Re: guest / host buffer sharing ...

[Stefan Hajnoczi <stefanha@gmail.com>]

On Sat, Nov 9, 2019 at 2:41 AM Stéphane Marchesin <marcheu@chromium.org> wrote:
>
> On Thu, Nov 7, 2019 at 11:35 PM Stefan Hajnoczi <stefanha@gmail.com> wrote:
> >
> > On Fri, Nov 8, 2019 at 8:22 AM Gerd Hoffmann <kraxel@redhat.com> wrote:
> > > > > Adding a list of common properties to the spec certainly makes sense,
> > > > > so everybody uses the same names.  Adding struct-ed properties for
> > > > > common use cases might be useful too.
> > > >
> > > > Why not define VIRTIO devices for wayland and friends?
> > >
> > > There is an out-of-tree implementation of that, so yes, that surely is
> > > an option.
> > >
> > > Wayland needs (a) shared buffers, mostly for gfx data, and (b) a stream
> > > pipe as control channel.  Pretty much the same for X11, except that
> > > shared buffers are optional because the X protocol can also squeeze all
> > > display updates through the stream pipe.
> > >
> > > So, if you want allow guests talk to the host display server you can run
> > > the stream pipe over vsock.  But there is nothing for the shared
> > > buffers ...
> > >
> > > We could replicate vsock functionality elsewhere.  I think that happened
> > > in the out-of-tree virtio-wayland implementation.  There also was some
> > > discussion about adding streams to virtio-gpu, slightly pimped up so you
> > > can easily pass around virtio-gpu resource references for buffer
> > > sharing.  But given that getting vsock right isn't exactly trivial
> > > (consider all the fairness issues when multiplexing multiple streams
> > > over a virtqueue for example) I don't think this is a good plan.
> >
> > I also think vsock isn't the right fit.
> >
>
> +1 we are using vsock right now and we have a few pains because of it.
>
> I think the high-level problem is that because it is a side channel,
> we don't see everything that happens to the buffer in one place
> (rendering + display) and we can't do things like reallocate the
> format accordingly if needed, or we can't do flushing etc. on that
> buffer where needed.

Do you think a VIRTIO device designed for your use case is an
appropriate solution?

I have been arguing that these use cases should be addressed with
dedicated VIRTIO devices, but I don't understand the use cases of
everyone on the CC list so maybe I'm missing something :).  If there
are reasons why having a VIRTIO device for your use case does not make
sense then it would be good to discuss them.  Blockers like "VIRTIO is
too heavyweight/complex for us because ...", "Our application can't
make use of VIRTIO devices because ...", etc would be important to
hear.

Stefan