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

[Kasireddy, Vivek]

Hi Gerd,

> -----Original Message-----
> From: Gerd Hoffmann <kraxel at redhat.com>
> Sent: Tuesday, February 09, 2021 12:45 AM
> To: Kasireddy, Vivek <vivek.kasireddy at intel.com>
> Cc: Daniel Vetter <daniel at ffwll.ch>; virtualization at lists.linux-foundation.org; dri-
> devel at lists.freedesktop.org; Vetter, Daniel <daniel.vetter at intel.com>;
> daniel.vetter at ffwll.ch; Kim, Dongwon <dongwon.kim at intel.com>;
> sumit.semwal at linaro.org; christian.koenig at amd.com; linux-media at vger.kernel.org
> Subject: Re: [RFC v3 2/3] virtio: Introduce Vdmabuf driver
> 
>   Hi,
> 
> > > > > 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).
> > [Kasireddy, Vivek] Although, I have not tested it yet but it looks like this will
> > throw a wrench in our solution as we use sg_next to iterate over all the struct page *
> > and get their PFNs. I wonder if there is any other clean way to get the PFNs of all
> > the pages associated with a dmabuf.
> 
> Well, there is no guarantee that dma-buf backing storage actually has
> struct page ...
[Kasireddy, Vivek] What if I do mmap() on the fd followed by mlock() or mmap()
followed by get_user_pages()? If it still fails, would ioremapping the device memory
and poking at the backing storage be an option? Or, if I bind the passthrough'd GPU device
to vfio-pci and tap into the memory region associated with the device memory, can it be
made to work? 

And, I noticed that for PFNs that do not have valid struct page associated with it, KVM
does a memremap() to access/map them. Is this an option?

> 
> > [Kasireddy, Vivek] To exclude such cases, would it not be OK to limit the scope
> > of this solution (Vdmabuf) to make it clear that the dma-buf has to live in Guest RAM?
> > Or, are there any ways to pin the dma-buf pages in Guest RAM to make this
> > solution work?
> 
> At that point it becomes (i915) driver-specific.  If you go that route
> it doesn't look that useful to use dma-bufs in the first place ...
[Kasireddy, Vivek] I prefer not to make this driver specific if possible.

> 
> > IIUC, Virtio GPU is used to present a virtual GPU to the Guest and all the rendering
> > commands are captured and forwarded to the Host GPU via Virtio.
> 
> 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] Is this the most viable path forward? I am not sure how complex or 
feasible it would be but I'll look into it.
Also, not using the rendering capabilities of virtio-gpu and turning it into a sharing only
device means there would be a giant mode switch with a lot of if() conditions sprinkled
across. Are you OK with that?

Thanks,
Vivek
> 
> take care,
>   Gerd