Hi Gerd, > -----Original Message----- > From: Gerd Hoffmann <kraxel@xxxxxxxxxx> > Sent: Tuesday, February 09, 2021 12:45 AM > To: Kasireddy, Vivek <vivek.kasireddy@xxxxxxxxx> > Cc: Daniel Vetter <daniel@xxxxxxxx>; virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx; dri- > devel@xxxxxxxxxxxxxxxxxxxxx; Vetter, Daniel <daniel.vetter@xxxxxxxxx>; > daniel.vetter@xxxxxxxx; Kim, Dongwon <dongwon.kim@xxxxxxxxx>; > sumit.semwal@xxxxxxxxxx; christian.koenig@xxxxxxx; linux-media@xxxxxxxxxxxxxxx > 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
