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

[Cornelia Huck <cohuck@redhat.com>]

On Wed, 10 Jan 2018 13:10:49 +0200
"Michael S. Tsirkin" <mst@redhat.com> wrote:

> On Wed, Jan 10, 2018 at 11:33:15AM +0100, Cornelia Huck wrote:
> > 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.  
> 
> Good point.
> There's exactly same number of chunks (descriptor, device and driver
> event suppression).
> 
> So the idea is to substitute descriptor for descriptor, device event
> suppression for available ring and driver event suppression for
> used ring.
> 
> That's why we don't need to change transports.
> 
> I will add text to clarify that.

I think we also need to update the text for the transports, as they
explicitly refer to avail/used.

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


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