[virglrenderer-devel] multiprocess model and GL

[Gerd Hoffmann]

On Mon, Jan 13, 2020 at 01:03:22PM -0800, Chia-I Wu wrote:
> Sorry I missed this email.
> 
> On Thu, Jan 9, 2020 at 12:54 PM Dave Airlie <airlied at gmail.com> wrote:
> >
> > This is just an aside to the issue discussion, but I was wondering
> > before we heavily consider a vulkan multi-process model, could
> > we/should we prove a GL multi-process model first?
> 
> I think there will just be the qemu process at first, with many GL
> contexts and one VkInstance for each guest VkInstance.  Then there
> will be a switch to run different VkInstance in different processes
> (unlikely, but I wonder if this can be a vk layer).  I did not plan
> for a multi-process GL model.  Do you have anything you want to prove
> from it?

Right now we have two models:
  - Everything in qemu.
  - Separate virgl process (see contrib/vhost-user-gpu/ in qemu),
    but still all gl contexts in a single process.

We could try to switch vhost-user-gpu to a one-process-per-context
model.  I think it makes sense to at least think about the resource
management implications this would have (it would make virgl work
simliar to vulkan):

 - We would need a master process.  It runs the virtqueues and manages
   the resources.
 - We would need a per-context process.
 - VIRTIO_GPU_CMD_CTX_ATTACH_RESOURCE would make master dma-buf export a
   resource and the per-context process import it.  Sharing resources
   works by calling VIRTIO_GPU_CMD_CTX_ATTACH_RESOURCE multipe times for
   different contexts, and therefore master passing the dma-buf to
   multiple per-context processes.

With vulkan resources would be created by the per-context process
instead, then exported and imported by the master process.  The master
process can allow guest access then (TRANSFER virtio commands, map into
address space, ...).  It could also pass it on to other per-context
processes, so they can import too, for sharing.

This also makes it pretty obvious how sharing between virgl and vulkan
would work:  Just use VIRTIO_GPU_CMD_CTX_ATTACH_RESOURCE + dma-bufs ...

cheers,
  Gerd