[v7] Add udmabuf misc device

[Gerd Hoffmann]

  Hi,

> > Well, no.  This is *not* about 3D, it's about software rendering, for
> > example cairo doing its work for gtk apps.  So the workflow would be
> > along these lines:
> >
> > (1) guest app allocates dumb drm buffer from virtio-gpu, renders to it.
> > (2) guest app passes the buffer to wayland guest proxy (which looks
> >     like a wayland server/compositor to the app, but it doesn't actually
> >     composite anything).
> > (3) wayland guest proxy passes buffer handle to wayland host proxy.
> > (4) qemu can then use the buffer handle to lookup the virtio-gpu
> >     buffer, then use udmabuf to create a host dma-buf for it.
> > (5) host dma-buf can be passed to host wayland server for display, so
> >     guest app window shows up seamlessly on the host.
> >
> > Details of the wayland protocol proxying are not hashed out yet.
> 
> Thanks for the clarification.  With dumb buffers, I guess the host can
> use DRM_FORMAT_MOD_LINEAR when sending the udmabuf-created buffer to
> the display.  Note there are certain cases where tiled buffers are
> map-able (Intel), and with some variation of
> virtio_gpu_resource_create_coherent, we could expose that to the
> guest.

The coherent mapping (host-allocated resources into guest address space)
is another thing which needs to be sorted out.

> That's the direction we're interested in going, though it
> looks like udmabuf is orthogonal to that.

Yes, udmabuf is the other way around, guest allocates resources and we
make them available as host dmabufs.

> What details on wayland protocol proxying still need to be worked out?
>  That's one component of the ChromiumOS solution (virtio_wl) that
> hasn't been up-streamed yet.  That method maps guest fds to host fds.

Tomeu Vizoso (Cc'ed) was looking into using virtio-serial as wayland
protocol transport between host and guest.  With udmabuf we can use
virtio-gpu drm objects to pass pixel buffers from guest to host, which
is one important building block for that.

Not sure what the current state is.

cheers,
  Gerd