Re: guest / host buffer sharing ...

["Stéphane Marchesin" <marcheu@chromium.org>]

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.

Best,
Stéphane

>
> Defining a virtio-wayland device makes sense to me: you get the guest
> RAM access via virtqueues, plus the VIRTIO infrastructure (device IDs,
> configuration space, feature bits, and existing reusable
> kernel/userspace/QEMU code).
>
> Stefan