On Fri, Sep 13, 2013 at 10:40:40AM +0930, Rusty Russell wrote:
> "Michael S. Tsirkin" <
mst@redhat.com> writes:
> > Transports can now lay out available/used/descriptor
> > regions in a flexible way.
> > This is useful for embedded systems to save memory,
> > and for large systems to reduce the need for
> > physically-contigious memory.
> >
> > This does not add a way to actually program this
> > in any of the transports, so it's not useful by
> > itself, a follow-up patch with add a way to
> > program this for PCI.
> >
> > Signed-off-by: Michael S. Tsirkin <
mst@redhat.com>
> > ---
> > virtio-v1.0-wd01-part1-specification.txt | 52 ++++++++++++++++++++++++++++----
> > 1 file changed, 46 insertions(+), 6 deletions(-)
> >
> > diff --git a/virtio-v1.0-wd01-part1-specification.txt b/virtio-v1.0-wd01-part1-specification.txt
> > index fcd9fd7..9a40973 100644
> > --- a/virtio-v1.0-wd01-part1-specification.txt
> > +++ b/virtio-v1.0-wd01-part1-specification.txt
> > @@ -223,6 +223,45 @@ transmit and one for receive. Each queue has a 16-bit queue size
> > parameter, which sets the number of entries and implies the total size
> > of the queue.
> >
> > +Each virtqueue consists of three parts:
> > +
> > + Descriptor Table
> > + Available Ring
> > + Used Ring
> > +
> > +where each part is physically-contiguous in guest memory,
> > +and has different alignment requirements.
> > +
> > +The Queue Size field controls the total number of bytes
> > +required for each part of the virtqueue.
> > +
> > +The memory aligment and size requirements, in bytes, of each part of the
> > +virtqueue are summarized in the following table (qsz is the Queue Size field):
> > +
> > ++------------+---------------------------------+
> > +| Virtqueue Part | Alignment | Size |
> > ++------------+---------------------------------+
> > ++------------+---------------------------------+
> > +| Descriptor Table | 16 | 16 * qsz |
>
> Alignment 16: isn't that overkill?
Why overkill? 16 bytes is exactly a single descriptor size,
aligning descriptor buffer to descriptor size seems sensible,
it simplifies translation: this we never cross a page boundary
in the middle of descriptor.
> > ++------------+---------------------------------+
> > +| Available Ring | 2 | 6 + 2 * qsz |
> > ++------------+---------------------------------+
> > +| Used Ring | 4 | 6 + 4 * qsz |
> > ++------------+---------------------------------+
> > +
> > +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.
> > +
> > +
> > +100.100.4.1. Legacy Interfaces: A Note on Virtqueue Layout
> > +--------------------------------------
> > +
> > +For Legacy Interfaces, several additional
> > +restrictions are placed on the virtqueue layout:
> > +
> > Each virtqueue occupies two or more physically-contiguous pages
> > (usually defined as 4096 bytes, but depending on the transport)
> > and consists of three parts:
> > @@ -241,9 +280,8 @@ required for the virtqueue according to the following formula:
> > + ALIGN(sizeof(u16)*3 + sizeof(struct vring_used_elem)*qsz);
> > }
> >
> > -This currently wastes some space with padding, but also allows future
> > -extensions such as the VIRTIO_RING_F_EVENT_IDX extension. The
> > -virtqueue layout structure looks like this:
> > +This wastes some space with padding.
> > +The legacy virtqueue layout structure therefore looks like this:
> >
> > struct vring {
> > // The actual descriptors (16 bytes each)
> > @@ -814,9 +852,11 @@ This is done as follows, for each virtqueue a device has:
> > always a power of 2. This controls how big the virtqueue is
> > (see "2.1.4. Virtqueues"). 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]
> > +3. Optionally, select a smaller virtqueue size and write it in the Queue Size
> > + field.
> > +
> > +3. Allocate and zero Descriptor Table, Available and Used rings for the
> > + virtqueue in contiguous physical memory.
> >
> > 4. Optionally, if MSI-X capability is present and enabled on the
> > device, select a vector to use to request interrupts triggered
>
> We should move most of this out to the PCI-specific section. For
> example, mmio has a QueueNumMax field instead, and ccw doesn't seem to
> have a way of negotiating.
My thinking is we should synchronize field names in transports, optionally
adding ways of negotiating.
E.g. I don't see why it won't be useful for ccw.
But can be done by a separate patch?
>
> Cheers,
> Rusty.