RE: [RFC v3 2/3] virtio: Introduce Vdmabuf driver

["Kasireddy, Vivek" <vivek.kasireddy@intel.com>]

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

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