Virtualization - Feb 2021 - [RFC v3 2/3] virtio: Introduce Vdmabuf driver

> > You don't have to use the rendering pipeline.  You can let the
i915 gpu
> > render into a dma-buf shared with virtio-gpu, then use virtio-gpu only
for
> > buffer sharing with the host.
> [Kasireddy, Vivek] Is this the most viable path forward? I am not sure how
complex or
> feasible it would be but I'll look into it.
> Also, not using the rendering capabilities of virtio-gpu and turning it
into a sharing only
> device means there would be a giant mode switch with a lot of if()
conditions sprinkled
> across. Are you OK with that?
Hmm, why a big mode switch?  You should be able to do that without
modifying the virtio-gpu guest driver.  On the host side qemu needs
some work to support the most recent virtio-gpu features like the
buffer uuids (assuming you use qemu userspace), right now those
are only supported by crosvm.

It might be useful to add support for display-less virtio-gpu, i.e.
"qemu -device virtio-gpu-pci,max_outputs=0".  Right now the linux
driver throws an error in case no output (crtc) is present.  Should
be fixable without too much effort though, effectively the sanity
check would have to be moved from driver initialization to commands
like SET_SCANOUT which manage the outputs.

take care,
  Gerd

Hi Gerd,
> > > You don't have to use the rendering pipeline.  You can let
the i915
> > > gpu render into a dma-buf shared with virtio-gpu, then use
> > > virtio-gpu only for buffer sharing with the host.
[Kasireddy, Vivek] Just to confirm my understanding of what you are suggesting,
are
you saying that we need to either have Weston allocate scanout buffers (GBM
surface/BO)
using virtio-gpu and render into them using i915; or have virtio-gpu allocate
pages and
export a dma-buf and have Weston create a GBM BO by calling gbm_bo_import(fd)
and
render into the BO using i915?
> Hmm, why a big mode switch?  You should be able to do that without
modifying the
> virtio-gpu guest driver.  On the host side qemu needs some work to support
the most
> recent virtio-gpu features like the buffer uuids (assuming you use qemu
userspace), right
> now those are only supported by crosvm.
[Kasireddy, Vivek] We are only interested in Qemu UI at the moment but if we
were to use
virtio-gpu, we are going to need to add one more vq and support for managing
buffers,
events, etc.

Thanks,
Vivek
> 
> It might be useful to add support for display-less virtio-gpu, i.e.
> "qemu -device virtio-gpu-pci,max_outputs=0".  Right now the linux
driver throws an error
> in case no output (crtc) is present.  Should be fixable without too much
effort though,
> effectively the sanity check would have to be moved from driver
initialization to
> commands like SET_SCANOUT which manage the outputs.
> 
> take care,
>   Gerd