On Sat, Nov 9, 2019 at 12:17 PM Tomasz Figa <tf...@chromium.org> wrote:
> On Sat, Nov 9, 2019 at 7:12 PM Stefan Hajnoczi <stefa...@gmail.com> wrote:
> > On Sat, Nov 9, 2019 at 2:41 AM St├ęphane Marchesin <marc...@chromium.org> 
> > wrote:
> > > On Thu, Nov 7, 2019 at 11:35 PM Stefan Hajnoczi <stefa...@gmail.com> 
> > > wrote:
> > > > On Fri, Nov 8, 2019 at 8:22 AM Gerd Hoffmann <kra...@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.
>
> Do you have any idea on how to model Wayland as a VIRTIO device?
>
> Stephane mentioned that we use vsock, but in fact we have our own
> VIRTIO device, except that it's semantically almost the same as vsock,
> with a difference being the ability to pass buffers and pipes across
> the VM boundary.

I know neither Wayland nor your use case :).

But we can discuss the design of your VIRTIO device.  Please post a
link to the code.

Stefan

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