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

[Gerd Hoffmann]

  Hi,

> > +/* extract pages referenced by sgt */
> > +static struct page **extr_pgs(struct sg_table *sgt, int *nents, int *last_len)
> 
> Nack, this doesn't work on dma-buf. And it'll blow up at runtime when you
> enable the very recently merged CONFIG_DMABUF_DEBUG (would be good to test
> with that, just to make sure).

> Aside from this, for virtio/kvm use-cases we've already merged the udmabuf
> driver. Does this not work for your usecase?

udmabuf can be used on the host side to make a collection of guest pages
available as host dmabuf.  It's part of the puzzle, but not a complete
solution.

As I understand it the intended workflow is this:

  (1) guest gpu driver exports some object as dma-buf
  (2) dma-buf is imported into this new driver.
  (3) driver sends the pages to the host.
  (4) hypervisor uses udmabuf to create a host dma-buf.
  (5) host dma-buf is passed on.

And step (3) is the problematic one as this will not
work in case the dma-buf doesn't live in guest ram but
in -- for example -- gpu device memory.

Reversing the driver roles in the guest (virtio driver
allocates pages and exports the dma-buf to the guest
gpu driver) should work fine.

Which btw is something you can do today with virtio-gpu.
Maybe it makes sense to have the option to run virtio-gpu
in render-only mode for that use case.

take care,
  Gerd