Re: [virtio] [PATCH v6 0/5] packed ring layout spec

[Cornelia Huck <cohuck@redhat.com>]

On Wed, 10 Jan 2018 11:47:53 +0200
"Michael S. Tsirkin" <mst@redhat.com> wrote:

> OK, this is in a shape where we could include it in the spec.
> Changes from v5:
> - scope reductions (see below). We can add more
>   features down the road, hopefully reduced scope will be enough
>   to finalize spec soon.
> - cleanup and integrate in the spec
> - pseudo-code
> 
> Deferred features:
> - dropped _F_DESC_LIST, 1.0 includes this unconditionally, we
>   can do same
> - dropped event structure change notifications - needed for
>   efficient hardware implementations but let's add this on top
> 
> 3 1st patches just move text around so all virtio 1.0
> things are in the same place. 2 last ones add the new layout
> 
> Option to mark descriptors as not generating events isn't
> yet implemented. Again, let's add this on top.
> 
> I also note that for hardware implementations, a different
> set of memory barriers is needed. Again, let's add this on top
> 
> not linking into conformance sections, will add after spec
> itself is approved.
> 
> Michael S. Tsirkin (5):
>   content: move 1.0 queue format out to a separate section
>   content: move ring text out to a separate file
>   content: move virtqueue operation description
>   packed virtqueues: more efficient virtqueue layout
>   packed-ring: add in order request support
> 
>  conformance.tex |   4 +-
>  content.tex     | 718 +++++---------------------------------------------------
>  packed-ring.tex | 692 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
>  split-ring.tex  | 666 ++++++++++++++++++++++++++++++++++++++++++++++++++++
>  4 files changed, 1423 insertions(+), 657 deletions(-)
>  create mode 100644 packed-ring.tex
>  create mode 100644 split-ring.tex
> 

One thing I just noticed: This does not seem to contain any changes for
transports. For ccw, we use different payloads in the SET_VQ command
for legacy (pre-virtio-1) and virtio-1 split layout. I think we would
need a new SET_VQ payload for the packed ring guarded by a new
revision, but I have not investigated it yet. I have not yet thought
through how this interacts with feature negotiation, either.

Otherwise, I don't see any reason why the packed ring would not work
fine for ccw as well.