* Gerd Hoffmann (kraxel@xxxxxxxxxx) wrote: > Hi, > > > > This is not about host memory, buffers are in guest ram, everything else > > > would make sharing those buffers between drivers inside the guest (as > > > dma-buf) quite difficult. > > > > Given it's just guest memory, can the guest just have a virt queue on > > which it places pointers to the memory it wants to share as elements in > > the queue? > > Well, good question. I'm actually wondering what the best approach is > to handle long-living, large buffers in virtio ... > > virtio-blk (and others) are using the approach you describe. They put a > pointer to the io request header, followed by pointer(s) to the io > buffers directly into the virtqueue. That works great with storage for > example. The queue entries are tagged being "in" or "out" (driver to > device or visa-versa), so the virtio transport can set up dma mappings > accordingly or even transparently copy data if needed. > > For long-living buffers where data can potentially flow both ways this > model doesn't fit very well though. So what virtio-gpu does instead is > transferring the scatter list as virtio payload. Does feel a bit > unclean as it doesn't really fit the virtio architecture. It assumes > the host can directly access guest memory for example (which is usually > the case but explicitly not required by virtio). It also requires > quirks in virtio-gpu to handle VIRTIO_F_IOMMU_PLATFORM properly, which > in theory should be handled fully transparently by the virtio-pci > transport. > > We could instead have a "create-buffer" command which adds the buffer > pointers as elements to the virtqueue as you describe. Then simply > continue using the buffer even after completing the "create-buffer" > command. Which isn't exactly clean either. It would likewise assume > direct access to guest memory, and it would likewise need quirks for > VIRTIO_F_IOMMU_PLATFORM as the virtio-pci transport tears down the dma > mappings for the virtqueue entries after command completion. > > Comments, suggestions, ideas? What about not completing the command while the device is using the memory? Dave > cheers, > Gerd > -- Dr. David Alan Gilbert / dgilbert@xxxxxxxxxx / Manchester, UK
