[virglrenderer-devel] coherent memory access for virgl

[Gerd Hoffmann]

> > Well, udmabuf can be used to get a linear mapping of a scattered
> > resource.  Now qemu can map that udmabuf, the pass on the one-elem-iov
> > to virglrenderer.  Or qemu can pass the udmabuf handle to virglrenderer.
> > 
> > The later has the advantage that virglrenderer can possibly do more with
> > the dmabuf than simply mmap()ing it (specifically let the gpu driver
> > import it).  The former has the advantage that it works without
> > virglrenderer changes.

> So at Collabora, we are also looking at reducing copies during texture uploads.
> We were thinking about the second conclusion: "qemu can pass the udmabuf handle to virglrenderer"
> and I'd like to recapitulate what has been discussed here on this regard.

> My understanding is that we'd require that the VMM allocates all the
> memory of the machine in a single memfd region.

Can be multiple memfd regions.  Otherwise correct.

> Then, when a client in the guest requests a texture upload, Mesa would create a virtio-gpu resource
> to wrap the buffer that the client passed with the texture data.
> virtio-gpu would pass the physical addresses backing that buffer to the virtio queue message
> for creating a buffer or attaching memory to it.
> When the VMM processes the message, it would create a dmabuf via udmabuf out of those physical addresses.

Yes.

> That dmabuf can then be mapped and the address can be directly passed to read_transfer_data in virglrenderer.

Yes.

> Did I get that right? If we agree with a plan, we will be happy to help with the implementation or the code review.

Some experimental code is there already:

  https://git.kraxel.org/cgit/linux/log/?h=drm-virtio-ttm
  https://git.kraxel.org/cgit/qemu/log/?h=sirius/virtio-api

Take care, both branches are moving targets as I'm busy hashing out the
virtio protocol changes needed.  The current way (simply adding flags)
will most likely not stay that way.

Also I havn't touched 3d mode and virglrenderer yet, right now I'm
testing with dumb gem objects in 2d mode.

cheers,
  Gerd