OASIS Virtual I/O Device (VIRTIO) TC

Simplify indirect descriptors ----------------------------- Key: VIRTIO-15 URL: http://tools.oasis-open.org/issues/browse/VIRTIO-15 Project: OASIS Virtual I/O Device (VIRTIO) TC Issue Type: Improvement Reporter: Rusty Russell The current definition of VIRTIO_DESC_F_INDIRECT is that it stacks: you can set that bit and VIRTIO_DESC_F_NEXT and after processing the (only single level!) of indirect descriptors, the device then processes the next descriptor as normal (which may also be indirect). No implementation I am aware of generates such multi-part indirect descriptors, but they could be useful if we allow huge indirect descriptors as proposed. Perhaps we should make the bit chain, so if any descriptor has that set, we start traversing that descriptor as a new table, and never return. This is simpler to implement, and backwards compatible as well. -- This message is automatically generated by JIRA. - If you think it was sent incorrectly contact one of the administrators: http://tools.oasis-open.org/issues/secure/Administrators.jspa - For more information on JIRA, see: http://www.atlassian.com/software/jira

OASIS Issues Tracker <workgroup_mailer@lists.oasis-open.org> writes:
> Simplify indirect descriptors
> -----------------------------
>
> Key: VIRTIO-15
> URL: http://tools.oasis-open.org/issues/browse/VIRTIO-15
> Project: OASIS Virtual I/O Device (VIRTIO) TC
> Issue Type: Improvement
> Reporter: Rusty Russell
>
>
> The current definition of VIRTIO_DESC_F_INDIRECT is that it stacks: you can set that bit and VIRTIO_DESC_F_NEXT and after processing the (only single level!) of indirect descriptors, the device then processes the next descriptor as normal (which may also be indirect). No implementation I am aware of generates such multi-part indirect descriptors, but they could be useful if we allow huge indirect descriptors as proposed.
>
> Perhaps we should make the bit chain, so if any descriptor has that set, we start traversing that descriptor as a new table, and never return. This is simpler to implement, and backwards compatible as well.
>

That would look something like this:

commit b6ebc21dba82817a2b238a073f30e15d4cc741a2
Author: Rusty Russell <rusty@au1.ibm.com>
Date: Tue Aug 20 16:08:07 2013 +0930

Simplify indirect descriptors

As per http://tools.oasis-open.org/issues/browse/VIRTIO-15.

Signed-off-by: Rusty Russell <rusty@au1.ibm.com>

diff --git a/virtio-v1.0-wd01-part1-specification.txt b/virtio-v1.0-wd01-part1-specification.txt
index d8302cb..8fc96b2 100644
--- a/virtio-v1.0-wd01-part1-specification.txt
+++ b/virtio-v1.0-wd01-part1-specification.txt
@@ -274,9 +274,9 @@ 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 FIXME: 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&VRING_DESC_F_INDIRECT on) that refers to memory buffer
-containing this indirect descriptor table; fields addr and len
+descriptors anywhere in memory, and insert a descriptor in
+virtqueue descriptor table with the VRING_DESC_F_INDIRECT flag set that refers
+those descriptors; 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,
@@ -290,10 +290,9 @@ which is a variable, so this code won't compile):
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&VRING_DESC_F_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&VRING_DESC_F_INDIRECT must be off). A single indirect descriptor
+(ie. without the VRING_DESC_F_NEXT flag set) signals the end of the descriptor
+chain. An indirect descriptor with the VRING_DESC_F_INDIRECT flag set
+refers to the next set of indirect descriptors. A single indirect descriptor
table can include both read-only and write-only descriptors;
write-only flag (flags&VRING_DESC_F_WRITE) in the descriptor that refers to it
is ignored.

OASIS Issues Tracker <workgroup_mailer@lists.oasis-open.org> writes: > Simplify indirect descriptors > ----------------------------- > > Key: VIRTIO-15 > URL: http://tools.oasis-open.org/issues/browse/VIRTIO-15 > Project: OASIS Virtual I/O Device (VIRTIO) TC > Issue Type: Improvement > Reporter: Rusty Russell > > > The current definition of VIRTIO_DESC_F_INDIRECT is that it stacks: you can set that bit and VIRTIO_DESC_F_NEXT and after processing the (only single level!) of indirect descriptors, the device then processes the next descriptor as normal (which may also be indirect). No implementation I am aware of generates such multi-part indirect descriptors, but they could be useful if we allow huge indirect descriptors as proposed. > > Perhaps we should make the bit chain, so if any descriptor has that set, we start traversing that descriptor as a new table, and never return. This is simpler to implement, and backwards compatible as well. > That would look something like this: commit b6ebc21dba82817a2b238a073f30e15d4cc741a2 Author: Rusty Russell <rusty@au1.ibm.com> Date: Tue Aug 20 16:08:07 2013 +0930 Simplify indirect descriptors As per http://tools.oasis-open.org/issues/browse/VIRTIO-15 . Signed-off-by: Rusty Russell <rusty@au1.ibm.com> diff --git a/virtio-v1.0-wd01-part1-specification.txt b/virtio-v1.0-wd01-part1-specification.txt index d8302cb..8fc96b2 100644 --- a/virtio-v1.0-wd01-part1-specification.txt +++ b/virtio-v1.0-wd01-part1-specification.txt @@ -274,9 +274,9 @@ 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 FIXME: 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&VRING_DESC_F_INDIRECT on) that refers to memory buffer -containing this indirect descriptor table; fields addr and len +descriptors anywhere in memory, and insert a descriptor in +virtqueue descriptor table with the VRING_DESC_F_INDIRECT flag set that refers +those descriptors; 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, @@ -290,10 +290,9 @@ which is a variable, so this code won't compile): 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&VRING_DESC_F_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&VRING_DESC_F_INDIRECT must be off). A single indirect descriptor +(ie. without the VRING_DESC_F_NEXT flag set) signals the end of the descriptor +chain. An indirect descriptor with the VRING_DESC_F_INDIRECT flag set +refers to the next set of indirect descriptors. A single indirect descriptor table can include both read-only and write-only descriptors; write-only flag (flags&VRING_DESC_F_WRITE) in the descriptor that refers to it is ignored.

Rusty Russell <rusty@au1.ibm.com> writes:
> OASIS Issues Tracker <workgroup_mailer@lists.oasis-open.org> writes:
>> Simplify indirect descriptors
>> -----------------------------
>>
>> Key: VIRTIO-15
>> URL: http://tools.oasis-open.org/issues/browse/VIRTIO-15
>> Project: OASIS Virtual I/O Device (VIRTIO) TC
>> Issue Type: Improvement
>> Reporter: Rusty Russell
>>
>>
>> The current definition of VIRTIO_DESC_F_INDIRECT is that it stacks: you can set that bit and VIRTIO_DESC_F_NEXT and after processing the (only single level!) of indirect descriptors, the device then processes the next descriptor as normal (which may also be indirect). No implementation I am aware of generates such multi-part indirect descriptors, but they could be useful if we allow huge indirect descriptors as proposed.
>>
>> Perhaps we should make the bit chain, so if any descriptor has that set, we start traversing that descriptor as a new table, and never return. This is simpler to implement, and backwards compatible as well.

This just simplifies them. I'll add a new issue to discuss chaining.

commit 20af5684afde54a96060a94faf582756b9d911fa
Author: Rusty Russell <rusty@au1.ibm.com>
Date: Mon Sep 9 17:48:02 2013 +0930

Simplify indirect descriptors.

You get a single table, no multiple tables (noone does this, but it's
a PITA to support in devices).

As per issue virtio-15

Signed-off-by: Rusty Russell <rusty@au1.ibm.com>

diff --git a/virtio-v1.0-wd01-part1-specification.txt b/virtio-v1.0-wd01-part1-specification.txt
index 6f7d4d5..afdb833 100644
--- a/virtio-v1.0-wd01-part1-specification.txt
+++ b/virtio-v1.0-wd01-part1-specification.txt
@@ -351,8 +351,8 @@ which is a variable, so this code won't compile):
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&VRING_DESC_F_NEXT off) signals the end of the indirect descriptor
-table, and transfers control back to the main virtqueue. An
+(with flags&VRING_DESC_F_NEXT off) signals the end of the descriptor.
+An
indirect descriptor can not refer to another indirect descriptor
table (flags&VRING_DESC_F_INDIRECT must be off). A single indirect descriptor
table can include both read-only and write-only descriptors;

Rusty Russell <rusty@au1.ibm.com> writes: > OASIS Issues Tracker <workgroup_mailer@lists.oasis-open.org> writes: >> Simplify indirect descriptors >> ----------------------------- >> >> Key: VIRTIO-15 >> URL: http://tools.oasis-open.org/issues/browse/VIRTIO-15 >> Project: OASIS Virtual I/O Device (VIRTIO) TC >> Issue Type: Improvement >> Reporter: Rusty Russell >> >> >> The current definition of VIRTIO_DESC_F_INDIRECT is that it stacks: you can set that bit and VIRTIO_DESC_F_NEXT and after processing the (only single level!) of indirect descriptors, the device then processes the next descriptor as normal (which may also be indirect). No implementation I am aware of generates such multi-part indirect descriptors, but they could be useful if we allow huge indirect descriptors as proposed. >> >> Perhaps we should make the bit chain, so if any descriptor has that set, we start traversing that descriptor as a new table, and never return. This is simpler to implement, and backwards compatible as well. This just simplifies them. I'll add a new issue to discuss chaining. commit 20af5684afde54a96060a94faf582756b9d911fa Author: Rusty Russell <rusty@au1.ibm.com> Date: Mon Sep 9 17:48:02 2013 +0930 Simplify indirect descriptors. You get a single table, no multiple tables (noone does this, but it's a PITA to support in devices). As per issue virtio-15 Signed-off-by: Rusty Russell <rusty@au1.ibm.com> diff --git a/virtio-v1.0-wd01-part1-specification.txt b/virtio-v1.0-wd01-part1-specification.txt index 6f7d4d5..afdb833 100644 --- a/virtio-v1.0-wd01-part1-specification.txt +++ b/virtio-v1.0-wd01-part1-specification.txt @@ -351,8 +351,8 @@ which is a variable, so this code won't compile): 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&VRING_DESC_F_NEXT off) signals the end of the indirect descriptor -table, and transfers control back to the main virtqueue. An +(with flags&VRING_DESC_F_NEXT off) signals the end of the descriptor. +An indirect descriptor can not refer to another indirect descriptor table (flags&VRING_DESC_F_INDIRECT must be off). A single indirect descriptor table can include both read-only and write-only descriptors;