.2.2 Virtio Header
>
> The virtio header looks as follows:
>
>
> +------------++---------------------+---------------------+----------+--------+---------+---------+---------+--------+
> | Bits || 32 | 32 | 32 | 16 | 16 | 16 | 8 | 8 |
> +------------++---------------------+---------------------+----------+--------+---------+---------+---------+--------+
> | Read/Write || R | R+W | R+W | R | R+W | R+W | R+W | R |
> +------------++---------------------+---------------------+----------+--------+---------+---------+---------+--------+
> | Purpose || Device | Guest | Queue | Queue | Queue | Queue | Device | ISR |
> | || Features bits 0:31 | Features bits 0:31 | Address | Size | Select | Notify | Status | Status |
> +------------++---------------------+---------------------+----------+--------+---------+---------+---------+--------+
>
>
> If MSI-X is enabled for the device, two additional fields
> immediately follow this header:[5]
>
>
> +------------++----------------+--------+
> | Bits || 16 | 16 |
> +----------------+--------+
> +------------++----------------+--------+
> | Read/Write || R+W | R+W |
> +------------++----------------+--------+
> | Purpose || Configuration | Queue |
> | (MSI-X) || Vector | Vector |
> +------------++----------------+--------+
>
>
> Immediately following these general headers, there may be
> device-specific headers:
>
>
> +------------++--------------------+
> | Bits || Device Specific |
> +--------------------+
> +------------++--------------------+
> | Read/Write || Device Specific |
> +------------++--------------------+
> | Purpose || Device Specific... |
> | || |
> +------------++--------------------+
>
>
> 2.2.2.1 Device Status
>
> The Device Status field is updated by the guest to indicate its
> progress. This provides a simple low-level diagnostic: it's most
> useful to imagine them hooked up to traffic lights on the console
> indicating the status of each device.
>
> The device can be reset by writing a 0 to this field, otherwise
> at least one bit should be set:
>
> ACKNOWLEDGE (1) Indicates that the guest OS has found the
> device and recognized it as a valid virtio device.
>
> DRIVER (2) Indicates that the guest OS knows how to drive the
> device. Under Linux, drivers can be loadable modules so there
> may be a significant (or infinite) delay before setting this
> bit.
>
> DRIVER_OK (4) Indicates that the driver is set up and ready to
> drive the device.
>
> FAILED (128) Indicates that something went wrong in the guest,
> and it has given up on the device. This could be an internal
> error, or the driver didn't like the device for some reason, or
> even a fatal error during device operation. The device must be
> reset before attempting to re-initialize.
>
> 2.2.2.2 Feature Bits
>
> The first configuration field indicates the features that the
> device supports. The bits are allocated as follows:
>
> 0 to 23 Feature bits for the specific device type
>
> 24 to 32 Feature bits reserved for extensions to the queue and
> feature negotiation mechanisms
>
> For example, feature bit 0 for a network device (i.e. Subsystem
> Device ID 1) indicates that the device supports checksumming of
> packets.
>
> The feature bits are negotiated: the device lists all the
> features it understands in the Device Features field, and the
> guest writes the subset that it understands into the Guest
> Features field. The only way to renegotiate is to reset the
> device.
>
> In particular, new fields in the device configuration header are
> indicated by offering a feature bit, so the guest can check
> before accessing that part of the configuration space.
>
> This allows for forwards and backwards compatibility: if the
> device is enhanced with a new feature bit, older guests will not
> write that feature bit back to the Guest Features field and it
> can go into backwards compatibility mode. Similarly, if a guest
> is enhanced with a feature that the device doesn't support, it
> will not see that feature bit in the Device Features field and
> can go into backwards compatibility mode (or, for poor
> implementations, set the FAILED Device Status bit).
>
> 2.2.2.3 Configuration/Queue Vectors
>
> When MSI-X capability is present and enabled in the device
> (through standard PCI configuration space) 4 bytes at byte offset
> 20 are used to map configuration change and queue interrupts to
> MSI-X vectors. In this case, the ISR Status field is unused, and
> device specific configuration starts at byte offset 24 in virtio
> header structure. When MSI-X capability is not enabled, device
> specific configuration starts at byte offset 20 in virtio header.
>
> Writing a valid MSI-X Table entry number, 0 to 0x7FF, to one of
> Configuration/Queue Vector registers, maps interrupts triggered
> by the configuration change/selected queue events respectively to
> the corresponding MSI-X vector. To disable interrupts for a
> specific event type, unmap it by writing a special NO_VECTOR
> value:
>
> /* Vector value used to disable MSI for queue */
>
> #define VIRTIO_MSI_NO_VECTOR 0xffff
>
> Reading these registers returns vector mapped to a given event,
> or NO_VECTOR if unmapped. All queue and configuration change
> events are unmapped by default.
>
> Note that mapping an event to vector might require allocating
> internal device resources, and might fail. Devices report such
> failures by returning the NO_VECTOR value when the relevant
> Vector field is read. After mapping an event to vector, the
> driver must verify success by reading the Vector field value: on
> success, the previously written value is returned, and on
> failure, NO_VECTOR is returned. If a mapping failure is detected,
> the driver can retry mapping with fewervectors, or disable MSI-X.
>
> 2.3 Virtqueue Configuration
>
> As a device can have zero or more virtqueues for bulk data
> transport (for example, the network driver has two), the driver
> needs to configure them as part of the device-specific
> configuration.
>
> This is done as follows, for each virtqueue a device has:
>
> 1. Write the virtqueue index (first queue is 0) to the Queue
> Select field.
>
> 2. Read the virtqueue size from the Queue Size field, which is
> always a power of 2. This controls how big the virtqueue is
> (see below). If this field is 0, the virtqueue does not exist.
>
> 3. Allocate and zero virtqueue in contiguous physical memory, on
> a 4096 byte alignment. Write the physical address, divided by
> 4096 to the Queue Address field.[6]
>
> 4. Optionally, if MSI-X capability is present and enabled on the
> device, select a vector to use to request interrupts triggered
> by virtqueue events. Write the MSI-X Table entry number
> corresponding to this vector in Queue Vector field. Read the
> Queue Vector field: on success, previously written value is
> returned; on failure, NO_VECTOR value is returned.
>
> The Queue Size field controls the total number of bytes required
> for the virtqueue according to the following formula:
>
> #define ALIGN(x) (((x) + 4095) & ~4095)
>
> static inline unsigned vring_size(unsigned int qsz)
> {
> return ALIGN(sizeof(struct vring_desc)*qsz + sizeof(u16)*(2 + qsz))
> + ALIGN(sizeof(struct vring_used_elem)*qsz);
> }
>
> This currently wastes some space with padding, but also allows
> future extensions. The virtqueue layout structure looks like this
> (qsz is the Queue Size field, which is a variable, so this code
> won't compile):
>
> struct vring {
> /* The actual descriptors (16 bytes each) */
> struct vring_desc desc[qsz];
>
> /* A ring of available descriptor heads with free-running index. */
> struct vring_avail avail;
>
> // Padding to the next 4096 boundary.
> char pad[];
>
> // A ring of used descriptor heads with free-running index.
> struct vring_used used;
> };
>
> 2.3.1 A Note on Virtqueue Endianness
>
> Note that the endian of these fields and everything else in the
> virtqueue is the native endian of the guest, not little-endian as
> PCI normally is. This makes for simpler guest code, and it is
> assumed that the host already has to be deeply aware of the guest
> endian so such an “endian-aware” device is not a significant
> issue.
>
> 2.3.2 Descriptor Table
>
> The descriptor table refers to the buffers the guest is using for
> the device. The addresses are physical addresses, and the buffers
> can be chained via the next field. Each descriptor describes a
> buffer which is read-only or write-only, but a chain of
> descriptors can contain both read-only and write-only buffers.
>
> No descriptor chain may be more than 2^32 bytes long in total.
>
> struct vring_desc {
> /* Address (guest-physical). */
> u64 addr;
> /* Length. */
> u32 len;
>
> /* This marks a buffer as continuing via the next field. */
> #define VRING_DESC_F_NEXT 1
> /* This marks a buffer as write-only (otherwise read-only). */
> #define VRING_DESC_F_WRITE 2
> /* This means the buffer contains a list of buffer descriptors. */
> #define VRING_DESC_F_INDIRECT 4
> /* The flags as indicated above. */
> u16 flags;
> /* Next field if flags & NEXT */
> u16 next;
> };
>
> The number of descriptors in the table is specified by the Queue
> Size field for this virtqueue.
>
> 2.3.3 Indirect Descriptors
>
> Some devices benefit by concurrently dispatching a large number
> of large requests. The VIRTIO_RING_F_INDIRECT_DESC feature can be
> used to allow this (see Appendix B: Reserved Feature Bits). To increase
> ring capacity it is possible to store a table of indirect
> descriptors anywhere in memory, and insert a descriptor in main
> virtqueue (with flags&INDIRECT on) that refers to memory buffer
> containing this indirect descriptor table; fields addr and len
> refer to the indirect table address and length in bytes,
> respectively. The indirect table layout structure looks like this
> (len is the length of the descriptor that refers to this table,
> which is a variable, so this code won't compile):
>
> struct indirect_descriptor_table {
> /* The actual descriptors (16 bytes each) */
> struct vring_desc desc[len / 16];
> };
>
> The first indirect descriptor is located at start of the indirect
> descriptor table (index 0), additional indirect descriptors are
> chained by next field. An indirect descriptor without next field
> (with flags&NEXT off) signals the end of the indirect descriptor
> table, and transfers control back to the main virtqueue. An
> indirect descriptor can not refer to another indirect descriptor
> table (flags&INDIRECT must be off). A single indirect descriptor
> table can include both read-only and write-only descriptors;
> write-only flag (flags&WRITE) in the descriptor that refers to it
> is ignored.
>
> 2.3.4 Available Ring
>
> The available ring refers to what descriptors we are offering the
> device: it refers to the head of a descriptor chain. The “flags” field
> is currently 0 or 1: 1 indicating that we do not need an interrupt
> when the device consumes a descriptor from the available
> ring. Alternatively, the guest can ask the device to delay interrupts
> until an entry with an index specified by the “ used_event” field is
> written in the used ring (equivalently, until the idx field in the
> used ring will reach the value used_event + 1). The method employed by
> the device is controlled by the VIRTIO_RING_F_EVENT_IDX feature bit
> (see Appendix B: Reserved Feature Bits). This interrupt suppression is
> merely an optimization; it may not suppress interrupts entirely.
>
> The “idx” field indicates where we would put the next descriptor
> entry (modulo the ring size). This starts at 0, and increases.
>
> struct vring_avail {
> #define VRING_AVAIL_F_NO_INTERRUPT 1
> u16 flags;
> u16 idx;
> u16 ring[qsz]; /* qsz is the Queue Size field read from device */
> u16 used_event;
> };
>
> 2.3.5 Used Ring
>
> The used ring is where the device returns buffers once it is done
> with them. The flags field can be used by the device to hint that
> no notification is necessary when the guest adds to the available
> ring. Alternatively, the “avail_event” field can be used by the
> device to hint that no notification is necessary until an entry
> with an index specified by the “avail_event” is written in the
> available ring (equivalently, until the idx field in the
> available ring will reach the value avail_event + 1). The method
> employed by the device is controlled by the guest through the
> VIRTIO_RING_F_EVENT_IDX feature bit (see Appendix B: Reserved
> Feature Bits).[7]
>
> Each entry in the ring is a pair: the head entry of the
> descriptor chain describing the buffer (this matches an entry
> placed in the available ring by the guest earlier), and the total
> of bytes written into the buffer. The latter is extremely useful
> for guests using untrusted buffers: if you do not know exactly
> how much has been written by the device, you usually have to zero
> the buffer to ensure no data leakage occurs.
>
> /* u32 is used here for ids for padding reasons. */
> struct vring_used_elem {
> /* Index of start of used descriptor chain. */
> u32 id;
> /* Total length of the descriptor chain which was used (written to) */
> u32 len;
> };
>
> struct vring_used {
> #define VRING_USED_F_NO_NOTIFY 1
> u16 flags;
> u16 idx;
> struct vring_used_elem ring[qsz];
> u16 avail_event;
> };
>
> 2.3.6 Helpers for Managing Virtqueues
>
> The Linux Kernel Source code contains the definitions above and
> helper routines in a more usable form, in
> include/linux/virtio_ring.h. This was explicitly licensed by IBM
> and Red Hat under the (3-clause) BSD license so that it can be
> freely used by all other projects, and is reproduced (with slight
> variation to remove Linux assumptions) in Appendix A.
>
> 2.4 Device Operation
>
> There are two parts to device operation: supplying new buffers to
> the device, and processing used buffers from the device. As an
> example, the virtio network device has two virtqueues: the
> transmit virtqueue and the receive virtqueue. The driver adds
> outgoing (read-only) packets to the transmit virtqueue, and then
> frees them after they are used. Similarly, incoming (write-only)
> buffers are added to the receive virtqueue, and processed after
> they are used.
>
> 2.4.1 Supplying Buffers to The Device
>
> Actual transfer of buffers from the guest OS to the device
> operates as follows:
>
> 1. Place the buffer(s) into free descriptor(s).
>
> (a) If there are no free descriptors, the guest may choose to
> notify the device even if notifications are suppressed (to
> reduce latency).[8]
>
> 2. Place the id of the buffer in the next ring entry of the
> available ring.
>
> 3. The steps (1) and (2) may be performed repeatedly if batching
> is possible.
>
> 4. A memory barrier should be executed to ensure the device sees
> the updated descriptor table and available ring before the next
> step.
>
> 5. The available “idx” field should be increased by the number of
> entries added to the available ring.
>
> 6. A memory barrier should be executed to ensure that we update
> the idx field before checking for notification suppression.
>
> 7. If notifications are not suppressed, the device should be
> notified of the new buffers.
>
> Note that the above code does not take precautions against the
> available ring buffer wrapping around: this is not possible since
> the ring buffer is the same size as the descriptor table, so step
> (1) will prevent such a condition.
>
> In addition, the maximum queue size is 32768 (it must be a power
> of 2 which fits in 16 bits), so the 16-bit “idx” value can always
> distinguish between a full and empty buffer.
>
> Here is a description of each stage in more detail.
>
> 2.4.1.1 Placing Buffers Into The Descriptor Table
>
> A buffer consists of zero or more read-only physically-contiguous
> elements followed by zero or more physically-contiguous
> write-only elements (it must have at least one element). This
> algorithm maps it into the descriptor table:
>
> 1. for each buffer element, b:
>
> (a) Get the next free descriptor table entry, d
>
> (b) Set d.addr to the physical address of the start of b
>
> (c) Set d.len to the length of b.
>
> (d) If b is write-only, set d.flags to VRING_DESC_F_WRITE,
> otherwise 0.
>
> (e) If there is a buffer element after this:
>
> i. Set d.next to the index of the next free descriptor
> element.
>
> ii. Set the VRING_DESC_F_NEXT bit in d.flags.
>
> In practice, the d.next fields are usually used to chain free
> descriptors, and a separate count kept to check there are enough
> free descriptors before beginning the mappings.
>
> 2.4.1.2 Updating The Available Ring
>
> The head of the buffer we mapped is the first d in the algorithm
> above. A naive implementation would do the following:
>
> avail->ring[avail->idx % qsz] = head;
>
> However, in general we can add many descriptors before we update
> the “idx” field (at which point they become visible to the
> device), so we keep a counter of how many we've added:
>
> avail->ring[(avail->idx + added++) % qsz] = head;
>
> 2.4.1.3 Updating The Index Field
>
> Once the idx field of the virtqueue is updated, the device will
> be able to access the descriptor entries we've created and the
> memory they refer to. This is why a memory barrier is generally
> used before the idx update, to ensure it sees the most up-to-date
> copy.
>
> The idx field always increments, and we let it wrap naturally at
> 65536:
>
> avail->idx += added;
>
> 2.4.1.4 Notifying The Device
>
> Device notification occurs by writing the 16-bit virtqueue index
> of this virtqueue to the Queue Notify field of the virtio header
> in the first I/O region of the PCI device. This can be expensive,
> however, so the device can suppress such notifications if it
> doesn't need them. We have to be careful to expose the new idx
> value before checking the suppression flag: it's OK to notify
> gratuitously, but not to omit a required notification. So again,
> we use a memory barrier here before reading the flags or the
> avail_event field.
>
> If the VIRTIO_F_RING_EVENT_IDX feature is not negotiated, and if
> the VRING_USED_F_NOTIFY flag is not set, we go ahead and write to
> the PCI configuration space.
>
> If the VIRTIO_F_RING_EVENT_IDX feature is negotiated, we read the
> avail_event field in the available ring structure. If the
> available index crossed_the avail_event field value since the
> last notification, we go ahead and write to the PCI configuration
> space. The avail_event field wraps naturally at 65536 as well:
>
> (u16)(new_idx - avail_event - 1) < (u16)(new_idx - old_idx)
>
> 2.4.2 Receiving Used Buffers From The Device
>
> Once the device has used a buffer (read from or written to it, or
> parts of both, depending on the nature of the virtqueue and the
> device), it sends an interrupt, following an algorithm very
> similar to the algorithm used for the driver to send the device a
> buffer:
>
> 1. Write the head descriptor number to the next field in the used
> ring.
>
> 2. Update the used ring idx.
>
> 3. Determine whether an interrupt is necessary:
>
> (a) If the VIRTIO_F_RING_EVENT_IDX feature is not negotiated:
> check if f the VRING_AVAIL_F_NO_INTERRUPT flag is not set in
> avail->flags
>
> (b) If the VIRTIO_F_RING_EVENT_IDX feature is negotiated: check
> whether the used index crossed the used_event field value
> since the last update. The used_event field wraps naturally
> at 65536 as well:
> (u16)(new_idx - used_event - 1) < (u16)(new_idx - old_idx)
>
> 4. If an interrupt is necessary:
>
> (a) If MSI-X capability is disabled:
>
> i. Set the lower bit of the ISR Status field for the device.
>
> ii. Send the appropriate PCI interrupt for the device.
>
> (b) If MSI-X capability is enabled:
>
> i. Request the appropriate MSI-X interrupt message for the
> device, Queue Vector field sets the MSI-X Table entry
> number.
>
> ii. If Queue Vector field value is NO_VECTOR, no interrupt
> message is requested for this event.
>
> The guest interrupt handler should:
>
> 1. If MSI-X capability is disabled: read the ISR Status field,
> which will reset it to zero. If the lower bit is zero, the
> interrupt was not for this device. Otherwise, the guest driver
> should look through the used rings of each virtqueue for the
> device, to see if any progress has been made by the device
> which requires servicing.
>
> 2. If MSI-X capability is enabled: look through the used rings of
> each virtqueue mapped to the specific MSI-X vector for the
> device, to see if any progress has been made by the device
> which requires servicing.
>
> For each ring, guest should then disable interrupts by writing
> VRING_AVAIL_F_NO_INTERRUPT flag in avail structure, if required.
> It can then process used ring entries finally enabling interrupts
> by clearing the VRING_AVAIL_F_NO_INTERRUPT flag or updating the
> EVENT_IDX field in the available structure, Guest should then
> execute a memory barrier, and then recheck the ring empty
> condition. This is necessary to handle the case where, after the
> last check and before enabling interrupts, an interrupt has been
> suppressed by the device:
>
> vring_disable_interrupts(vq);
>
> for (;;) {
> if (vq->last_seen_used != vring->used.idx) {
> vring_enable_interrupts(vq);
> mb();
>
> if (vq->last_seen_used != vring->used.idx)
> break;
> }
>
> struct vring_used_elem *e = vring.used->ring[vq->last_seen_used%vsz];
> process_buffer(e);
> vq->last_seen_used++;
> }
>
> 2.4.3 Dealing With Configuration Changes
>
> Some virtio PCI devices can change the device configuration
> state, as reflected in the virtio header in the PCI configuration
> space. In this case:
>
> 1. If MSI-X capability is disabled: an interrupt is delivered and
> the second highest bit is set in the ISR Status field to
> indicate that the driver should re-examine the configuration
> space. Note that a single interrupt can indicate both that one
> or more virtqueue has been used and that the configuration
> space has changed: even if the config bit is set, virtqueues
> must be scanned.
>
> 2. If MSI-X capability is enabled: an interrupt message is
> requested. The Configuration Vector field sets the MSI-X Table
> entry number to use. If Configuration Vector field value is
> NO_VECTOR, no interrupt message is requested for this event.
>
>
> Creating New Device Types
>
> Various considerations are necessary when creating a new device
> type:
>
> How Many Virtqueues?
>
> It is possible that a very simple device will operate entirely
> through its configuration space, but most will need at least one
> virtqueue in which it will place requests. A device with both
> input and output (eg. console and network devices described here)
> need two queues: one which the driver fills with buffers to
> receive input, and one which the driver places buffers to
> transmit output.
>
> What Configuration Space Layout?
>
> Configuration space is generally used for rarely-changing or
> initialization-time parameters. But it is a limited resource, so
> it might be better to use a virtqueue to update configuration
> information (the network device does this for filtering,
> otherwise the table in the config space could potentially be very
> large).
>
> Note that this space is generally the guest's native endian,
> rather than PCI's little-endian.
>
> What Device Number?
>
> Currently device numbers are assigned quite freely: a simple
> request mail to the author of this document or the Linux
> virtualization mailing list[9] will be sufficient to secure a unique one.
>
> Meanwhile for experimental drivers, use 65535 and work backwards.
>
> How many MSI-X vectors?
>
> Using the optional MSI-X capability devices can speed up
> interrupt processing by removing the need to read ISR Status
> register by guest driver (which might be an expensive operation),
> reducing interrupt sharing between devices and queues within the
> device, and handling interrupts from multiple CPUs. However, some
> systems impose a limit (which might be as low as 256) on the
> total number of MSI-X vectors that can be allocated to all
> devices. Devices and/or device drivers should take this into
> account, limiting the number of vectors used unless the device is
> expected to cause a high volume of interrupts. Devices can
> control the number of vectors used by limiting the MSI-X Table
> Size or not presenting MSI-X capability in PCI configuration
> space. Drivers can control this by mapping events to as small
> number of vectors as possible, or disabling MSI-X capability
> altogether.
>
> Message Framing
>
> The descriptors used for a buffer should not effect the semantics
> of the message, except for the total length of the buffer. For
> example, a network buffer consists of a 10 byte header followed
> by the network packet. Whether this is presented in the ring
> descriptor chain as (say) a 10 byte buffer and a 1514 byte
> buffer, or a single 1524 byte buffer, or even three buffers,
> should have no effect.
>
> In particular, no implementation should use the descriptor
> boundaries to determine the size of any header in a request.[10]
>
> Device Improvements
>
> Any change to configuration space, or new virtqueues, or
> behavioural changes, should be indicated by negotiation of a new
> feature bit. This establishes clarity[11] and avoids future expansion problems.
>
> Clusters of functionality which are always implemented together
> can use a single bit, but if one feature makes sense without the
> others they should not be gratuitously grouped together to
> conserve feature bits. We can always extend the spec when the
> first person needs more than 24 feature bits for their device.
>
>
>
>
> Appendix A: virtio_ring.h
>
> #ifndef VIRTIO_RING_H
> #define VIRTIO_RING_H
> /* An interface for efficient virtio implementation.
> *
> * This header is BSD licensed so anyone can use the definitions
> * to implement compatible drivers/servers.
> *
> * Copyright 2007, 2009, IBM Corporation
> * Copyright 2011, Red Hat, Inc
> * All rights reserved.
> *
> * Redistribution and use in source and binary forms, with or without
> * modification, are permitted provided that the following conditions
> * are met:
> * 1. Redistributions of source code must retain the above copyright
> * notice, this list of conditions and the following disclaimer.
> * 2. Redistributions in binary form must reproduce the above copyright
> * notice, this list of conditions and the following disclaimer in the
> * documentation and/or other materials provided with the distribution.
> * 3. Neither the name of IBM nor the names of its contributors
> * may be used to endorse or promote products derived from this software
> * without specific prior written permission.
> * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS ``AS IS'' AND
> * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
> * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
> * ARE DISCLAIMED. IN NO EVENT SHALL IBM OR CONTRIBUTORS BE LIABLE
> * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
> * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
> * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
> * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
> * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
> * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
> * SUCH DAMAGE.
> */
>
> /* This marks a buffer as continuing via the next field. */
> #define VRING_DESC_F_NEXT 1
> /* This marks a buffer as write-only (otherwise read-only). */
> #define VRING_DESC_F_WRITE 2
>
> /* The Host uses this in used->flags to advise the Guest: don't kick me
> * when you add a buffer. It's unreliable, so it's simply an
> * optimization. Guest will still kick if it's out of buffers. */
> #define VRING_USED_F_NO_NOTIFY 1
> /* The Guest uses this in avail->flags to advise the Host: don't
> * interrupt me when you consume a buffer. It's unreliable, so it's
> * simply an optimization. */
> #define VRING_AVAIL_F_NO_INTERRUPT 1
>
> /* Virtio ring descriptors: 16 bytes.
> * These can chain together via "next". */
> struct vring_desc {
> /* Address (guest-physical). */
> uint64_t addr;
> /* Length. */
> uint32_t len;
> /* The flags as indicated above. */
> uint16_t flags;
> /* We chain unused descriptors via this, too */
> uint16_t next;
> };
>
> struct vring_avail {
> uint16_t flags;
> uint16_t idx;
> uint16_t ring[];
> uint16_t used_event;
> };
>
> /* u32 is used here for ids for padding reasons. */
> struct vring_used_elem {
> /* Index of start of used descriptor chain. */
> uint32_t id;
> /* Total length of the descriptor chain which was written to. */
> uint32_t len;
> };
>
> struct vring_used {
> uint16_t flags;
> uint16_t idx;
> struct vring_used_elem ring[];
> uint16_t avail_event;
> };
>
> struct vring {
> unsigned int num;
>
> struct vring_desc *desc;
> struct vring_avail *avail;
> struct vring_used *used;
> };
>
> /* The standard layout for the ring is a continuous chunk of memory which
> * looks like this. We assume num is a power of 2.
> *
> * struct vring {
> * // The actual descriptors (16 bytes each)
> * struct vring_desc desc[num];
> *
> * // A ring of available descriptor heads with free-running index.
> * __u16 avail_flags;
> * __u16 avail_idx;
> * __u16 available[num];
> *
> * // Padding to the next align boundary.
> * char pad[];
> *
> * // A ring of used descriptor heads with free-running index.
> * __u16 used_flags;
> * __u16 EVENT_IDX;
> * struct vring_used_elem used[num];
> * };
> * Note: for virtio PCI, align is 4096.
> */
> static inline void vring_init(struct vring *vr, unsigned int num, void *p,
> unsigned long align)
> {
> vr->num = num;
> vr->desc = p;
> vr->avail = p + num*sizeof(struct vring_desc);
> vr->used = (void *)(((unsigned long)&vr->avail->ring[num]
> + align-1)
> & ~(align - 1));
> }
>
> static inline unsigned vring_size(unsigned int num, unsigned long align)
> {
> return ((sizeof(struct vring_desc)*num + sizeof(uint16_t)*(2+num)
> + align - 1) & ~(align - 1))
> + sizeof(uint16_t)*3 + sizeof(struct vring_used_elem)*num;
> }
>
> static inline int vring_need_event(uint16_t event_idx, uint16_t new_idx, uint16_t old_idx)
> {
> return (uint16_t)(new_idx - event_idx - 1) < (uint16_t)(new_idx - old_idx);
> }
> #endif /* VIRTIO_RING_H */
>
>
> Appendix B: Reserved Feature Bits
>
> Currently there are five device-independent feature bits defined:
>
> VIRTIO_F_NOTIFY_ON_EMPTY (24) Negotiating this feature
> indicates that the driver wants an interrupt if the device runs
> out of available descriptors on a virtqueue, even though
> interrupts are suppressed using the VRING_AVAIL_F_NO_INTERRUPT
> flag or the used_event field. An example of this is the
> networking driver: it doesn't need to know every time a packet
> is transmitted, but it does need to free the transmitted
> packets a finite time after they are transmitted. It can avoid
> using a timer if the device interrupts it when all the packets
> are transmitted.
>
> VIRTIO_F_RING_INDIRECT_DESC (28) Negotiating this feature indicates
> that the driver can use descriptors with the VRING_DESC_F_INDIRECT
> flag set, as described in 2.3.3 Indirect Descriptors.
>
> VIRTIO_F_RING_EVENT_IDX(29) This feature enables the used_event
> and the avail_event fields. If set, it indicates that the
> device should ignore the flags field in the available ring
> structure. Instead, the used_event field in this structure is
> used by guest to suppress device interrupts. Further, the
> driver should ignore the flags field in the used ring
> structure. Instead, the avail_event field in this structure is
> used by the device to suppress notifications. If unset, the
> driver should ignore the used_event field; the device should
> ignore the avail_event field; the flags field is used
>
> Appendix C: Network Device
>
> The virtio network device is a virtual ethernet card, and is the
> most complex of the devices supported so far by virtio. It has
> enhanced rapidly and demonstrates clearly how support for new
> features should be added to an existing device. Empty buffers are
> placed in one virtqueue for receiving packets, and outgoing
> packets are enqueued into another for transmission in that order.
> A third command queue is used to control advanced filtering
> features.
>
> Configuration
>
> Subsystem Device ID 1
>
> Virtqueues 0:receiveq. 1:transmitq. 2:controlq[12]
>
> Feature bits
>
> VIRTIO_NET_F_CSUM (0) Device handles packets with partial checksum
>
> VIRTIO_NET_F_GUEST_CSUM (1) Guest handles packets with partial checksum
>
> VIRTIO_NET_F_MAC (5) Device has given MAC address.
>
> VIRTIO_NET_F_GSO (6) (Deprecated) device handles packets with
> any GSO type.[13]
>
> VIRTIO_NET_F_GUEST_TSO4 (7) Guest can receive TSOv4.
>
> VIRTIO_NET_F_GUEST_TSO6 (8) Guest can receive TSOv6.
>
> VIRTIO_NET_F_GUEST_ECN (9) Guest can receive TSO with ECN.
>
> VIRTIO_NET_F_GUEST_UFO (10) Guest can receive UFO.
>
> VIRTIO_NET_F_HOST_TSO4 (11) Device can receive TSOv4.
>
> VIRTIO_NET_F_HOST_TSO6 (12) Device can receive TSOv6.
>
> VIRTIO_NET_F_HOST_ECN (13) Device can receive TSO with ECN.
>
> VIRTIO_NET_F_HOST_UFO (14) Device can receive UFO.
>
> VIRTIO_NET_F_MRG_RXBUF (15) Guest can merge receive buffers.
>
> VIRTIO_NET_F_STATUS (16) Configuration status field is
> available.
>
> VIRTIO_NET_F_CTRL_VQ (17) Control channel is available.
>
> VIRTIO_NET_F_CTRL_RX (18) Control channel RX mode support.
>
> VIRTIO_NET_F_CTRL_VLAN (19) Control channel VLAN filtering.
>
> VIRTIO_NET_F_GUEST_ANNOUNCE(21) Guest can send gratuitous
> packets.
>
> Device configuration layout Two configuration fields are
> currently defined. The mac address field always exists (though
> is only valid if VIRTIO_NET_F_MAC is set), and the status field
> only exists if VIRTIO_NET_F_STATUS is set. Two read-only bits
> are currently defined for the status field:
> VIRTIO_NET_S_LINK_UP and VIRTIO_NET_S_ANNOUNCE.
>
> #define VIRTIO_NET_S_LINK_UP 1
> #define VIRTIO_NET_S_ANNOUNCE 2
>
> struct virtio_net_config {
> u8 mac[6];
> u16 status;
> };
>
> Device Initialization
>
> 1. The initialization routine should identify the receive and
> transmission virtqueues.
>
> 2. If the VIRTIO_NET_F_MAC feature bit is set, the configuration
> space “mac” entry indicates the “physical” address of the the
> network card, otherwise a private MAC address should be
> assigned. All guests are expected to negotiate this feature if
> it is set.
>
> 3. If the VIRTIO_NET_F_CTRL_VQ feature bit is negotiated,
> identify the control virtqueue.
>
> 4. If the VIRTIO_NET_F_STATUS feature bit is negotiated, the link
> status can be read from the bottom bit of the “status” config
> field. Otherwise, the link should be assumed active.
>
> 5. The receive virtqueue should be filled with receive buffers.
> This is described in detail below in “Setting Up Receive
> Buffers”.
>
> 6. A driver can indicate that it will generate checksumless
> packets by negotating the VIRTIO_NET_F_CSUM feature. This “
> checksum offload” is a common feature on modern network cards.
>
> 7. If that feature is negotiated[14], a driver can use TCP or UDP
> segmentation offload by negotiating the VIRTIO_NET_F_HOST_TSO4 (IPv4
> TCP), VIRTIO_NET_F_HOST_TSO6 (IPv6 TCP) and VIRTIO_NET_F_HOST_UFO
> (UDP fragmentation) features. It should not send TCP packets
> requiring segmentation offload which have the Explicit Congestion
> Notification bit set, unless the VIRTIO_NET_F_HOST_ECN feature is
> negotiated.[15]
>
> 8. The converse features are also available: a driver can save
> the virtual device some work by negotiating these features.[16]
> The VIRTIO_NET_F_GUEST_CSUM feature indicates that partially
> checksummed packets can be received, and if it can do that then
> the VIRTIO_NET_F_GUEST_TSO4, VIRTIO_NET_F_GUEST_TSO6,
> VIRTIO_NET_F_GUEST_UFO and VIRTIO_NET_F_GUEST_ECN are the input
> equivalents of the features described above. See “Receiving
> Packets” below.
>
> Device Operation
>
> Packets are transmitted by placing them in the transmitq, and
> buffers for incoming packets are placed in the receiveq. In each
> case, the packet itself is preceeded by a header:
>
> struct virtio_net_hdr {
> #define VIRTIO_NET_HDR_F_NEEDS_CSUM 1
> u8 flags;
> #define VIRTIO_NET_HDR_GSO_NONE 0
> #define VIRTIO_NET_HDR_GSO_TCPV4 1
> #define VIRTIO_NET_HDR_GSO_UDP 3
> #define VIRTIO_NET_HDR_GSO_TCPV6 4
> #define VIRTIO_NET_HDR_GSO_ECN 0x80
> u8 gso_type;
> u16 hdr_len;
> u16 gso_size;
> u16 csum_start;
> u16 csum_offset;
> /* Only if VIRTIO_NET_F_MRG_RXBUF: */
> u16 num_buffers
> };
>
> The controlq is used to control device features such as
> filtering.
>
> Packet Transmission
>
> Transmitting a single packet is simple, but varies depending on
> the different features the driver negotiated.
>
> 1. If the driver negotiated VIRTIO_NET_F_CSUM, and the packet has
> not been fully checksummed, then the virtio_net_hdr's fields
> are set as follows. Otherwise, the packet must be fully
> checksummed, and flags is zero.
>
> • flags has the VIRTIO_NET_HDR_F_NEEDS_CSUM set,
>
> • csum_start is set to the offset within the packet to begin checksumming,
> and
>
> • csum_offset indicates how many bytes after the csum_start the
> new (16 bit ones' complement) checksum should be placed.[17]
>
> 2. If the driver negotiated
> VIRTIO_NET_F_HOST_TSO4, TSO6 or UFO, and the packet requires
> TCP segmentation or UDP fragmentation, then the “gso_type”
> field is set to VIRTIO_NET_HDR_GSO_TCPV4, TCPV6 or UDP.
> (Otherwise, it is set to VIRTIO_NET_HDR_GSO_NONE). In this
> case, packets larger than 1514 bytes can be transmitted: the
> metadata indicates how to replicate the packet header to cut it
> into smaller packets. The other gso fields are set:
>
> • hdr_len is a hint to the device as to how much of the header
> needs to be kept to copy into each packet, usually set to the
> length of the headers, including the transport header.[18]
>
> • gso_size is the maximum size of each packet beyond that
> header (ie. MSS).
>
> • If the driver negotiated the VIRTIO_NET_F_HOST_ECN feature,
> the VIRTIO_NET_HDR_GSO_ECN bit may be set in “gso_type” as
> well, indicating that the TCP packet has the ECN bit set.[19]
>
> 3. If the driver negotiated the VIRTIO_NET_F_MRG_RXBUF feature,
> the num_buffers field is set to zero.
>
> 4. The header and packet are added as one output buffer to the
> transmitq, and the device is notified of the new entry (see 2.4.1.4
> Notifying The Device).[20]
>
> Packet Transmission Interrupt
>
> Often a driver will suppress transmission interrupts using the
> VRING_AVAIL_F_NO_INTERRUPT flag (see 2.4.2 Receiving Used Buffers From
> The Device) and check for used packets in the transmit path of following
> packets. However, it will still receive interrupts if the
> VIRTIO_F_NOTIFY_ON_EMPTY feature is negotiated, indicating that
> the transmission queue is completely emptied.
>
> The normal behavior in this interrupt handler is to retrieve and
> new descriptors from the used ring and free the corresponding
> headers and packets.
>
> Setting Up Receive Buffers
>
> It is generally a good idea to keep the receive virtqueue as
> fully populated as possible: if it runs out, network performance
> will suffer.
>
> If the VIRTIO_NET_F_GUEST_TSO4, VIRTIO_NET_F_GUEST_TSO6 or
> VIRTIO_NET_F_GUEST_UFO features are used, the Guest will need to
> accept packets of up to 65550 bytes long (the maximum size of a
> TCP or UDP packet, plus the 14 byte ethernet header), otherwise
> 1514 bytes. So unless VIRTIO_NET_F_MRG_RXBUF is negotiated, every
> buffer in the receive queue needs to be at least this length [20a]
>
> If VIRTIO_NET_F_MRG_RXBUF is negotiated, each buffer must be at
> least the size of the struct virtio_net_hdr.
>
> Packet Receive Interrupt
>
> When a packet is copied into a buffer in the receiveq, the
> optimal path is to disable further interrupts for the receiveq
> (see [sub:Receiving-Used-Buffers]) and process packets until no
> more are found, then re-enable them.
>
> Processing packet involves:
>
> 1. If the driver negotiated the VIRTIO_NET_F_MRG_RXBUF feature,
> then the “num_buffers” field indicates how many descriptors
> this packet is spread over (including this one). This allows
> receipt of large packets without having to allocate large
> buffers. In this case, there will be at least “num_buffers” in
> the used ring, and they should be chained together to form a
> single packet. The other buffers will not begin with a struct
> virtio_net_hdr.
>
> 2. If the VIRTIO_NET_F_MRG_RXBUF feature was not negotiated, or
> the “num_buffers” field is one, then the entire packet will be
> contained within this buffer, immediately following the struct
> virtio_net_hdr.
>
> 3. If the VIRTIO_NET_F_GUEST_CSUM feature was negotiated, the
> VIRTIO_NET_HDR_F_NEEDS_CSUM bit in the “flags” field may be
> set: if so, the checksum on the packet is incomplete and the “
> csum_start” and “csum_offset” fields indicate how to calculate
> it (see Packet Transmission point 1).
>
> 4. If the VIRTIO_NET_F_GUEST_TSO4, TSO6 or UFO options were
> negotiated, then the “gso_type” may be something other than
> VIRTIO_NET_HDR_GSO_NONE, and the “gso_size” field indicates the
> desired MSS (see Packet Transmission point 2).
>
> Control Virtqueue
>
> The driver uses the control virtqueue (if VIRTIO_NET_F_VTRL_VQ is
> negotiated) to send commands to manipulate various features of
> the device which would not easily map into the configuration
> space.
>
> All commands are of the following form:
>
> struct virtio_net_ctrl {
> u8 class;
> u8 command;
> u8 command-specific-data[];
> u8 ack;
> };
>
> /* ack values */
> #define VIRTIO_NET_OK 0
> #define VIRTIO_NET_ERR 1
>
> The class, command and command-specific-data are set by the
> driver, and the device sets the ack byte. There is little it can
> do except issue a diagnostic if the ack byte is not
> VIRTIO_NET_OK.
>
> Packet Receive Filtering
>
> If the VIRTIO_NET_F_CTRL_RX feature is negotiated, the driver can
> send control commands for promiscuous mode, multicast receiving,
> and filtering of MAC addresses.
>
> Note that in general, these commands are best-effort: unwanted
> packets may still arrive.
>
> Setting Promiscuous Mode
>
> #define VIRTIO_NET_CTRL_RX 0
> #define VIRTIO_NET_CTRL_RX_PROMISC 0
> #define VIRTIO_NET_CTRL_RX_ALLMULTI 1
>
> The class VIRTIO_NET_CTRL_RX has two commands:
> VIRTIO_NET_CTRL_RX_PROMISC turns promiscuous mode on and off, and
> VIRTIO_NET_CTRL_RX_ALLMULTI turns all-multicast receive on and
> off. The command-specific-data is one byte containing 0 (off) or
> 1 (on).
>
> Setting MAC Address Filtering
>
> struct virtio_net_ctrl_mac {
> u32 entries;
> u8 macs[entries][ETH_ALEN];
> };
>
> #define VIRTIO_NET_CTRL_MAC 1
> #define VIRTIO_NET_CTRL_MAC_TABLE_SET 0
>
> The device can filter incoming packets by any number of destination
> MAC addresses.[21] This table is set using the class
> VIRTIO_NET_CTRL_MAC and the command VIRTIO_NET_CTRL_MAC_TABLE_SET. The
> command-specific-data is two variable length tables of 6-byte MAC
> addresses. The first table contains unicast addresses, and the second
> contains multicast addresses.
>
> VLAN Filtering
>
> If the driver negotiates the VIRTION_NET_F_CTRL_VLAN feature, it
> can control a VLAN filter table in the device.
>
> #define VIRTIO_NET_CTRL_VLAN 2
> #define VIRTIO_NET_CTRL_VLAN_ADD 0
> #define VIRTIO_NET_CTRL_VLAN_DEL 1
>
> Both the VIRTIO_NET_CTRL_VLAN_ADD and VIRTIO_NET_CTRL_VLAN_DEL
> command take a 16-bit VLAN id as the command-specific-data.
>
> Gratuitous Packet Sending
>
> If the driver negotiates the VIRTIO_NET_F_GUEST_ANNOUNCE (depends
> on VIRTIO_NET_F_CTRL_VQ), it can ask the guest to send gratuitous
> packets; this is usually done after the guest has been physically
> migrated, and needs to announce its presence on the new network
> links. (As hypervisor does not have the knowledge of guest
> network configuration (eg. tagged vlan) it is simplest to prod
> the guest in this way).
>
> #define VIRTIO_NET_CTRL_ANNOUNCE 3
> #define VIRTIO_NET_CTRL_ANNOUNCE_ACK 0
>
> The Guest needs to check VIRTIO_NET_S_ANNOUNCE bit in status
> field when it notices the changes of device configuration. The
> command VIRTIO_NET_CTRL_ANNOUNCE_ACK is used to indicate that
> driver has recevied the notification and device would clear the
> VIRTIO_NET_S_ANNOUNCE bit in the status filed after it received
> this command.
>
> Processing this notification involves:
>
> 1. Sending the gratuitous packets or marking there are pending
> gratuitous packets to be sent and letting deferred routine to
> send them.
>
> 2. Sending VIRTIO_NET_CTRL_ANNOUNCE_ACK command through control
> vq.
>
> 3. .
>
> Appendix D: Block Device
>
> The virtio block device is a simple virtual block device (ie.
> disk). Read and write requests (and other exotic requests) are
> placed in the queue, and serviced (probably out of order) by the
> device except where noted.
>
> Configuration
>
> Subsystem Device ID 2
>
> Virtqueues 0:requestq.
>
> Feature bits
>
> VIRTIO_BLK_F_BARRIER (0) Host supports request barriers.
>
> VIRTIO_BLK_F_SIZE_MAX (1) Maximum size of any single segment is
> in “size_max”.
>
> VIRTIO_BLK_F_SEG_MAX (2) Maximum number of segments in a
> request is in “seg_max”.
>
> VIRTIO_BLK_F_GEOMETRY (4) Disk-style geometry specified in “
> geometry”.
>
> VIRTIO_BLK_F_RO (5) Device is read-only.
>
> VIRTIO_BLK_F_BLK_SIZE (6) Block size of disk is in “blk_size”.
>
> VIRTIO_BLK_F_SCSI (7) Device supports scsi packet commands.
>
> VIRTIO_BLK_F_FLUSH (9) Cache flush command support.
>
> Device configuration layout The capacity of the device
> (expressed in 512-byte sectors) is always present. The
> availability of the others all depend on various feature bits
> as indicated above.
>
> struct virtio_blk_config {
> u64 capacity;
> u32 size_max;
> u32 seg_max;
> struct virtio_blk_geometry {
> u16 cylinders;
> u8 heads;
> u8 sectors;
> } geometry;
> u32 blk_size;
> };
>
> Device Initialization
>
> 1. The device size should be read from the “capacity”
> configuration field. No requests should be submitted which goes
> beyond this limit.
>
> 2. If the VIRTIO_BLK_F_BLK_SIZE feature is negotiated, the
> blk_size field can be read to determine the optimal sector size
> for the driver to use. This does not effect the units used in
> the protocol (always 512 bytes), but awareness of the correct
> value can effect performance.
>
> 3. If the VIRTIO_BLK_F_RO feature is set by the device, any write
> requests will fail.
>
> Device Operation
>
> The driver queues requests to the virtqueue, and they are used by
> the device (not necessarily in order). Each request is of form:
>
> struct virtio_blk_req {
> u32 type;
> u32 ioprio;
> u64 sector;
> char data[][512];
> u8 status;
> };
>
> If the device has VIRTIO_BLK_F_SCSI feature, it can also support
> scsi packet command requests, each of these requests is of form:
>
> struct virtio_scsi_pc_req {
> u32 type;
> u32 ioprio;
> u64 sector;
> char cmd[];
> char data[][512];
> #define SCSI_SENSE_BUFFERSIZE 96
> u8 sense[SCSI_SENSE_BUFFERSIZE];
> u32 errors;
> u32 data_len;
> u32 sense_len;
> u32 residual;
> u8 status;
> };
>
> The type of the request is either a read (VIRTIO_BLK_T_IN), a write
> (VIRTIO_BLK_T_OUT), a scsi packet command (VIRTIO_BLK_T_SCSI_CMD or
> VIRTIO_BLK_T_SCSI_CMD_OUT[22]) or a flush (VIRTIO_BLK_T_FLUSH or
> VIRTIO_BLK_T_FLUSH_OUT[23]). If the device has VIRTIO_BLK_F_BARRIER
> feature the high bit (VIRTIO_BLK_T_BARRIER) indicates that this
> request acts as a barrier and that all preceeding requests must be
> complete before this one, and all following requests must not be
> started until this is complete. Note that a barrier does not flush
> caches in the underlying backend device in host, and thus does not
> serve as data consistency guarantee. Driver must use FLUSH request to
> flush the host cache.
>
> #define VIRTIO_BLK_T_IN 0
> #define VIRTIO_BLK_T_OUT 1
> #define VIRTIO_BLK_T_SCSI_CMD 2
> #define VIRTIO_BLK_T_SCSI_CMD_OUT 3
> #define VIRTIO_BLK_T_FLUSH 4
> #define VIRTIO_BLK_T_FLUSH_OUT 5
> #define VIRTIO_BLK_T_BARRIER 0x80000000
>
> The ioprio field is a hint about the relative priorities of
> requests to the device: higher numbers indicate more important
> requests.
>
> The sector number indicates the offset (multiplied by 512) where
> the read or write is to occur. This field is unused and set to 0
> for scsi packet commands and for flush commands.
>
> The cmd field is only present for scsi packet command requests,
> and indicates the command to perform. This field must reside in a
> single, separate read-only buffer; command length can be derived
> from the length of this buffer.
>
> Note that these first three (four for scsi packet commands)
> fields are always read-only: the data field is either read-only
> or write-only, depending on the request. The size of the read or
> write can be derived from the total size of the request buffers.
>
> The sense field is only present for scsi packet command requests,
> and indicates the buffer for scsi sense data.
>
> The data_len field is only present for scsi packet command
> requests, this field is deprecated, and should be ignored by the
> driver. Historically, devices copied data length there.
>
> The sense_len field is only present for scsi packet command
> requests and indicates the number of bytes actually written to
> the sense buffer.
>
> The residual field is only present for scsi packet command
> requests and indicates the residual size, calculated as data
> length - number of bytes actually transferred.
>
> The final status byte is written by the device: either
> VIRTIO_BLK_S_OK for success, VIRTIO_BLK_S_IOERR for host or guest
> error or VIRTIO_BLK_S_UNSUPP for a request unsupported by host:
>
> #define VIRTIO_BLK_S_OK 0
> #define VIRTIO_BLK_S_IOERR 1
> #define VIRTIO_BLK_S_UNSUPP 2
>
> Historically, devices assumed that the fields type, ioprio and
> sector reside in a single, separate read-only buffer; the fields
> errors, data_len, sense_len and residual reside in a single,
> separate write-only buffer; the sense field in a separate
> write-only buffer of size 96 bytes, by itself; the fields errors,
> data_len, sense_len and residual in a single write-only buffer;
> and the status field is a separate read-only buffer of size 1
> byte, by itself.
>
> Appendix E: Console Device
>
> The virtio console device is a simple device for data input and
> output. A device may have one or more ports. Each port has a pair
> of input and output virtqueues. Moreover, a device has a pair of
> control IO virtqueues. The control virtqueues are used to
> communicate information between the device and the driver about
> ports being opened and closed on either side of the connection,
> indication from the host about whether a particular port is a
> console port, adding new ports, port hot-plug/unplug, etc., and
> indication from the guest about whether a port or a device was
> successfully added, port open/close, etc.. For data IO, one or
> more empty buffers are placed in the receive queue for incoming
> data and outgoing characters are placed in the transmit queue.
>
> Configuration
>
> Subsystem Device ID 3
>
> Virtqueues 0:receiveq(port0). 1:transmitq(port0), 2:control
> receiveq[24], 3:control transmitq, 4:receiveq(port1), 5:transmitq(port1),
> ...
>
> Feature bits
>
> VIRTIO_CONSOLE_F_SIZE (0) Configuration cols and rows fields
> are valid.
>
> VIRTIO_CONSOLE_F_MULTIPORT(1) Device has support for multiple
> ports; configuration fields nr_ports and max_nr_ports are
> valid and control virtqueues will be used.
>
> Device configuration layout The size of the console is supplied
> in the configuration space if the VIRTIO_CONSOLE_F_SIZE feature
> is set. Furthermore, if the VIRTIO_CONSOLE_F_MULTIPORT feature
> is set, the maximum number of ports supported by the device can
> be fetched.
>
> struct virtio_console_config {
> u16 cols;
> u16 rows;
> u32 max_nr_ports;
> };
>
> Device Initialization
>
> 1. If the VIRTIO_CONSOLE_F_SIZE feature is negotiated, the driver
> can read the console dimensions from the configuration fields.
>
> 2. If the VIRTIO_CONSOLE_F_MULTIPORT feature is negotiated, the
> driver can spawn multiple ports, not all of which may be
> attached to a console. Some could be generic ports. In this
> case, the control virtqueues are enabled and according to the
> max_nr_ports configuration-space value, the appropriate number
> of virtqueues are created. A control message indicating the
> driver is ready is sent to the host. The host can then send
> control messages for adding new ports to the device. After
> creating and initializing each port, a
> VIRTIO_CONSOLE_PORT_READY control message is sent to the host
> for that port so the host can let us know of any additional
> configuration options set for that port.
>
> 3. The receiveq for each port is populated with one or more
> receive buffers.
>
> Device Operation
>
> 1. For output, a buffer containing the characters is placed in
> the port's transmitq.[25]
>
> 2. When a buffer is used in the receiveq (signalled by an
> interrupt), the contents is the input to the port associated
> with the virtqueue for which the notification was received.
>
> 3. If the driver negotiated the VIRTIO_CONSOLE_F_SIZE feature, a
> configuration change interrupt may occur. The updated size can
> be read from the configuration fields.
>
> 4. If the driver negotiated the VIRTIO_CONSOLE_F_MULTIPORT
> feature, active ports are announced by the host using the
> VIRTIO_CONSOLE_PORT_ADD control message. The same message is
> used for port hot-plug as well.
>
> 5. If the host specified a port `name', a sysfs attribute is
> created with the name filled in, so that udev rules can be
> written that can create a symlink from the port's name to the
> char device for port discovery by applications in the guest.
>
> 6. Changes to ports' state are effected by control messages.
> Appropriate action is taken on the port indicated in the
> control message. The layout of the structure of the control
> buffer and the events associated are:
>
> struct virtio_console_control {
> uint32_t id; /* Port number */
> uint16_t event; /* The kind of control event */
> uint16_t value; /* Extra information for the event */
> };
>
> /* Some events for the internal messages (control packets) */
> #define VIRTIO_CONSOLE_DEVICE_READY 0
> #define VIRTIO_CONSOLE_PORT_ADD 1
> #define VIRTIO_CONSOLE_PORT_REMOVE 2
> #define VIRTIO_CONSOLE_PORT_READY 3
> #define VIRTIO_CONSOLE_CONSOLE_PORT 4
> #define VIRTIO_CONSOLE_RESIZE 5
> #define VIRTIO_CONSOLE_PORT_OPEN 6
> #define VIRTIO_CONSOLE_PORT_NAME 7
>
> Appendix F: Entropy Device
>
> The virtio entropy device supplies high-quality randomness for
> guest use.
>
> Configuration
>
> Subsystem Device ID 4
>
> Virtqueues 0:requestq.
>
> Feature bits None currently defined
>
> Device configuration layout None currently defined.
>
> Device Initialization
>
> 1. The virtqueue is initialized
>
> Device Operation
>
> When the driver requires random bytes, it places the descriptor
> of one or more buffers in the queue. It will be completely filled
> by random data by the device.
>
> Appendix G: Memory Balloon Device
>
> The virtio memory balloon device is a primitive device for
> managing guest memory: the device asks for a certain amount of
> memory, and the guest supplies it (or withdraws it, if the device
> has more than it asks for). This allows the guest to adapt to
> changes in allowance of underlying physical memory. If the
> feature is negotiated, the device can also be used to communicate
> guest memory statistics to the host.
>
> Configuration
>
> Subsystem Device ID 5
>
> Virtqueues 0:inflateq. 1:deflateq. 2:statsq.[26]
>
> Feature bits
>
> VIRTIO_BALLOON_F_MUST_TELL_HOST (0) Host must be told before
> pages from the balloon are used.
>
> VIRTIO_BALLOON_F_STATS_VQ (1) A virtqueue for reporting guest
> memory statistics is present.
>
> Device configuration layout Both fields of this configuration
> are always available. Note that they are little endian, despite
> convention that device fields are guest endian:
>
> struct virtio_balloon_config {
> u32 num_pages;
> u32 actual;
> };
>
> Device Initialization
>
> 1. The inflate and deflate virtqueues are identified.
>
> 2. If the VIRTIO_BALLOON_F_STATS_VQ feature bit is negotiated:
>
> (a) Identify the stats virtqueue.
>
> (b) Add one empty buffer to the stats virtqueue and notify the
> host.
>
> Device operation begins immediately.
>
> Device Operation
>
> Memory Ballooning The device is driven by the receipt of a
> configuration change interrupt.
>
> 1. The “num_pages” configuration field is examined. If this is
> greater than the “actual” number of pages, memory must be given
> to the balloon. If it is less than the “actual” number of
> pages, memory may be taken back from the balloon for general
> use.
>
> 2. To supply memory to the balloon (aka. inflate):
>
> (a) The driver constructs an array of addresses of unused memory
> pages. These addresses are divided by 4096[27] and the descriptor
> describing the resulting 32-bit array is added to the inflateq.
>
> 3. To remove memory from the balloon (aka. deflate):
>
> (a) The driver constructs an array of addresses of memory pages
> it has previously given to the balloon, as described above.
> This descriptor is added to the deflateq.
>
> (b) If the VIRTIO_BALLOON_F_MUST_TELL_HOST feature is set, the
> guest may not use these requested pages until that descriptor
> in the deflateq has been used by the device.
>
> (c) Otherwise, the guest may begin to re-use pages previously
> given to the balloon before the device has acknowledged their
> withdrawl. [28]
>
> 4. In either case, once the device has completed the inflation or
> deflation, the “actual” field of the configuration should be
> updated to reflect the new number of pages in the balloon.[29]
>
> Memory Statistics
>
> The stats virtqueue is atypical because communication is driven
> by the device (not the driver). The channel becomes active at
> driver initialization time when the driver adds an empty buffer
> and notifies the device. A request for memory statistics proceeds
> as follows:
>
> 1. The device pushes the buffer onto the used ring and sends an
> interrupt.
>
> 2. The driver pops the used buffer and discards it.
>
> 3. The driver collects memory statistics and writes them into a
> new buffer.
>
> 4. The driver adds the buffer to the virtqueue and notifies the
> device.
>
> 5. The device pops the buffer (retaining it to initiate a
> subsequent request) and consumes the statistics.
>
> Memory Statistics Format Each statistic consists of a 16 bit
> tag and a 64 bit value. Both quantities are represented in the
> native endian of the guest. All statistics are optional and the
> driver may choose which ones to supply. To guarantee backwards
> compatibility, unsupported statistics should be omitted.
>
> struct virtio_balloon_stat {
> #define VIRTIO_BALLOON_S_SWAP_IN 0
> #define VIRTIO_BALLOON_S_SWAP_OUT 1
> #define VIRTIO_BALLOON_S_MAJFLT 2
> #define VIRTIO_BALLOON_S_MINFLT 3
> #define VIRTIO_BALLOON_S_MEMFREE 4
> #define VIRTIO_BALLOON_S_MEMTOT 5
> u16 tag;
> u64 val;
> } __attribute__((packed));
>
> Tags
>
> VIRTIO_BALLOON_S_SWAP_IN The amount of memory that has been
> swapped in (in bytes).
>
> VIRTIO_BALLOON_S_SWAP_OUT The amount of memory that has been
> swapped out to disk (in bytes).
>
> VIRTIO_BALLOON_S_MAJFLT The number of major page faults that
> have occurred.
>
> VIRTIO_BALLOON_S_MINFLT The number of minor page faults that
> have occurred.
>
> VIRTIO_BALLOON_S_MEMFREE The amount of memory not being used
> for any purpose (in bytes).
>
> VIRTIO_BALLOON_S_MEMTOT The total amount of memory available
> (in bytes).
>
> Appendix I: SCSI Host Device
>
> The virtio SCSI host device groups together one or more virtual
> logical units (such as disks), and allows communicating to them
> using the SCSI protocol. An instance of the device represents a
> SCSI host to which many targets and LUNs are attached.
>
> The virtio SCSI device services two kinds of requests:
>
> • command requests for a logical unit;
>
> • task management functions related to a logical unit, target or
> command.
>
> The device is also able to send out notifications about added and
> removed logical units. Together, these capabilities provide a
> SCSI transport protocol that uses virtqueues as the transfer
> medium. In the transport protocol, the virtio driver acts as the
> initiator, while the virtio SCSI host provides one or more
> targets that receive and process the requests.
>
> Configuration
>
> Subsystem Device ID 8
>
> Virtqueues 0:controlq; 1:eventq; 2..n:request queues.
>
> Feature bits
>
> VIRTIO_SCSI_F_INOUT (0) A single request can include both
> read-only and write-only data buffers.
>
> VIRTIO_SCSI_F_HOTPLUG (1) The host should enable
> hot-plug/hot-unplug of new LUNs and targets on the SCSI bus.
>
> Device configuration layout All fields of this configuration
> are always available. sense_size and cdb_size are writable by
> the guest.
>
> struct virtio_scsi_config {
> u32 num_queues;
> u32 seg_max;
> u32 max_sectors;
> u32 cmd_per_lun;
> u32 event_info_size;
> u32 sense_size;
> u32 cdb_size;
> u16 max_channel;
> u16 max_target;
> u32 max_lun;
> };
>
> num_queues is the total number of request virtqueues exposed by
> the device. The driver is free to use only one request queue,
> or it can use more to achieve better performance.
>
> seg_max is the maximum number of segments that can be in a
> command. A bidirectional command can include seg_max input
> segments and seg_max output segments.
>
> max_sectors is a hint to the guest about the maximum transfer
> size it should use.
>
> cmd_per_lun is a hint to the guest about the maximum number of
> linked commands it should send to one LUN. The actual value
> to be used is the minimum of cmd_per_lun and the virtqueue
> size.
>
> event_info_size is the maximum size that the device will fill
> for buffers that the driver places in the eventq. The driver
> should always put buffers at least of this size. It is
> written by the device depending on the set of negotated
> features.
>
> sense_size is the maximum size of the sense data that the
> device will write. The default value is written by the device
> and will always be 96, but the driver can modify it. It is
> restored to the default when the device is reset.
>
> cdb_size is the maximum size of the CDB that the driver will
> write. The default value is written by the device and will
> always be 32, but the driver can likewise modify it. It is
> restored to the default when the device is reset.
>
> max_channel, max_target and max_lun can be used by the driver
> as hints to constrain scanning the logical units on the
> host.h
>
> Device Initialization
>
> The initialization routine should first of all discover the
> device's virtqueues.
>
> If the driver uses the eventq, it should then place at least a
> buffer in the eventq.
>
> The driver can immediately issue requests (for example, INQUIRY
> or REPORT LUNS) or task management functions (for example, I_T
> RESET).
>
> Device Operation: request queues
>
> The driver queues requests to an arbitrary request queue, and
> they are used by the device on that same queue. It is the
> responsibility of the driver to ensure strict request ordering
> for commands placed on different queues, because they will be
> consumed with no order constraints.
>
> Requests have the following format:
>
> struct virtio_scsi_req_cmd {
> // Read-only
> u8 lun[8];
> u64 id;
> u8 task_attr;
> u8 prio;
> u8 crn;
> char cdb[cdb_size];
> char dataout[];
> // Write-only part
> u32 sense_len;
> u32 residual;
> u16 status_qualifier;
> u8 status;
> u8 response;
> u8 sense[sense_size];
> char datain[];
> };
>
>
> /* command-specific response values */
> #define VIRTIO_SCSI_S_OK 0
> #define VIRTIO_SCSI_S_OVERRUN 1
> #define VIRTIO_SCSI_S_ABORTED 2
> #define VIRTIO_SCSI_S_BAD_TARGET 3
> #define VIRTIO_SCSI_S_RESET 4
> #define VIRTIO_SCSI_S_BUSY 5
> #define VIRTIO_SCSI_S_TRANSPORT_FAILURE 6
> #define VIRTIO_SCSI_S_TARGET_FAILURE 7
> #define VIRTIO_SCSI_S_NEXUS_FAILURE 8
> #define VIRTIO_SCSI_S_FAILURE 9
>
> /* task_attr */
> #define VIRTIO_SCSI_S_SIMPLE 0
> #define VIRTIO_SCSI_S_ORDERED 1
> #define VIRTIO_SCSI_S_HEAD 2
> #define VIRTIO_SCSI_S_ACA 3
>
> The lun field addresses a target and logical unit in the
> virtio-scsi device's SCSI domain. The only supported format for
> the LUN field is: first byte set to 1, second byte set to target,
> third and fourth byte representing a single level LUN structure,
> followed by four zero bytes. With this representation, a
> virtio-scsi device can serve up to 256 targets and 16384 LUNs per
> target.
>
> The id field is the command identifier (“tag”).
>
> task_attr, prio and crn should be left to zero. task_attr defines
> the task attribute as in the table above, but all task attributes
> may be mapped to SIMPLE by the device; crn may also be provided
> by clients, but is generally expected to be 0. The maximum CRN
> value defined by the protocol is 255, since CRN is stored in an
> 8-bit integer.
>
> All of these fields are defined in SAM. They are always
> read-only, as are the cdb and dataout field. The cdb_size is
> taken from the configuration space.
>
> sense and subsequent fields are always write-only. The sense_len
> field indicates the number of bytes actually written to the sense
> buffer. The residual field indicates the residual size,
> calculated as “data_length - number_of_transferred_bytes”, for
> read or write operations. For bidirectional commands, the
> number_of_transferred_bytes includes both read and written bytes.
> A residual field that is less than the size of datain means that
> the dataout field was processed entirely. A residual field that
> exceeds the size of datain means that the dataout field was
> processed partially and the datain field was not processed at
> all.
>
> The status byte is written by the device to be the status code as
> defined in SAM.
>
> The response byte is written by the device to be one of the
> following:
>
> VIRTIO_SCSI_S_OK when the request was completed and the status
> byte is filled with a SCSI status code (not necessarily
> "GOOD").
>
> VIRTIO_SCSI_S_OVERRUN if the content of the CDB requires
> transferring more data than is available in the data buffers.
>
> VIRTIO_SCSI_S_ABORTED if the request was cancelled due to an
> ABORT TASK or ABORT TASK SET task management function.
>
> VIRTIO_SCSI_S_BAD_TARGET if the request was never processed
> because the target indicated by the lun field does not exist.
>
> VIRTIO_SCSI_S_RESET if the request was cancelled due to a bus
> or device reset (including a task management function).
>
> VIRTIO_SCSI_S_TRANSPORT_FAILURE if the request failed due to a
> problem in the connection between the host and the target
> (severed link).
>
> VIRTIO_SCSI_S_TARGET_FAILURE if the target is suffering a
> failure and the guest should not retry on other paths.
>
> VIRTIO_SCSI_S_NEXUS_FAILURE if the nexus is suffering a failure
> but retrying on other paths might yield a different result.
>
> VIRTIO_SCSI_S_BUSY if the request failed but retrying on the
> same path should work.
>
> VIRTIO_SCSI_S_FAILURE for other host or guest error. In
> particular, if neither dataout nor datain is empty, and the
> VIRTIO_SCSI_F_INOUT feature has not been negotiated, the
> request will be immediately returned with a response equal to
> VIRTIO_SCSI_S_FAILURE.
>
> Device Operation: controlq
>
> The controlq is used for other SCSI transport operations.
> Requests have the following format:
>
> struct virtio_scsi_ctrl {
> u32 type;
> ...
> u8 response;
> };
>
> /* response values valid for all commands */
> #define VIRTIO_SCSI_S_OK 0
> #define VIRTIO_SCSI_S_BAD_TARGET 3
> #define VIRTIO_SCSI_S_BUSY 5
> #define VIRTIO_SCSI_S_TRANSPORT_FAILURE 6
> #define VIRTIO_SCSI_S_TARGET_FAILURE 7
> #define VIRTIO_SCSI_S_NEXUS_FAILURE 8
> #define VIRTIO_SCSI_S_FAILURE 9
> #define VIRTIO_SCSI_S_INCORRECT_LUN 12
>
> The type identifies the remaining fields.
>
> The following commands are defined:
>
> Task management function
> #define VIRTIO_SCSI_T_TMF 0
>
> #define VIRTIO_SCSI_T_TMF_ABORT_TASK 0
> #define VIRTIO_SCSI_T_TMF_ABORT_TASK_SET 1
> #define VIRTIO_SCSI_T_TMF_CLEAR_ACA 2
> #define VIRTIO_SCSI_T_TMF_CLEAR_TASK_SET 3
> #define VIRTIO_SCSI_T_TMF_I_T_NEXUS_RESET 4
> #define VIRTIO_SCSI_T_TMF_LOGICAL_UNIT_RESET 5
> #define VIRTIO_SCSI_T_TMF_QUERY_TASK 6
> #define VIRTIO_SCSI_T_TMF_QUERY_TASK_SET 7
>
> struct virtio_scsi_ctrl_tmf
> {
> // Read-only part
> u32 type;
> u32 subtype;
> u8 lun[8];
> u64 id;
> // Write-only part
> u8 response;
> }
>
> /* command-specific response values */
> #define VIRTIO_SCSI_S_FUNCTION_COMPLETE 0
> #define VIRTIO_SCSI_S_FUNCTION_SUCCEEDED 10
> #define VIRTIO_SCSI_S_FUNCTION_REJECTED 11
>
> The type is VIRTIO_SCSI_T_TMF; the subtype field defines. All
> fields except response are filled by the driver. The subtype
> field must always be specified and identifies the requested
> task management function.
>
> Other fields may be irrelevant for the requested TMF; if so,
> they are ignored but they should still be present. The lun
> field is in the same format specified for request queues; the
> single level LUN is ignored when the task management function
> addresses a whole I_T nexus. When relevant, the value of the id
> field is matched against the id values passed on the requestq.
>
> The outcome of the task management function is written by the
> device in the response field. The command-specific response
> values map 1-to-1 with those defined in SAM.
>
> Asynchronous notification query
>
> #define VIRTIO_SCSI_T_AN_QUERY 1
>
> struct virtio_scsi_ctrl_an {
> // Read-only part
> u32 type;
> u8 lun[8];
> u32 event_requested;
> // Write-only part
> u32 event_actual;
> u8 response;
> }
>
> #define VIRTIO_SCSI_EVT_ASYNC_OPERATIONAL_CHANGE 2
> #define VIRTIO_SCSI_EVT_ASYNC_POWER_MGMT 4
> #define VIRTIO_SCSI_EVT_ASYNC_EXTERNAL_REQUEST 8
> #define VIRTIO_SCSI_EVT_ASYNC_MEDIA_CHANGE 16
> #define VIRTIO_SCSI_EVT_ASYNC_MULTI_HOST 32
> #define VIRTIO_SCSI_EVT_ASYNC_DEVICE_BUSY 64
>
> By sending this command, the driver asks the device which
> events the given LUN can report, as described in paragraphs 6.6
> and A.6 of the SCSI MMC specification. The driver writes the
> events it is interested in into the event_requested; the device
> responds by writing the events that it supports into
> event_actual.
>
> The type is VIRTIO_SCSI_T_AN_QUERY. The lun and event_requested
> fields are written by the driver. The event_actual and response
> fields are written by the device.
>
> No command-specific values are defined for the response byte.
>
> Asynchronous notification subscription
> #define VIRTIO_SCSI_T_AN_SUBSCRIBE 2
>
> struct virtio_scsi_ctrl_an {
> // Read-only part
> u32 type;
> u8 lun[8];
> u32 event_requested;
> // Write-only part
> u32 event_actual;
> u8 response;
> }
>
> By sending this command, the driver asks the specified LUN to
> report events for its physical interface, again as described in
> the SCSI MMC specification. The driver writes the events it is
> interested in into the event_requested; the device responds by
> writing the events that it supports into event_actual.
>
> Event types are the same as for the asynchronous notification
> query message.
>
> The type is VIRTIO_SCSI_T_AN_SUBSCRIBE. The lun and
> event_requested fields are written by the driver. The
> event_actual and response fields are written by the device.
>
> No command-specific values are defined for the response byte.
>
> Device Operation: eventq
>
> The eventq is used by the device to report information on logical
> units that are attached to it. The driver should always leave a
> few buffers ready in the eventq. In general, the device will not
> queue events to cope with an empty eventq, and will end up
> dropping events if it finds no buffer ready. However, when
> reporting events for many LUNs (e.g. when a whole target
> disappears), the device can throttle events to avoid dropping
> them. For this reason, placing 10-15 buffers on the event queue
> should be enough.
>
> Buffers are placed in the eventq and filled by the device when
> interesting events occur. The buffers should be strictly
> write-only (device-filled) and the size of the buffers should be
> at least the value given in the device's configuration
> information.
>
> Buffers returned by the device on the eventq will be referred to
> as "events" in the rest of this section. Events have the
> following format:
>
> #define VIRTIO_SCSI_T_EVENTS_MISSED 0x80000000
>
> struct virtio_scsi_event {
> // Write-only part
> u32 event;
> ...
> }
>
> If bit 31 is set in the event field, the device failed to report
> an event due to missing buffers. In this case, the driver should
> poll the logical units for unit attention conditions, and/or do
> whatever form of bus scan is appropriate for the guest operating
> system.
>
> Other data that the device writes to the buffer depends on the
> contents of the event field. The following events are defined:
>
> No event
> #define VIRTIO_SCSI_T_NO_EVENT 0
>
> This event is fired in the following cases:
>
> • When the device detects in the eventq a buffer that is
> shorter than what is indicated in the configuration field, it
> might use it immediately and put this dummy value in the
> event field. A well-written driver will never observe this
> situation.
>
> • When events are dropped, the device may signal this event as
> soon as the drivers makes a buffer available, in order to
> request action from the driver. In this case, of course, this
> event will be reported with the VIRTIO_SCSI_T_EVENTS_MISSED
> flag.
>
> Transport reset
> #define VIRTIO_SCSI_T_TRANSPORT_RESET 1
>
> struct virtio_scsi_event_reset {
> // Write-only part
> u32 event;
> u8 lun[8];
> u32 reason;
> }
>
> #define VIRTIO_SCSI_EVT_RESET_HARD 0
> #define VIRTIO_SCSI_EVT_RESET_RESCAN 1
> #define VIRTIO_SCSI_EVT_RESET_REMOVED 2
>
> By sending this event, the device signals that a logical unit
> on a target has been reset, including the case of a new device
> appearing or disappearing on the bus.The device fills in all
> fields. The event field is set to
> VIRTIO_SCSI_T_TRANSPORT_RESET. The lun field addresses a
> logical unit in the SCSI host.
>
> The reason value is one of the three #define values appearing
> above:
>
> • VIRTIO_SCSI_EVT_RESET_REMOVED (“LUN/target removed”) is used
> if the target or logical unit is no longer able to receive
> commands.
>
> • VIRTIO_SCSI_EVT_RESET_HARD (“LUN hard reset”) is used if the
> logical unit has been reset, but is still present.
>
> • VIRTIO_SCSI_EVT_RESET_RESCAN (“rescan LUN/target”) is used if
> a target or logical unit has just appeared on the device.
>
> The “removed” and “rescan” events, when sent for LUN 0, may
> apply to the entire target. After receiving them the driver
> should ask the initiator to rescan the target, in order to
> detect the case when an entire target has appeared or
> disappeared. These two events will never be reported unless the
> VIRTIO_SCSI_F_HOTPLUG feature was negotiated between the host
> and the guest.
>
> Events will also be reported via sense codes (this obviously
> does not apply to newly appeared buses or targets, since the
> application has never discovered them):
>
> • “LUN/target removed” maps to sense key ILLEGAL REQUEST, asc
> 0x25, ascq 0x00 (LOGICAL UNIT NOT SUPPORTED)
>
> • “LUN hard reset” maps to sense key UNIT ATTENTION, asc 0x29
> (POWER ON, RESET OR BUS DEVICE RESET OCCURRED)
>
> • “rescan LUN/target” maps to sense key UNIT ATTENTION, asc
> 0x3f, ascq 0x0e (REPORTED LUNS DATA HAS CHANGED)
>
> The preferred way to detect transport reset is always to use
> events, because sense codes are only seen by the driver when it
> sends a SCSI command to the logical unit or target. However, in
> case events are dropped, the initiator will still be able to
> synchronize with the actual state of the controller if the
> driver asks the initiator to rescan of the SCSI bus. During the
> rescan, the initiator will be able to observe the above sense
> codes, and it will process them as if it the driver had
> received the equivalent event.
>
> Asynchronous notification
> #define VIRTIO_SCSI_T_ASYNC_NOTIFY 2
>
> struct virtio_scsi_event_an {
> // Write-only part
> u32 event;
> u8 lun[8];
> u32 reason;
> }
>
> By sending this event, the device signals that an asynchronous
> event was fired from a physical interface.
>
> All fields are written by the device. The event field is set to
> VIRTIO_SCSI_T_ASYNC_NOTIFY. The lun field addresses a logical
> unit in the SCSI host. The reason field is a subset of the
> events that the driver has subscribed to via the "Asynchronous
> notification subscription" command.
>
> When dropped events are reported, the driver should poll for
> asynchronous events manually using SCSI commands.
>
> Appendix X: virtio-mmio
>
> Virtual environments without PCI support (a common situation in
> embedded devices models) might use simple memory mapped device (“
> virtio-mmio”) instead of the PCI device.
>
> The memory mapped virtio device behaviour is based on the PCI
> device specification. Therefore most of operations like device
> initialization, queues configuration and buffer transfers are
> nearly identical. Existing differences are described in the
> following sections.
>
> Device Initialization
>
> Instead of using the PCI IO space for virtio header, the “
> virtio-mmio” device provides a set of memory mapped control
> registers, all 32 bits wide, followed by device-specific
> configuration space. The following list presents their layout:
>
> • Offset from the device base address | Direction | Name
> Description
>
> • 0x000 | R | MagicValue
> “virt” string.
>
> • 0x004 | R | Version
> Device version number. Currently must be 1.
>
> • 0x008 | R | DeviceID
> Virtio Subsystem Device ID (ie. 1 for network card).
>
> • 0x00c | R | VendorID
> Virtio Subsystem Vendor ID.
>
> • 0x010 | R | HostFeatures
> Flags representing features the device supports.
> Reading from this register returns 32 consecutive flag bits,
> first bit depending on the last value written to
> HostFeaturesSel register. Access to this register returns bits HostFeaturesSel*32
>
> to (HostFeaturesSel*32)+31, eg. feature bits 0 to 31 if
> HostFeaturesSel is set to 0 and features bits 32 to 63 if
> HostFeaturesSel is set to 1. Also see [sub:Feature-Bits]
>
> • 0x014 | W | HostFeaturesSel
> Device (Host) features word selection.
> Writing to this register selects a set of 32 device feature bits
> accessible by reading from HostFeatures register. Device driver
> must write a value to the HostFeaturesSel register before
> reading from the HostFeatures register.
>
> • 0x020 | W | GuestFeatures
> Flags representing device features understood and activated by
> the driver.
> Writing to this register sets 32 consecutive flag bits, first
> bit depending on the last value written to GuestFeaturesSel
> register. Access to this register sets bits GuestFeaturesSel*32
> to (GuestFeaturesSel*32)+31, eg. feature bits 0 to 31 if
> GuestFeaturesSel is set to 0 and features bits 32 to 63 if
> GuestFeaturesSel is set to 1. Also see [sub:Feature-Bits]
>
> • 0x024 | W | GuestFeaturesSel
> Activated (Guest) features word selection.
> Writing to this register selects a set of 32 activated feature
> bits accessible by writing to the GuestFeatures register.
> Device driver must write a value to the GuestFeaturesSel
> register before writing to the GuestFeatures register.
>
> • 0x028 | W | GuestPageSize
> Guest page size.
> Device driver must write the guest page size in bytes to the
> register during initialization, before any queues are used.
> This value must be a power of 2 and is used by the Host to
> calculate Guest address of the first queue page (see QueuePFN).
>
> • 0x030 | W | QueueSel
> Virtual queue index (first queue is 0).
> Writing to this register selects the virtual queue that the
> following operations on QueueNum, QueueAlign and QueuePFN apply
> to.
>
> • 0x034 | R | QueueNumMax
> Maximum virtual queue size.
> Reading from the register returns the maximum size of the queue
> the Host is ready to process or zero (0x0) if the queue is not
> available. This applies to the queue selected by writing to
> QueueSel and is allowed only when QueuePFN is set to zero
> (0x0), so when the queue is not actively used.
>
> • 0x038 | W | QueueNum
> Virtual queue size.
> Queue size is a number of elements in the queue, therefore size
> of the descriptor table and both available and used rings.
> Writing to this register notifies the Host what size of the
> queue the Guest will use. This applies to the queue selected by
> writing to QueueSel.
>
> • 0x03c | W | QueueAlign
> Used Ring alignment in the virtual queue.
> Writing to this register notifies the Host about alignment
> boundary of the Used Ring in bytes. This value must be a power
> of 2 and applies to the queue selected by writing to QueueSel.
>
> • 0x040 | RW | QueuePFN
> Guest physical page number of the virtual queue.
> Writing to this register notifies the host about location of the
> virtual queue in the Guest's physical address space. This value
> is the index number of a page starting with the queue
> Descriptor Table. Value zero (0x0) means physical address zero
> (0x00000000) and is illegal. When the Guest stops using the
> queue it must write zero (0x0) to this register.
> Reading from this register returns the currently used page
> number of the queue, therefore a value other than zero (0x0)
> means that the queue is in use.
> Both read and write accesses apply to the queue selected by
> writing to QueueSel.
>
> • 0x050 | W | QueueNotify
> Queue notifier.
> Writing a queue index to this register notifies the Host that
> there are new buffers to process in the queue.
>
> • 0x60 | R | InterruptStatus
> Interrupt status.
> Reading from this register returns a bit mask of interrupts
> asserted by the device. An interrupt is asserted if the
> corresponding bit is set, ie. equals one (1).
>
> – Bit 0 | Used Ring Update
> This interrupt is asserted when the Host has updated the Used
> Ring in at least one of the active virtual queues.
>
> – Bit 1 | Configuration change
> This interrupt is asserted when configuration of the device has
> changed.
>
> • 0x064 | W | InterruptACK
> Interrupt acknowledge.
> Writing to this register notifies the Host that the Guest
> finished handling interrupts. Set bits in the value clear the
> corresponding bits of the InterruptStatus register.
>
> • 0x070 | RW | Status
> Device status.
> Reading from this register returns the current device status
> flags.
> Writing non-zero values to this register sets the status flags,
> indicating the Guest progress. Writing zero (0x0) to this
> register triggers a device reset.
> Also see [sub:Device-Initialization-Sequence]
>
> • 0x100+ | RW | Config
> Device-specific configuration space starts at an offset 0x100
> and is accessed with byte alignment. Its meaning and size
> depends on the device and the driver.
>
> Virtual queue size is a number of elements in the queue,
> therefore size of the descriptor table and both available and
> used rings.
>
> The endianness of the registers follows the native endianness of
> the Guest. Writing to registers described as “R” and reading from
> registers described as “W” is not permitted and can cause
> undefined behavior.
>
> The device initialization is performed as described in 2.2.1 Device
> Initialization Sequence with one exception: the Guest must notify the
> Host about its page size, writing the size in bytes to GuestPageSize
> register before the initialization is finished.
>
> The memory mapped virtio devices generate single interrupt only,
> therefore no special configuration is required.
>
> Virtqueue Configuration
>
> The virtual queue configuration is performed in a similar way to
> the one described in 2.3 Virtqueue Configuration with a few
> additional operations:
>
> 1. Select the queue writing its index (first queue is 0) to the
> QueueSel register.
>
> 2. Check if the queue is not already in use: read QueuePFN
> register, returned value should be zero (0x0).
>
> 3. Read maximum queue size (number of elements) from the
> QueueNumMax register. If the returned value is zero (0x0) the
> queue is not available.
>
> 4. Allocate and zero the queue pages in contiguous virtual
> memory, aligning the Used Ring to an optimal boundary (usually
> page size). Size of the allocated queue may be smaller than or
> equal to the maximum size returned by the Host.
>
> 5. Notify the Host about the queue size by writing the size to
> QueueNum register.
>
> 6. Notify the Host about the used alignment by writing its value
> in bytes to QueueAlign register.
>
> 7. Write the physical number of the first page of the queue to
> the QueuePFN register.
>
> The queue and the device are ready to begin normal operations
> now.
>
> Device Operation
>
> The memory mapped virtio device behaves in the same way as
> described in 2.4 Device Operation, with the following
> exceptions:
>
> 1. The device is notified about new buffers available in a queue
> by writing the queue index to register QueueNum instead of the
> virtio header in PCI I/O space (2.4.1.4 Notifying The Device).
>
> 2. The memory mapped virtio device is using single, dedicated
> interrupt signal, which is raised when at least one of the
> interrupts described in the InterruptStatus register
> description is asserted. After receiving an interrupt, the
> driver must read the InterruptStatus register to check what
> caused the interrupt (see the register description). After the
> interrupt is handled, the driver must acknowledge it by writing
> a bit mask corresponding to the serviced interrupt to the
> InterruptACK register.
>
>
> FOOTNOTES:
> [1] This lack of page-sharing implies that the implementation of the
> device (e.g. the hypervisor or host) needs full access to the
> guest memory. Communication with untrusted parties (i.e.
> inter-guest communication) requires copying.
>
> [2] The Linux implementation further separates the PCI virtio code
> from the specific virtio drivers: these drivers are shared with
> the non-PCI implementations (currently lguest and S/390).
>
> [3] The actual value within this range is ignored
>
> [4] Historically, drivers have used the device before steps 5 and 6.
> This is only allowed if the driver does not use any features
> which would alter this early use of the device.
>
> [5] ie. once you enable MSI-X on the device, the other fields move.
> If you turn it off again, they move back!
>
> [6] The 4096 is based on the x86 page size, but it's also large
> enough to ensure that the separate parts of the virtqueue are on
> separate cache lines.
>
> [7] These fields are kept here because this is the only part of the
> virtqueue written by the device
>
> [8] The Linux drivers do this only for read-only buffers: for
> write-only buffers, it is assumed that the driver is merely
> trying to keep the receive buffer ring full, and no notification
> of this expected condition is necessary.
>
> [9] https://lists.linux-foundation.org/mailman/listinfo/virtualization
>
> [10] The current qemu device implementations mistakenly insist that
> the first descriptor cover the header in these cases exactly, so
> a cautious driver should arrange it so.
>
> [11] Even if it does mean documenting design or implementation
> mistakes!
>
> [12] Only if VIRTIO_NET_F_CTRL_VQ set
>
> [13] It was supposed to indicate segmentation offload support, but
> upon further investigation it became clear that multiple bits
> were required.
>
> [14] ie. VIRTIO_NET_F_HOST_TSO* and VIRTIO_NET_F_HOST_UFO are
> dependent on VIRTIO_NET_F_CSUM; a dvice which offers the offload
> features must offer the checksum feature, and a driver which
> accepts the offload features must accept the checksum feature.
> Similar logic applies to the VIRTIO_NET_F_GUEST_TSO4 features
> depending on VIRTIO_NET_F_GUEST_CSUM.
>
> [15] This is a common restriction in real, older network cards.
>
> [16] For example, a network packet transported between two guests on
> the same system may not require checksumming at all, nor segmentation,
> if both guests are amenable.
>
> [17] For example, consider a partially checksummed TCP (IPv4) packet.
> It will have a 14 byte ethernet header and 20 byte IP header
> followed by the TCP header (with the TCP checksum field 16 bytes
> into that header). csum_start will be 14+20 = 34 (the TCP
> checksum includes the header), and csum_offset will be 16. The
> value in the TCP checksum field should be initialized to the sum
> of the TCP pseudo header, so that replacing it by the ones'
> complement checksum of the TCP header and body will give the
> correct result.
>
> [18] Due to various bugs in implementations, this field is not useful
> as a guarantee of the transport header size.
>
> [19] This case is not handled by some older hardware, so is called out
> specifically in the protocol.
>
> [20] Note that the header will be two bytes longer for the
> VIRTIO_NET_F_MRG_RXBUF case.
>
> [20a] Obviously each one can be split across multiple descriptor
> elements.
>
> [21] Since there are no guarentees, it can use a hash filter or
> silently switch to allmulti or promiscuous mode if it is given too
> many addresses.
>
> [22] The SCSI_CMD and SCSI_CMD_OUT types are equivalent, the device
> does not distinguish between them.
>
> [23] The FLUSH and FLUSH_OUT types are equivalent, the device does not
> distinguish between them
>
> [24] Ports 2 onwards only if VIRTIO_CONSOLE_F_MULTIPORT is set.
>
> [25] Because this is high importance and low bandwidth, the current
> Linux implementation polls for the buffer to be used, rather than
> waiting for an interrupt, simplifying the implementation
> significantly. However, for generic serial ports with the
> O_NONBLOCK flag set, the polling limitation is relaxed and the
> consumed buffers are freed upon the next write or poll call or
> when a port is closed or hot-unplugged.
>
> [26] Only if VIRTIO_BALLON_F_STATS_VQ set.
>
> [27] This is historical, and independent of the guest page size
>
> [28] In this case, deflation advice is merely a courtesy
>
> [29] As updates to configuration space are not atomic, this field
> isn't particularly reliable, but can be used to diagnose buggy guests.
>
>
> ---------------------------------------------------------------------
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> generates this mail. Follow this link to all your TCs in OASIS at:
> https://www.oasis-open.org/apps/org/workgroup/portal/my_workgroups.php

On (Wed) 31 Jul 2013 [12:44:02], Michael S. Tsirkin wrote: > On Wed, Jul 31, 2013 at 01:33:39PM +0930, Rusty Russell wrote: > > Rusty Russell <rusty@au1.ibm.com> writes: > > > ----------------- > > > Action Item Subject: Create text version of virtio 0.9.5 document > > > > OK, I've attached this below. This is exactly the spec as 0.9.5 > > reformatted into text so we can work it. > > I copied virtio-comment which is an open-list. > > Hope that's OK with everyone. > People aren't auto-subscribed there but if everyone is > willing to subscribe to virtio-comment, we could > reserve the virtio@ members-only list to administrative > issues only? I'm fine with this. Amit

On (Wed) 31 Jul 2013 [12:44:02], Michael S. Tsirkin wrote:
> On Wed, Jul 31, 2013 at 01:33:39PM +0930, Rusty Russell wrote:
> > Rusty Russell <rusty@au1.ibm.com> writes:
> > > -----------------
> > > Action Item Subject: Create text version of virtio 0.9.5 document
> >
> > OK, I've attached this below. This is exactly the spec as 0.9.5
> > reformatted into text so we can work it.
>
> I copied virtio-comment which is an open-list.
>
> Hope that's OK with everyone.
> People aren't auto-subscribed there but if everyone is
> willing to subscribe to virtio-comment, we could
> reserve the virtio@ members-only list to administrative
> issues only?

I'm fine with this.

Amit

"Michael S. Tsirkin" <mst@redhat.com> writes:
> On Wed, Jul 31, 2013 at 01:33:39PM +0930, Rusty Russell wrote:
>> Rusty Russell <rusty@au1.ibm.com> writes:
>> > -----------------
>> > Action Item Subject: Create text version of virtio 0.9.5 document
>>
>> OK, I've attached this below. This is exactly the spec as 0.9.5
>> reformatted into text so we can work it.
>
> I copied virtio-comment which is an open-list.
>
> Hope that's OK with everyone.
> People aren't auto-subscribed there but if everyone is
> willing to subscribe to virtio-comment, we could
> reserve the virtio@ members-only list to administrative
> issues only?

Just to be clear, only TC members can post, but everyone can read the
list.

But I'm happy to use virtio-comment, too. In fact, I need to get some
of my s390 colleages to comment on the endian debate...

Cheers,
Rusty.

"Michael S. Tsirkin" <mst@redhat.com> writes: > On Wed, Jul 31, 2013 at 01:33:39PM +0930, Rusty Russell wrote: >> Rusty Russell <rusty@au1.ibm.com> writes: >> > ----------------- >> > Action Item Subject: Create text version of virtio 0.9.5 document >> >> OK, I've attached this below. This is exactly the spec as 0.9.5 >> reformatted into text so we can work it. > > I copied virtio-comment which is an open-list. > > Hope that's OK with everyone. > People aren't auto-subscribed there but if everyone is > willing to subscribe to virtio-comment, we could > reserve the virtio@ members-only list to administrative > issues only? Just to be clear, only TC members can post, but everyone can read the list. But I'm happy to use virtio-comment, too. In fact, I need to get some of my s390 colleages to comment on the endian debate... Cheers, Rusty.

Rusty Russell <rusty@au1.ibm.com> writes: > Rusty Russell <rusty@au1.ibm.com> writes: >> ----------------- >> Action Item Subject: Create text version of virtio 0.9.5 document > > OK, I've attached this below. This is exactly the spec as 0.9.5 > reformatted into text so we can work it. > > I'm still waiting for virtio to be added to the OASIS issue tracking > system, where I will be prompting you all to open an issue for every > change we want to consider. > > The obvious issues I want to open are: > > o Major rework to make PCI an appendix, and core bus-independent. We should revisit the use of PCI vendor/device IDs for virtio-pci. Currently we use the Red Hat/Qumranet vendor ID. This has led to a situation where only Red Hat can produce a signed set of drivers for Windows (since they own the vendor ID). These drivers are released under proprietary and discriminatory terms. As it stands today, it is impossible to create a signed driver for Windows that is spec compliant unless you are Red Hat. Can we work with Oasis to get a vendor neutral PCI ID created to avoid this problem in the future? Regards, Anthony Liguori > o Update spec with changes/fixes since 0.9.5 > - This is easy where contributors are already members, > trickier for others. > > In addition here's a brain dump: > > o Endian for config space > - LE everywhere? > o Endian for ring > - LE as well? > o Allow arbitrary descriptor layouts / message framing. > o Method to stop activity on a queue? > o Size descriptor table independent of ringsize? > o Remove VIRTIO_F_NOTIFY_ON_EMPTY? > o Remove limit on # indirect descriptors > - Some other limit? > o Simplify indirect desc > - No return to top level on end of desc array > o Allow chained indirect desc > - indirect bit use to chain? > > Net: > o Remove VIRTIO_NET_F_GSO? > > Block: > o Remove VIRTIO_BLK_F_SCSI? > o Revisit flush/barrier semantics? > > PCI: > o New capability layout > o Allowing non-zero revision numbers? > o Remove 'align' and use explicit addresses for used/avail. > > Balloon: > o Fix endianness > o Remove VIRTIO_BALLOON_F_MUST_TELL_HOST? > o Remove outgoing page queue? > > Cheers, > Rusty. > > ==== > This document describes the specifications of the “virtio” family > of PCI devices. These are devices > are found in virtual environments, > yet by design they are not all that different from physical PCI > devices, and this document treats them as such. This allows the > guest to use standard PCI drivers and discovery mechanisms. > > The purpose of virtio and this specification is that virtual > environments and guests should have a straightforward, efficient, > standard and extensible mechanism for virtual devices, rather > than boutique per-environment or per-OS mechanisms. > > Straightforward: Virtio PCI devices use normal PCI mechanisms > of interrupts and DMA which should be familiar to any device > driver author. There is no exotic page-flipping or COW > mechanism: it's just a PCI device.[1] > > Efficient: Virtio PCI devices consist of rings of descriptors > for input and output, which are neatly separated to avoid cache > effects from both guest and device writing to the same cache > lines. > > Standard: Virtio PCI makes no assumptions about the environment > in which it operates, beyond supporting PCI. In fact the virtio > devices specified in the appendices do not require PCI at all: > they have been implemented on non-PCI buses.[2] > > Extensible: Virtio PCI devices contain feature bits which are > acknowledged by the guest operating system during device setup. > This allows forwards and backwards compatibility: the device > offers all the features it knows about, and the driver > acknowledges those it understands and wishes to use. > > 1.1 Virtqueues > > The mechanism for bulk data transport on virtio PCI devices is > pretentiously called a virtqueue. Each device can have zero or > more virtqueues: for example, the network device has one for > transmit and one for receive. > > Each virtqueue occupies two or more physically-contiguous pages > (defined, for the purposes of this specification, as 4096 bytes), > and consists of three parts: > > > +-------------------+-----------------------------------+-----------+ > Descriptor Table Available Ring (padding) Used Ring > +-------------------+-----------------------------------+-----------+ > > > When the driver wants to send a buffer to the device, it fills in > a slot in the descriptor table (or chains several together), and > writes the descriptor index into the available ring. It then > notifies the device. When the device has finished a buffer, it > writes the descriptor into the used ring, and sends an interrupt. > > Specification > > 2.1 PCI Discovery > > Any PCI device with Vendor ID 0x1AF4, and Device ID 0x1000 through > 0x103F inclusive is a virtio device[3]. The device must also have a > Revision ID of 0 to match this specification. > > The Subsystem Device ID indicates which virtio device is > supported by the device. The Subsystem Vendor ID should reflect > the PCI Vendor ID of the environment (it's currently only used > for informational purposes by the guest). > > > +----------------------+--------------------+---------------+ > Subsystem Device ID Virtio Device Specification > +----------------------+--------------------+---------------+ > +----------------------+--------------------+---------------+ > 1 network card Appendix C > +----------------------+--------------------+---------------+ > 2 block device Appendix D > +----------------------+--------------------+---------------+ > 3 console Appendix E > +----------------------+--------------------+---------------+ > 4 entropy source Appendix F > +----------------------+--------------------+---------------+ > 5 memory ballooning Appendix G > +----------------------+--------------------+---------------+ > 6 ioMemory - > +----------------------+--------------------+---------------+ > 7 rpmsg - > +----------------------+--------------------+---------------+ > 8 SCSI host Appendix I > +----------------------+--------------------+---------------+ > 9 9P transport - > +----------------------+--------------------+---------------+ > 10 mac80211 wlan - > +----------------------+--------------------+---------------+ > > > 2.2 Device Configuration > > To configure the device, we use the first I/O region of the PCI > device. This contains a virtio header followed by a > device-specific region. > > There may be different widths of accesses to the I/O region; the > “natural” access method for each field in the virtio header must be > used (i.e. 32-bit accesses for 32-bit fields, etc), but the > device-specific region can be accessed using any width accesses, and > should obtain the same results. > > Note that this is possible because while the virtio header is PCI > (i.e. little) endian, the device-specific region is encoded in > the native endian of the guest (where such distinction is > applicable). > > 2.2.1 Device Initialization Sequence > > We start with an overview of device initialization, then expand > on the details of the device and how each step is preformed. > > 1. Reset the device. This is not required on initial start up. > > 2. The ACKNOWLEDGE status bit is set: we have noticed the device. > > 3. The DRIVER status bit is set: we know how to drive the device. > > 4. Device-specific setup, including reading the Device Feature > Bits, discovery of virtqueues for the device, optional MSI-X > setup, and reading and possibly writing the virtio > configuration space. > > 5. The subset of Device Feature Bits understood by the driver is > written to the device. > > 6. The DRIVER_OK status bit is set. > > 7. The device can now be used (ie. buffers added to the > virtqueues)[4] > > If any of these steps go irrecoverably wrong, the guest should > set the FAILED status bit to indicate that it has given up on the > device (it can reset the device later to restart if desired). > > We now cover the fields required for general setup in detail. > > 2.2.2 Virtio Header > > The virtio header looks as follows: > > > +------------++---------------------+---------------------+----------+--------+---------+---------+---------+--------+ > Bits 32 32 32 16 16 16 8 8 > +------------++---------------------+---------------------+----------+--------+---------+---------+---------+--------+ > Read/Write R R+W R+W R R+W R+W R+W R > +------------++---------------------+---------------------+----------+--------+---------+---------+---------+--------+ > Purpose Device Guest Queue Queue Queue Queue Device ISR > Features bits 0:31 Features bits 0:31 Address Size Select Notify Status Status > +------------++---------------------+---------------------+----------+--------+---------+---------+---------+--------+ > > > If MSI-X is enabled for the device, two additional fields > immediately follow this header:[5] > > > +------------++----------------+--------+ > Bits 16 16 > +----------------+--------+ > +------------++----------------+--------+ > Read/Write R+W R+W > +------------++----------------+--------+ > Purpose Configuration Queue > (MSI-X) Vector Vector > +------------++----------------+--------+ > > > Immediately following these general headers, there may be > device-specific headers: > > > +------------++--------------------+ > Bits Device Specific > +--------------------+ > +------------++--------------------+ > Read/Write Device Specific > +------------++--------------------+ > Purpose Device Specific... > > +------------++--------------------+ > > > 2.2.2.1 Device Status > > The Device Status field is updated by the guest to indicate its > progress. This provides a simple low-level diagnostic: it's most > useful to imagine them hooked up to traffic lights on the console > indicating the status of each device. > > The device can be reset by writing a 0 to this field, otherwise > at least one bit should be set: > > ACKNOWLEDGE (1) Indicates that the guest OS has found the > device and recognized it as a valid virtio device. > > DRIVER (2) Indicates that the guest OS knows how to drive the > device. Under Linux, drivers can be loadable modules so there > may be a significant (or infinite) delay before setting this > bit. > > DRIVER_OK (4) Indicates that the driver is set up and ready to > drive the device. > > FAILED (128) Indicates that something went wrong in the guest, > and it has given up on the device. This could be an internal > error, or the driver didn't like the device for some reason, or > even a fatal error during device operation. The device must be > reset before attempting to re-initialize. > > 2.2.2.2 Feature Bits > > The first configuration field indicates the features that the > device supports. The bits are allocated as follows: > > 0 to 23 Feature bits for the specific device type > > 24 to 32 Feature bits reserved for extensions to the queue and > feature negotiation mechanisms > > For example, feature bit 0 for a network device (i.e. Subsystem > Device ID 1) indicates that the device supports checksumming of > packets. > > The feature bits are negotiated: the device lists all the > features it understands in the Device Features field, and the > guest writes the subset that it understands into the Guest > Features field. The only way to renegotiate is to reset the > device. > > In particular, new fields in the device configuration header are > indicated by offering a feature bit, so the guest can check > before accessing that part of the configuration space. > > This allows for forwards and backwards compatibility: if the > device is enhanced with a new feature bit, older guests will not > write that feature bit back to the Guest Features field and it > can go into backwards compatibility mode. Similarly, if a guest > is enhanced with a feature that the device doesn't support, it > will not see that feature bit in the Device Features field and > can go into backwards compatibility mode (or, for poor > implementations, set the FAILED Device Status bit). > > 2.2.2.3 Configuration/Queue Vectors > > When MSI-X capability is present and enabled in the device > (through standard PCI configuration space) 4 bytes at byte offset > 20 are used to map configuration change and queue interrupts to > MSI-X vectors. In this case, the ISR Status field is unused, and > device specific configuration starts at byte offset 24 in virtio > header structure. When MSI-X capability is not enabled, device > specific configuration starts at byte offset 20 in virtio header. > > Writing a valid MSI-X Table entry number, 0 to 0x7FF, to one of > Configuration/Queue Vector registers, maps interrupts triggered > by the configuration change/selected queue events respectively to > the corresponding MSI-X vector. To disable interrupts for a > specific event type, unmap it by writing a special NO_VECTOR > value: > > /* Vector value used to disable MSI for queue */ > > #define VIRTIO_MSI_NO_VECTOR 0xffff > > Reading these registers returns vector mapped to a given event, > or NO_VECTOR if unmapped. All queue and configuration change > events are unmapped by default. > > Note that mapping an event to vector might require allocating > internal device resources, and might fail. Devices report such > failures by returning the NO_VECTOR value when the relevant > Vector field is read. After mapping an event to vector, the > driver must verify success by reading the Vector field value: on > success, the previously written value is returned, and on > failure, NO_VECTOR is returned. If a mapping failure is detected, > the driver can retry mapping with fewervectors, or disable MSI-X. > > 2.3 Virtqueue Configuration > > As a device can have zero or more virtqueues for bulk data > transport (for example, the network driver has two), the driver > needs to configure them as part of the device-specific > configuration. > > This is done as follows, for each virtqueue a device has: > > 1. Write the virtqueue index (first queue is 0) to the Queue > Select field. > > 2. Read the virtqueue size from the Queue Size field, which is > always a power of 2. This controls how big the virtqueue is > (see below). If this field is 0, the virtqueue does not exist. > > 3. Allocate and zero virtqueue in contiguous physical memory, on > a 4096 byte alignment. Write the physical address, divided by > 4096 to the Queue Address field.[6] > > 4. Optionally, if MSI-X capability is present and enabled on the > device, select a vector to use to request interrupts triggered > by virtqueue events. Write the MSI-X Table entry number > corresponding to this vector in Queue Vector field. Read the > Queue Vector field: on success, previously written value is > returned; on failure, NO_VECTOR value is returned. > > The Queue Size field controls the total number of bytes required > for the virtqueue according to the following formula: > > #define ALIGN(x) (((x) + 4095) & ~4095) > > static inline unsigned vring_size(unsigned int qsz) > { > return ALIGN(sizeof(struct vring_desc)*qsz + sizeof(u16)*(2 + qsz)) > + ALIGN(sizeof(struct vring_used_elem)*qsz); > } > > This currently wastes some space with padding, but also allows > future extensions. The virtqueue layout structure looks like this > (qsz is the Queue Size field, which is a variable, so this code > won't compile): > > struct vring { > /* The actual descriptors (16 bytes each) */ > struct vring_desc desc[qsz]; > > /* A ring of available descriptor heads with free-running index. */ > struct vring_avail avail; > > // Padding to the next 4096 boundary. > char pad[]; > > // A ring of used descriptor heads with free-running index. > struct vring_used used; > }; > > 2.3.1 A Note on Virtqueue Endianness > > Note that the endian of these fields and everything else in the > virtqueue is the native endian of the guest, not little-endian as > PCI normally is. This makes for simpler guest code, and it is > assumed that the host already has to be deeply aware of the guest > endian so such an “endian-aware” device is not a significant > issue. > > 2.3.2 Descriptor Table > > The descriptor table refers to the buffers the guest is using for > the device. The addresses are physical addresses, and the buffers > can be chained via the next field. Each descriptor describes a > buffer which is read-only or write-only, but a chain of > descriptors can contain both read-only and write-only buffers. > > No descriptor chain may be more than 2^32 bytes long in total. > > struct vring_desc { > /* Address (guest-physical). */ > u64 addr; > /* Length. */ > u32 len; > > /* This marks a buffer as continuing via the next field. */ > #define VRING_DESC_F_NEXT 1 > /* This marks a buffer as write-only (otherwise read-only). */ > #define VRING_DESC_F_WRITE 2 > /* This means the buffer contains a list of buffer descriptors. */ > #define VRING_DESC_F_INDIRECT 4 > /* The flags as indicated above. */ > u16 flags; > /* Next field if flags & NEXT */ > u16 next; > }; > > The number of descriptors in the table is specified by the Queue > Size field for this virtqueue. > > 2.3.3 Indirect Descriptors > > Some devices benefit by concurrently dispatching a large number > of large requests. The VIRTIO_RING_F_INDIRECT_DESC feature can be > used to allow this (see Appendix B: Reserved Feature Bits). To increase > ring capacity it is possible to store a table of indirect > descriptors anywhere in memory, and insert a descriptor in main > virtqueue (with flags&INDIRECT on) that refers to memory buffer > containing this indirect descriptor table; fields addr and len > refer to the indirect table address and length in bytes, > respectively. The indirect table layout structure looks like this > (len is the length of the descriptor that refers to this table, > which is a variable, so this code won't compile): > > struct indirect_descriptor_table { > /* The actual descriptors (16 bytes each) */ > struct vring_desc desc[len / 16]; > }; > > The first indirect descriptor is located at start of the indirect > descriptor table (index 0), additional indirect descriptors are > chained by next field. An indirect descriptor without next field > (with flags&NEXT off) signals the end of the indirect descriptor > table, and transfers control back to the main virtqueue. An > indirect descriptor can not refer to another indirect descriptor > table (flags&INDIRECT must be off). A single indirect descriptor > table can include both read-only and write-only descriptors; > write-only flag (flags&WRITE) in the descriptor that refers to it > is ignored. > > 2.3.4 Available Ring > > The available ring refers to what descriptors we are offering the > device: it refers to the head of a descriptor chain. The “flags” field > is currently 0 or 1: 1 indicating that we do not need an interrupt > when the device consumes a descriptor from the available > ring. Alternatively, the guest can ask the device to delay interrupts > until an entry with an index specified by the “ used_event” field is > written in the used ring (equivalently, until the idx field in the > used ring will reach the value used_event + 1). The method employed by > the device is controlled by the VIRTIO_RING_F_EVENT_IDX feature bit > (see Appendix B: Reserved Feature Bits). This interrupt suppression is > merely an optimization; it may not suppress interrupts entirely. > > The “idx” field indicates where we would put the next descriptor > entry (modulo the ring size). This starts at 0, and increases. > > struct vring_avail { > #define VRING_AVAIL_F_NO_INTERRUPT 1 > u16 flags; > u16 idx; > u16 ring[qsz]; /* qsz is the Queue Size field read from device */ > u16 used_event; > }; > > 2.3.5 Used Ring > > The used ring is where the device returns buffers once it is done > with them. The flags field can be used by the device to hint that > no notification is necessary when the guest adds to the available > ring. Alternatively, the “avail_event” field can be used by the > device to hint that no notification is necessary until an entry > with an index specified by the “avail_event” is written in the > available ring (equivalently, until the idx field in the > available ring will reach the value avail_event + 1). The method > employed by the device is controlled by the guest through the > VIRTIO_RING_F_EVENT_IDX feature bit (see Appendix B: Reserved > Feature Bits).[7] > > Each entry in the ring is a pair: the head entry of the > descriptor chain describing the buffer (this matches an entry > placed in the available ring by the guest earlier), and the total > of bytes written into the buffer. The latter is extremely useful > for guests using untrusted buffers: if you do not know exactly > how much has been written by the device, you usually have to zero > the buffer to ensure no data leakage occurs. > > /* u32 is used here for ids for padding reasons. */ > struct vring_used_elem { > /* Index of start of used descriptor chain. */ > u32 id; > /* Total length of the descriptor chain which was used (written to) */ > u32 len; > }; > > struct vring_used { > #define VRING_USED_F_NO_NOTIFY 1 > u16 flags; > u16 idx; > struct vring_used_elem ring[qsz]; > u16 avail_event; > }; > > 2.3.6 Helpers for Managing Virtqueues > > The Linux Kernel Source code contains the definitions above and > helper routines in a more usable form, in > include/linux/virtio_ring.h. This was explicitly licensed by IBM > and Red Hat under the (3-clause) BSD license so that it can be > freely used by all other projects, and is reproduced (with slight > variation to remove Linux assumptions) in Appendix A. > > 2.4 Device Operation > > There are two parts to device operation: supplying new buffers to > the device, and processing used buffers from the device. As an > example, the virtio network device has two virtqueues: the > transmit virtqueue and the receive virtqueue. The driver adds > outgoing (read-only) packets to the transmit virtqueue, and then > frees them after they are used. Similarly, incoming (write-only) > buffers are added to the receive virtqueue, and processed after > they are used. > > 2.4.1 Supplying Buffers to The Device > > Actual transfer of buffers from the guest OS to the device > operates as follows: > > 1. Place the buffer(s) into free descriptor(s). > > (a) If there are no free descriptors, the guest may choose to > notify the device even if notifications are suppressed (to > reduce latency).[8] > > 2. Place the id of the buffer in the next ring entry of the > available ring. > > 3. The steps (1) and (2) may be performed repeatedly if batching > is possible. > > 4. A memory barrier should be executed to ensure the device sees > the updated descriptor table and available ring before the next > step. > > 5. The available “idx” field should be increased by the number of > entries added to the available ring. > > 6. A memory barrier should be executed to ensure that we update > the idx field before checking for notification suppression. > > 7. If notifications are not suppressed, the device should be > notified of the new buffers. > > Note that the above code does not take precautions against the > available ring buffer wrapping around: this is not possible since > the ring buffer is the same size as the descriptor table, so step > (1) will prevent such a condition. > > In addition, the maximum queue size is 32768 (it must be a power > of 2 which fits in 16 bits), so the 16-bit “idx” value can always > distinguish between a full and empty buffer. > > Here is a description of each stage in more detail. > > 2.4.1.1 Placing Buffers Into The Descriptor Table > > A buffer consists of zero or more read-only physically-contiguous > elements followed by zero or more physically-contiguous > write-only elements (it must have at least one element). This > algorithm maps it into the descriptor table: > > 1. for each buffer element, b: > > (a) Get the next free descriptor table entry, d > > (b) Set d.addr to the physical address of the start of b > > (c) Set d.len to the length of b. > > (d) If b is write-only, set d.flags to VRING_DESC_F_WRITE, > otherwise 0. > > (e) If there is a buffer element after this: > > i. Set d.next to the index of the next free descriptor > element. > > ii. Set the VRING_DESC_F_NEXT bit in d.flags. > > In practice, the d.next fields are usually used to chain free > descriptors, and a separate count kept to check there are enough > free descriptors before beginning the mappings. > > 2.4.1.2 Updating The Available Ring > > The head of the buffer we mapped is the first d in the algorithm > above. A naive implementation would do the following: > > avail->ring[avail->idx % qsz] = head; > > However, in general we can add many descriptors before we update > the “idx” field (at which point they become visible to the > device), so we keep a counter of how many we've added: > > avail->ring[(avail->idx + added++) % qsz] = head; > > 2.4.1.3 Updating The Index Field > > Once the idx field of the virtqueue is updated, the device will > be able to access the descriptor entries we've created and the > memory they refer to. This is why a memory barrier is generally > used before the idx update, to ensure it sees the most up-to-date > copy. > > The idx field always increments, and we let it wrap naturally at > 65536: > > avail->idx += added; > > 2.4.1.4 Notifying The Device > > Device notification occurs by writing the 16-bit virtqueue index > of this virtqueue to the Queue Notify field of the virtio header > in the first I/O region of the PCI device. This can be expensive, > however, so the device can suppress such notifications if it > doesn't need them. We have to be careful to expose the new idx > value before checking the suppression flag: it's OK to notify > gratuitously, but not to omit a required notification. So again, > we use a memory barrier here before reading the flags or the > avail_event field. > > If the VIRTIO_F_RING_EVENT_IDX feature is not negotiated, and if > the VRING_USED_F_NOTIFY flag is not set, we go ahead and write to > the PCI configuration space. > > If the VIRTIO_F_RING_EVENT_IDX feature is negotiated, we read the > avail_event field in the available ring structure. If the > available index crossed_the avail_event field value since the > last notification, we go ahead and write to the PCI configuration > space. The avail_event field wraps naturally at 65536 as well: > > (u16)(new_idx - avail_event - 1) < (u16)(new_idx - old_idx) > > 2.4.2 Receiving Used Buffers From The Device > > Once the device has used a buffer (read from or written to it, or > parts of both, depending on the nature of the virtqueue and the > device), it sends an interrupt, following an algorithm very > similar to the algorithm used for the driver to send the device a > buffer: > > 1. Write the head descriptor number to the next field in the used > ring. > > 2. Update the used ring idx. > > 3. Determine whether an interrupt is necessary: > > (a) If the VIRTIO_F_RING_EVENT_IDX feature is not negotiated: > check if f the VRING_AVAIL_F_NO_INTERRUPT flag is not set in > avail->flags > > (b) If the VIRTIO_F_RING_EVENT_IDX feature is negotiated: check > whether the used index crossed the used_event field value > since the last update. The used_event field wraps naturally > at 65536 as well: > (u16)(new_idx - used_event - 1) < (u16)(new_idx - old_idx) > > 4. If an interrupt is necessary: > > (a) If MSI-X capability is disabled: > > i. Set the lower bit of the ISR Status field for the device. > > ii. Send the appropriate PCI interrupt for the device. > > (b) If MSI-X capability is enabled: > > i. Request the appropriate MSI-X interrupt message for the > device, Queue Vector field sets the MSI-X Table entry > number. > > ii. If Queue Vector field value is NO_VECTOR, no interrupt > message is requested for this event. > > The guest interrupt handler should: > > 1. If MSI-X capability is disabled: read the ISR Status field, > which will reset it to zero. If the lower bit is zero, the > interrupt was not for this device. Otherwise, the guest driver > should look through the used rings of each virtqueue for the > device, to see if any progress has been made by the device > which requires servicing. > > 2. If MSI-X capability is enabled: look through the used rings of > each virtqueue mapped to the specific MSI-X vector for the > device, to see if any progress has been made by the device > which requires servicing. > > For each ring, guest should then disable interrupts by writing > VRING_AVAIL_F_NO_INTERRUPT flag in avail structure, if required. > It can then process used ring entries finally enabling interrupts > by clearing the VRING_AVAIL_F_NO_INTERRUPT flag or updating the > EVENT_IDX field in the available structure, Guest should then > execute a memory barrier, and then recheck the ring empty > condition. This is necessary to handle the case where, after the > last check and before enabling interrupts, an interrupt has been > suppressed by the device: > > vring_disable_interrupts(vq); > > for (;;) { > if (vq->last_seen_used != vring->used.idx) { > vring_enable_interrupts(vq); > mb(); > > if (vq->last_seen_used != vring->used.idx) > break; > } > > struct vring_used_elem *e = vring.used->ring[vq->last_seen_used%vsz]; > process_buffer(e); > vq->last_seen_used++; > } > > 2.4.3 Dealing With Configuration Changes > > Some virtio PCI devices can change the device configuration > state, as reflected in the virtio header in the PCI configuration > space. In this case: > > 1. If MSI-X capability is disabled: an interrupt is delivered and > the second highest bit is set in the ISR Status field to > indicate that the driver should re-examine the configuration > space. Note that a single interrupt can indicate both that one > or more virtqueue has been used and that the configuration > space has changed: even if the config bit is set, virtqueues > must be scanned. > > 2. If MSI-X capability is enabled: an interrupt message is > requested. The Configuration Vector field sets the MSI-X Table > entry number to use. If Configuration Vector field value is > NO_VECTOR, no interrupt message is requested for this event. > > > Creating New Device Types > > Various considerations are necessary when creating a new device > type: > > How Many Virtqueues? > > It is possible that a very simple device will operate entirely > through its configuration space, but most will need at least one > virtqueue in which it will place requests. A device with both > input and output (eg. console and network devices described here) > need two queues: one which the driver fills with buffers to > receive input, and one which the driver places buffers to > transmit output. > > What Configuration Space Layout? > > Configuration space is generally used for rarely-changing or > initialization-time parameters. But it is a limited resource, so > it might be better to use a virtqueue to update configuration > information (the network device does this for filtering, > otherwise the table in the config space could potentially be very > large). > > Note that this space is generally the guest's native endian, > rather than PCI's little-endian. > > What Device Number? > > Currently device numbers are assigned quite freely: a simple > request mail to the author of this document or the Linux > virtualization mailing list[9] will be sufficient to secure a unique one. > > Meanwhile for experimental drivers, use 65535 and work backwards. > > How many MSI-X vectors? > > Using the optional MSI-X capability devices can speed up > interrupt processing by removing the need to read ISR Status > register by guest driver (which might be an expensive operation), > reducing interrupt sharing between devices and queues within the > device, and handling interrupts from multiple CPUs. However, some > systems impose a limit (which might be as low as 256) on the > total number of MSI-X vectors that can be allocated to all > devices. Devices and/or device drivers should take this into > account, limiting the number of vectors used unless the device is > expected to cause a high volume of interrupts. Devices can > control the number of vectors used by limiting the MSI-X Table > Size or not presenting MSI-X capability in PCI configuration > space. Drivers can control this by mapping events to as small > number of vectors as possible, or disabling MSI-X capability > altogether. > > Message Framing > > The descriptors used for a buffer should not effect the semantics > of the message, except for the total length of the buffer. For > example, a network buffer consists of a 10 byte header followed > by the network packet. Whether this is presented in the ring > descriptor chain as (say) a 10 byte buffer and a 1514 byte > buffer, or a single 1524 byte buffer, or even three buffers, > should have no effect. > > In particular, no implementation should use the descriptor > boundaries to determine the size of any header in a request.[10] > > Device Improvements > > Any change to configuration space, or new virtqueues, or > behavioural changes, should be indicated by negotiation of a new > feature bit. This establishes clarity[11] and avoids future expansion problems. > > Clusters of functionality which are always implemented together > can use a single bit, but if one feature makes sense without the > others they should not be gratuitously grouped together to > conserve feature bits. We can always extend the spec when the > first person needs more than 24 feature bits for their device. > > > > > Appendix A: virtio_ring.h > > #ifndef VIRTIO_RING_H > #define VIRTIO_RING_H > /* An interface for efficient virtio implementation. > * > * This header is BSD licensed so anyone can use the definitions > * to implement compatible drivers/servers. > * > * Copyright 2007, 2009, IBM Corporation > * Copyright 2011, Red Hat, Inc > * All rights reserved. > * > * Redistribution and use in source and binary forms, with or without > * modification, are permitted provided that the following conditions > * are met: > * 1. Redistributions of source code must retain the above copyright > * notice, this list of conditions and the following disclaimer. > * 2. Redistributions in binary form must reproduce the above copyright > * notice, this list of conditions and the following disclaimer in the > * documentation and/or other materials provided with the distribution. > * 3. Neither the name of IBM nor the names of its contributors > * may be used to endorse or promote products derived from this software > * without specific prior written permission. > * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS ``AS IS'' AND > * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE > * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE > * ARE DISCLAIMED. IN NO EVENT SHALL IBM OR CONTRIBUTORS BE LIABLE > * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL > * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS > * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) > * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT > * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY > * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF > * SUCH DAMAGE. > */ > > /* This marks a buffer as continuing via the next field. */ > #define VRING_DESC_F_NEXT 1 > /* This marks a buffer as write-only (otherwise read-only). */ > #define VRING_DESC_F_WRITE 2 > > /* The Host uses this in used->flags to advise the Guest: don't kick me > * when you add a buffer. It's unreliable, so it's simply an > * optimization. Guest will still kick if it's out of buffers. */ > #define VRING_USED_F_NO_NOTIFY 1 > /* The Guest uses this in avail->flags to advise the Host: don't > * interrupt me when you consume a buffer. It's unreliable, so it's > * simply an optimization. */ > #define VRING_AVAIL_F_NO_INTERRUPT 1 > > /* Virtio ring descriptors: 16 bytes. > * These can chain together via "next". */ > struct vring_desc { > /* Address (guest-physical). */ > uint64_t addr; > /* Length. */ > uint32_t len; > /* The flags as indicated above. */ > uint16_t flags; > /* We chain unused descriptors via this, too */ > uint16_t next; > }; > > struct vring_avail { > uint16_t flags; > uint16_t idx; > uint16_t ring[]; > uint16_t used_event; > }; > > /* u32 is used here for ids for padding reasons. */ > struct vring_used_elem { > /* Index of start of used descriptor chain. */ > uint32_t id; > /* Total length of the descriptor chain which was written to. */ > uint32_t len; > }; > > struct vring_used { > uint16_t flags; > uint16_t idx; > struct vring_used_elem ring[]; > uint16_t avail_event; > }; > > struct vring { > unsigned int num; > > struct vring_desc *desc; > struct vring_avail *avail; > struct vring_used *used; > }; > > /* The standard layout for the ring is a continuous chunk of memory which > * looks like this. We assume num is a power of 2. > * > * struct vring { > * // The actual descriptors (16 bytes each) > * struct vring_desc desc[num]; > * > * // A ring of available descriptor heads with free-running index. > * __u16 avail_flags; > * __u16 avail_idx; > * __u16 available[num]; > * > * // Padding to the next align boundary. > * char pad[]; > * > * // A ring of used descriptor heads with free-running index. > * __u16 used_flags; > * __u16 EVENT_IDX; > * struct vring_used_elem used[num]; > * }; > * Note: for virtio PCI, align is 4096. > */ > static inline void vring_init(struct vring *vr, unsigned int num, void *p, > unsigned long align) > { > vr->num = num; > vr->desc = p; > vr->avail = p + num*sizeof(struct vring_desc); > vr->used = (void *)(((unsigned long)&vr->avail->ring[num] > + align-1) > & ~(align - 1)); > } > > static inline unsigned vring_size(unsigned int num, unsigned long align) > { > return ((sizeof(struct vring_desc)*num + sizeof(uint16_t)*(2+num) > + align - 1) & ~(align - 1)) > + sizeof(uint16_t)*3 + sizeof(struct vring_used_elem)*num; > } > > static inline int vring_need_event(uint16_t event_idx, uint16_t new_idx, uint16_t old_idx) > { > return (uint16_t)(new_idx - event_idx - 1) < (uint16_t)(new_idx - old_idx); > } > #endif /* VIRTIO_RING_H */ > > > Appendix B: Reserved Feature Bits > > Currently there are five device-independent feature bits defined: > > VIRTIO_F_NOTIFY_ON_EMPTY (24) Negotiating this feature > indicates that the driver wants an interrupt if the device runs > out of available descriptors on a virtqueue, even though > interrupts are suppressed using the VRING_AVAIL_F_NO_INTERRUPT > flag or the used_event field. An example of this is the > networking driver: it doesn't need to know every time a packet > is transmitted, but it does need to free the transmitted > packets a finite time after they are transmitted. It can avoid > using a timer if the device interrupts it when all the packets > are transmitted. > > VIRTIO_F_RING_INDIRECT_DESC (28) Negotiating this feature indicates > that the driver can use descriptors with the VRING_DESC_F_INDIRECT > flag set, as described in 2.3.3 Indirect Descriptors. > > VIRTIO_F_RING_EVENT_IDX(29) This feature enables the used_event > and the avail_event fields. If set, it indicates that the > device should ignore the flags field in the available ring > structure. Instead, the used_event field in this structure is > used by guest to suppress device interrupts. Further, the > driver should ignore the flags field in the used ring > structure. Instead, the avail_event field in this structure is > used by the device to suppress notifications. If unset, the > driver should ignore the used_event field; the device should > ignore the avail_event field; the flags field is used > > Appendix C: Network Device > > The virtio network device is a virtual ethernet card, and is the > most complex of the devices supported so far by virtio. It has > enhanced rapidly and demonstrates clearly how support for new > features should be added to an existing device. Empty buffers are > placed in one virtqueue for receiving packets, and outgoing > packets are enqueued into another for transmission in that order. > A third command queue is used to control advanced filtering > features. > > Configuration > > Subsystem Device ID 1 > > Virtqueues 0:receiveq. 1:transmitq. 2:controlq[12] > > Feature bits > > VIRTIO_NET_F_CSUM (0) Device handles packets with partial checksum > > VIRTIO_NET_F_GUEST_CSUM (1) Guest handles packets with partial checksum > > VIRTIO_NET_F_MAC (5) Device has given MAC address. > > VIRTIO_NET_F_GSO (6) (Deprecated) device handles packets with > any GSO type.[13] > > VIRTIO_NET_F_GUEST_TSO4 (7) Guest can receive TSOv4. > > VIRTIO_NET_F_GUEST_TSO6 (8) Guest can receive TSOv6. > > VIRTIO_NET_F_GUEST_ECN (9) Guest can receive TSO with ECN. > > VIRTIO_NET_F_GUEST_UFO (10) Guest can receive UFO. > > VIRTIO_NET_F_HOST_TSO4 (11) Device can receive TSOv4. > > VIRTIO_NET_F_HOST_TSO6 (12) Device can receive TSOv6. > > VIRTIO_NET_F_HOST_ECN (13) Device can receive TSO with ECN. > > VIRTIO_NET_F_HOST_UFO (14) Device can receive UFO. > > VIRTIO_NET_F_MRG_RXBUF (15) Guest can merge receive buffers. > > VIRTIO_NET_F_STATUS (16) Configuration status field is > available. > > VIRTIO_NET_F_CTRL_VQ (17) Control channel is available. > > VIRTIO_NET_F_CTRL_RX (18) Control channel RX mode support. > > VIRTIO_NET_F_CTRL_VLAN (19) Control channel VLAN filtering. > > VIRTIO_NET_F_GUEST_ANNOUNCE(21) Guest can send gratuitous > packets. > > Device configuration layout Two configuration fields are > currently defined. The mac address field always exists (though > is only valid if VIRTIO_NET_F_MAC is set), and the status field > only exists if VIRTIO_NET_F_STATUS is set. Two read-only bits > are currently defined for the status field: > VIRTIO_NET_S_LINK_UP and VIRTIO_NET_S_ANNOUNCE. > > #define VIRTIO_NET_S_LINK_UP 1 > #define VIRTIO_NET_S_ANNOUNCE 2 > > struct virtio_net_config { > u8 mac[6]; > u16 status; > }; > > Device Initialization > > 1. The initialization routine should identify the receive and > transmission virtqueues. > > 2. If the VIRTIO_NET_F_MAC feature bit is set, the configuration > space “mac” entry indicates the “physical” address of the the > network card, otherwise a private MAC address should be > assigned. All guests are expected to negotiate this feature if > it is set. > > 3. If the VIRTIO_NET_F_CTRL_VQ feature bit is negotiated, > identify the control virtqueue. > > 4. If the VIRTIO_NET_F_STATUS feature bit is negotiated, the link > status can be read from the bottom bit of the “status” config > field. Otherwise, the link should be assumed active. > > 5. The receive virtqueue should be filled with receive buffers. > This is described in detail below in “Setting Up Receive > Buffers”. > > 6. A driver can indicate that it will generate checksumless > packets by negotating the VIRTIO_NET_F_CSUM feature. This “ > checksum offload” is a common feature on modern network cards. > > 7. If that feature is negotiated[14], a driver can use TCP or UDP > segmentation offload by negotiating the VIRTIO_NET_F_HOST_TSO4 (IPv4 > TCP), VIRTIO_NET_F_HOST_TSO6 (IPv6 TCP) and VIRTIO_NET_F_HOST_UFO > (UDP fragmentation) features. It should not send TCP packets > requiring segmentation offload which have the Explicit Congestion > Notification bit set, unless the VIRTIO_NET_F_HOST_ECN feature is > negotiated.[15] > > 8. The converse features are also available: a driver can save > the virtual device some work by negotiating these features.[16] > The VIRTIO_NET_F_GUEST_CSUM feature indicates that partially > checksummed packets can be received, and if it can do that then > the VIRTIO_NET_F_GUEST_TSO4, VIRTIO_NET_F_GUEST_TSO6, > VIRTIO_NET_F_GUEST_UFO and VIRTIO_NET_F_GUEST_ECN are the input > equivalents of the features described above. See “Receiving > Packets” below. > > Device Operation > > Packets are transmitted by placing them in the transmitq, and > buffers for incoming packets are placed in the receiveq. In each > case, the packet itself is preceeded by a header: > > struct virtio_net_hdr { > #define VIRTIO_NET_HDR_F_NEEDS_CSUM 1 > u8 flags; > #define VIRTIO_NET_HDR_GSO_NONE 0 > #define VIRTIO_NET_HDR_GSO_TCPV4 1 > #define VIRTIO_NET_HDR_GSO_UDP 3 > #define VIRTIO_NET_HDR_GSO_TCPV6 4 > #define VIRTIO_NET_HDR_GSO_ECN 0x80 > u8 gso_type; > u16 hdr_len; > u16 gso_size; > u16 csum_start; > u16 csum_offset; > /* Only if VIRTIO_NET_F_MRG_RXBUF: */ > u16 num_buffers > }; > > The controlq is used to control device features such as > filtering. > > Packet Transmission > > Transmitting a single packet is simple, but varies depending on > the different features the driver negotiated. > > 1. If the driver negotiated VIRTIO_NET_F_CSUM, and the packet has > not been fully checksummed, then the virtio_net_hdr's fields > are set as follows. Otherwise, the packet must be fully > checksummed, and flags is zero. > > • flags has the VIRTIO_NET_HDR_F_NEEDS_CSUM set, > > • csum_start is set to the offset within the packet to begin checksumming, > and > > • csum_offset indicates how many bytes after the csum_start the > new (16 bit ones' complement) checksum should be placed.[17] > > 2. If the driver negotiated > VIRTIO_NET_F_HOST_TSO4, TSO6 or UFO, and the packet requires > TCP segmentation or UDP fragmentation, then the “gso_type” > field is set to VIRTIO_NET_HDR_GSO_TCPV4, TCPV6 or UDP. > (Otherwise, it is set to VIRTIO_NET_HDR_GSO_NONE). In this > case, packets larger than 1514 bytes can be transmitted: the > metadata indicates how to replicate the packet header to cut it > into smaller packets. The other gso fields are set: > > • hdr_len is a hint to the device as to how much of the header > needs to be kept to copy into each packet, usually set to the > length of the headers, including the transport header.[18] > > • gso_size is the maximum size of each packet beyond that > header (ie. MSS). > > • If the driver negotiated the VIRTIO_NET_F_HOST_ECN feature, > the VIRTIO_NET_HDR_GSO_ECN bit may be set in “gso_type” as > well, indicating that the TCP packet has the ECN bit set.[19] > > 3. If the driver negotiated the VIRTIO_NET_F_MRG_RXBUF feature, > the num_buffers field is set to zero. > > 4. The header and packet are added as one output buffer to the > transmitq, and the device is notified of the new entry (see 2.4.1.4 > Notifying The Device).[20] > > Packet Transmission Interrupt > > Often a driver will suppress transmission interrupts using the > VRING_AVAIL_F_NO_INTERRUPT flag (see 2.4.2 Receiving Used Buffers From > The Device) and check for used packets in the transmit path of following > packets. However, it will still receive interrupts if the > VIRTIO_F_NOTIFY_ON_EMPTY feature is negotiated, indicating that > the transmission queue is completely emptied. > > The normal behavior in this interrupt handler is to retrieve and > new descriptors from the used ring and free the corresponding > headers and packets. > > Setting Up Receive Buffers > > It is generally a good idea to keep the receive virtqueue as > fully populated as possible: if it runs out, network performance > will suffer. > > If the VIRTIO_NET_F_GUEST_TSO4, VIRTIO_NET_F_GUEST_TSO6 or > VIRTIO_NET_F_GUEST_UFO features are used, the Guest will need to > accept packets of up to 65550 bytes long (the maximum size of a > TCP or UDP packet, plus the 14 byte ethernet header), otherwise > 1514 bytes. So unless VIRTIO_NET_F_MRG_RXBUF is negotiated, every > buffer in the receive queue needs to be at least this length [20a] > > If VIRTIO_NET_F_MRG_RXBUF is negotiated, each buffer must be at > least the size of the struct virtio_net_hdr. > > Packet Receive Interrupt > > When a packet is copied into a buffer in the receiveq, the > optimal path is to disable further interrupts for the receiveq > (see [sub:Receiving-Used-Buffers]) and process packets until no > more are found, then re-enable them. > > Processing packet involves: > > 1. If the driver negotiated the VIRTIO_NET_F_MRG_RXBUF feature, > then the “num_buffers” field indicates how many descriptors > this packet is spread over (including this one). This allows > receipt of large packets without having to allocate large > buffers. In this case, there will be at least “num_buffers” in > the used ring, and they should be chained together to form a > single packet. The other buffers will not begin with a struct > virtio_net_hdr. > > 2. If the VIRTIO_NET_F_MRG_RXBUF feature was not negotiated, or > the “num_buffers” field is one, then the entire packet will be > contained within this buffer, immediately following the struct > virtio_net_hdr. > > 3. If the VIRTIO_NET_F_GUEST_CSUM feature was negotiated, the > VIRTIO_NET_HDR_F_NEEDS_CSUM bit in the “flags” field may be > set: if so, the checksum on the packet is incomplete and the “ > csum_start” and “csum_offset” fields indicate how to calculate > it (see Packet Transmission point 1). > > 4. If the VIRTIO_NET_F_GUEST_TSO4, TSO6 or UFO options were > negotiated, then the “gso_type” may be something other than > VIRTIO_NET_HDR_GSO_NONE, and the “gso_size” field indicates the > desired MSS (see Packet Transmission point 2). > > Control Virtqueue > > The driver uses the control virtqueue (if VIRTIO_NET_F_VTRL_VQ is > negotiated) to send commands to manipulate various features of > the device which would not easily map into the configuration > space. > > All commands are of the following form: > > struct virtio_net_ctrl { > u8 class; > u8 command; > u8 command-specific-data[]; > u8 ack; > }; > > /* ack values */ > #define VIRTIO_NET_OK 0 > #define VIRTIO_NET_ERR 1 > > The class, command and command-specific-data are set by the > driver, and the device sets the ack byte. There is little it can > do except issue a diagnostic if the ack byte is not > VIRTIO_NET_OK. > > Packet Receive Filtering > > If the VIRTIO_NET_F_CTRL_RX feature is negotiated, the driver can > send control commands for promiscuous mode, multicast receiving, > and filtering of MAC addresses. > > Note that in general, these commands are best-effort: unwanted > packets may still arrive. > > Setting Promiscuous Mode > > #define VIRTIO_NET_CTRL_RX 0 > #define VIRTIO_NET_CTRL_RX_PROMISC 0 > #define VIRTIO_NET_CTRL_RX_ALLMULTI 1 > > The class VIRTIO_NET_CTRL_RX has two commands: > VIRTIO_NET_CTRL_RX_PROMISC turns promiscuous mode on and off, and > VIRTIO_NET_CTRL_RX_ALLMULTI turns all-multicast receive on and > off. The command-specific-data is one byte containing 0 (off) or > 1 (on). > > Setting MAC Address Filtering > > struct virtio_net_ctrl_mac { > u32 entries; > u8 macs[entries][ETH_ALEN]; > }; > > #define VIRTIO_NET_CTRL_MAC 1 > #define VIRTIO_NET_CTRL_MAC_TABLE_SET 0 > > The device can filter incoming packets by any number of destination > MAC addresses.[21] This table is set using the class > VIRTIO_NET_CTRL_MAC and the command VIRTIO_NET_CTRL_MAC_TABLE_SET. The > command-specific-data is two variable length tables of 6-byte MAC > addresses. The first table contains unicast addresses, and the second > contains multicast addresses. > > VLAN Filtering > > If the driver negotiates the VIRTION_NET_F_CTRL_VLAN feature, it > can control a VLAN filter table in the device. > > #define VIRTIO_NET_CTRL_VLAN 2 > #define VIRTIO_NET_CTRL_VLAN_ADD 0 > #define VIRTIO_NET_CTRL_VLAN_DEL 1 > > Both the VIRTIO_NET_CTRL_VLAN_ADD and VIRTIO_NET_CTRL_VLAN_DEL > command take a 16-bit VLAN id as the command-specific-data. > > Gratuitous Packet Sending > > If the driver negotiates the VIRTIO_NET_F_GUEST_ANNOUNCE (depends > on VIRTIO_NET_F_CTRL_VQ), it can ask the guest to send gratuitous > packets; this is usually done after the guest has been physically > migrated, and needs to announce its presence on the new network > links. (As hypervisor does not have the knowledge of guest > network configuration (eg. tagged vlan) it is simplest to prod > the guest in this way). > > #define VIRTIO_NET_CTRL_ANNOUNCE 3 > #define VIRTIO_NET_CTRL_ANNOUNCE_ACK 0 > > The Guest needs to check VIRTIO_NET_S_ANNOUNCE bit in status > field when it notices the changes of device configuration. The > command VIRTIO_NET_CTRL_ANNOUNCE_ACK is used to indicate that > driver has recevied the notification and device would clear the > VIRTIO_NET_S_ANNOUNCE bit in the status filed after it received > this command. > > Processing this notification involves: > > 1. Sending the gratuitous packets or marking there are pending > gratuitous packets to be sent and letting deferred routine to > send them. > > 2. Sending VIRTIO_NET_CTRL_ANNOUNCE_ACK command through control > vq. > > 3. . > > Appendix D: Block Device > > The virtio block device is a simple virtual block device (ie. > disk). Read and write requests (and other exotic requests) are > placed in the queue, and serviced (probably out of order) by the > device except where noted. > > Configuration > > Subsystem Device ID 2 > > Virtqueues 0:requestq. > > Feature bits > > VIRTIO_BLK_F_BARRIER (0) Host supports request barriers. > > VIRTIO_BLK_F_SIZE_MAX (1) Maximum size of any single segment is > in “size_max”. > > VIRTIO_BLK_F_SEG_MAX (2) Maximum number of segments in a > request is in “seg_max”. > > VIRTIO_BLK_F_GEOMETRY (4) Disk-style geometry specified in “ > geometry”. > > VIRTIO_BLK_F_RO (5) Device is read-only. > > VIRTIO_BLK_F_BLK_SIZE (6) Block size of disk is in “blk_size”. > > VIRTIO_BLK_F_SCSI (7) Device supports scsi packet commands. > > VIRTIO_BLK_F_FLUSH (9) Cache flush command support. > > Device configuration layout The capacity of the device > (expressed in 512-byte sectors) is always present. The > availability of the others all depend on various feature bits > as indicated above. > > struct virtio_blk_config { > u64 capacity; > u32 size_max; > u32 seg_max; > struct virtio_blk_geometry { > u16 cylinders; > u8 heads; > u8 sectors; > } geometry; > u32 blk_size; > }; > > Device Initialization > > 1. The device size should be read from the “capacity” > configuration field. No requests should be submitted which goes > beyond this limit. > > 2. If the VIRTIO_BLK_F_BLK_SIZE feature is negotiated, the > blk_size field can be read to determine the optimal sector size > for the driver to use. This does not effect the units used in > the protocol (always 512 bytes), but awareness of the correct > value can effect performance. > > 3. If the VIRTIO_BLK_F_RO feature is set by the device, any write > requests will fail. > > Device Operation > > The driver queues requests to the virtqueue, and they are used by > the device (not necessarily in order). Each request is of form: > > struct virtio_blk_req { > u32 type; > u32 ioprio; > u64 sector; > char data[][512]; > u8 status; > }; > > If the device has VIRTIO_BLK_F_SCSI feature, it can also support > scsi packet command requests, each of these requests is of form: > > struct virtio_scsi_pc_req { > u32 type; > u32 ioprio; > u64 sector; > char cmd[]; > char data[][512]; > #define SCSI_SENSE_BUFFERSIZE 96 > u8 sense[SCSI_SENSE_BUFFERSIZE]; > u32 errors; > u32 data_len; > u32 sense_len; > u32 residual; > u8 status; > }; > > The type of the request is either a read (VIRTIO_BLK_T_IN), a write > (VIRTIO_BLK_T_OUT), a scsi packet command (VIRTIO_BLK_T_SCSI_CMD or > VIRTIO_BLK_T_SCSI_CMD_OUT[22]) or a flush (VIRTIO_BLK_T_FLUSH or > VIRTIO_BLK_T_FLUSH_OUT[23]). If the device has VIRTIO_BLK_F_BARRIER > feature the high bit (VIRTIO_BLK_T_BARRIER) indicates that this > request acts as a barrier and that all preceeding requests must be > complete before this one, and all following requests must not be > started until this is complete. Note that a barrier does not flush > caches in the underlying backend device in host, and thus does not > serve as data consistency guarantee. Driver must use FLUSH request to > flush the host cache. > > #define VIRTIO_BLK_T_IN 0 > #define VIRTIO_BLK_T_OUT 1 > #define VIRTIO_BLK_T_SCSI_CMD 2 > #define VIRTIO_BLK_T_SCSI_CMD_OUT 3 > #define VIRTIO_BLK_T_FLUSH 4 > #define VIRTIO_BLK_T_FLUSH_OUT 5 > #define VIRTIO_BLK_T_BARRIER 0x80000000 > > The ioprio field is a hint about the relative priorities of > requests to the device: higher numbers indicate more important > requests. > > The sector number indicates the offset (multiplied by 512) where > the read or write is to occur. This field is unused and set to 0 > for scsi packet commands and for flush commands. > > The cmd field is only present for scsi packet command requests, > and indicates the command to perform. This field must reside in a > single, separate read-only buffer; command length can be derived > from the length of this buffer. > > Note that these first three (four for scsi packet commands) > fields are always read-only: the data field is either read-only > or write-only, depending on the request. The size of the read or > write can be derived from the total size of the request buffers. > > The sense field is only present for scsi packet command requests, > and indicates the buffer for scsi sense data. > > The data_len field is only present for scsi packet command > requests, this field is deprecated, and should be ignored by the > driver

Anthony Liguori <aliguori@us.ibm.com> writes: > Rusty Russell <rusty@au1.ibm.com> writes: > >> Rusty Russell <rusty@au1.ibm.com> writes: >>> ----------------- >>> Action Item Subject: Create text version of virtio 0.9.5 document >> >> OK, I've attached this below. This is exactly the spec as 0.9.5 >> reformatted into text so we can work it. >> >> I'm still waiting for virtio to be added to the OASIS issue tracking >> system, where I will be prompting you all to open an issue for every >> change we want to consider. >> >> The obvious issues I want to open are: >> >> o Major rework to make PCI an appendix, and core bus-independent. > > We should revisit the use of PCI vendor/device IDs for virtio-pci. > > Currently we use the Red Hat/Qumranet vendor ID. This has led to a > situation where only Red Hat can produce a signed set of drivers for > Windows (since they own the vendor ID). These drivers are released > under proprietary and discriminatory terms. As it stands today, it is > impossible to create a signed driver for Windows that is spec compliant > unless you are Red Hat. > > Can we work with Oasis to get a vendor neutral PCI ID created to avoid > this problem in the future? Excellent point. I'm not sure PCI IDs *can* be vendor neutral? Beyond RH allowing others to use theirs as they've done. Perhaps the spec should not specify them, except to note that RH have allowed compliant software implementations within their range, perhaps even requiring all compliant drivers to recognize those. It's certainly commonplace on Linux to add new PCI IDs to existing drivers. Is this already a solved problem? (Hopeful question!) Cheers, Rusty. PS. Issue tracking will be up next week; once it is I'll post across from this thread.