About new backend for GPU compute ROCm in qemu

Hi Michael, Alex, Dmitry, Akihiko,

I'm bringing AMD GPU compute ROCm based on virtio. I posted a ROCm over 
virtio
implementation to virglrenderer nine months ago (MR !1568 [1]). The ROCm 
side has
been supportted by ROCm offical.

Current implementation is a virtio gpu context type capset handled inside
virglrenderer, sharing the display path. That's an awkward fit, many
compute GPUs have no display engine at all.

Beyond that, sharing the display path is increasingly painful:

   - Compute hammers the queues more than graphics, so sharing
     virtio gpu's single control queue with display/virgl causes contention
     and display stutter.
   - Compute contexts need far more blob / shared memory than a display one.
   - Maybe needs a wider ROCm / compute stack, cause the render model 
fits poorly:
     rocprofiler (PC sampling, SQTT/SPM, counters, high bandwidth streams)
     and ROCgdb (wave control, address watch, async exceptions an
     out of band channel that must not block display).
   - Events, faults and GPU reset/SMI are async and don't map onto fences.
   - All of this is hard to extend cleanly inside a display capset.

On the QEMU/host side, would something like this be OK? One step, two parts:

   - a dedicated headless virtio gpu instance for compute.
   - that instance served by a separate ROCm backend library loaded
     in-process by QEMU.

That reuses the existing pluggable backend model, a second virtio gpu + a
backend library. It doesn't add dedicated queues for debug/profiling 
currently.

Waiting for reply and  happy to share more detail. Thanks!

[1] https://gitlab.freedesktop.org/virgl/virglrenderer/-/merge_requests/1568

Regards,
Honglei