On Thu, 2025-09-25 at 03:56 +0000, Kasireddy, Vivek wrote: > Hi Christian, > > > Subject: Re: [PATCH v4 1/5] PCI/P2PDMA: Don't enforce ACS check for > > device > > functions of Intel GPUs > > > > > > > > > > On 23.09.25 15:38, Jason Gunthorpe wrote: > > > > > On Tue, Sep 23, 2025 at 03:28:53PM +0200, Christian König > > > > > wrote: > > > > > > On 23.09.25 15:12, Jason Gunthorpe wrote: > > > > > > > > When you want to communicate addresses in a device > > > > > > > > specific > > address > > > > > > > > space you need a device specific type for that and not > > > > > > > > abuse > > > > > > > > phys_addr_t. > > > > > > > > > > > > > > I'm not talking about abusing phys_addr_t, I'm talking > > > > > > > about putting a > > > > > > > legitimate CPU address in there. > > > > > > > > > > > > > > You can argue it is hack in Xe to reverse engineer the > > > > > > > VRAM offset > > > > > > > from a CPU physical, and I would be sympathetic, but it > > > > > > > does allow > > > > > > > VFIO to be general not specialized to Xe. > > > > > > > > > > > > No, exactly that doesn't work for all use cases. That's why > > > > > > I'm > > > > > > pushing back so hard on using phys_addr_t or CPU addresses. > > > > > > > > > > > > See the CPU address is only valid temporary because the VF > > > > > > BAR is > > > > > > only a window into the device memory. > > > > > > > > > > I know, generally yes. > > > > > > > > > > But there should be no way that a VFIO VF driver in the > > > > > hypervisor > > > > > knows what is currently mapped to the VF's BAR. The only way > > > > > I can > > > > > make sense of what Xe is doing here is if the VF BAR is a > > > > > static > > > > > aperture of the VRAM.. > > > > > > > > > > Would be nice to know the details. > > > > > > > > Yeah, that's why i asked how VFIO gets the information which > > > > parts of the > > > > it's BAR should be part of the DMA-buf? > > > > > > > > That would be really interesting to know. > > > As Jason guessed, we are relying on the GPU VF being a Large BAR > > > device here. In other words, as you suggested, this will not work > > > if the > > > VF BAR size is not as big as its actual VRAM portion. We can > > > certainly add > > > this check but we have not seen either the GPU PF or VF getting > > > detected > > > as Small BAR devices in various test environments. > > > > > > So, given the above, once a VF device is bound to vfio-pci driver > > > and > > > assigned to a Guest VM (launched via Qemu), Qemu's vfio layer > > > maps > > > all the VF's resources including the BARs. This mapping info > > > (specifically > > HVA) > > > is leveraged (by Qemu) to identity the offset at which the Guest > > > VM's buffer > > > is located (in the BAR) and this info is then provided to vfio- > > > pci kernel driver > > > which finally creates the dmabuf (with BAR Addresses). > > > > In that case I strongly suggest to add a private DMA-buf interface > > for the DMA- > > bufs exported by vfio-pci which returns which BAR and offset the > > DMA-buf > > represents.
@Christian, Is what you're referring to here the "dma_buf private interconnect" we've been discussing previously, now only between vfio- pci and any interested importers instead of private to a known exporter and importer? If so I have a POC I can post as an RFC on a way to negotiate such an interconnect. > Does this private dmabuf interface already exist or does it need to > be created > from the ground up? > > If it already exists, could you please share an example/reference of > how you > have used it with amdgpu or other drivers? > > If it doesn't exist, I was wondering if it should be based on any > particular best > practices/ideas (or design patterns) that already exist in other > drivers? @Vivek, another question: Also on the guest side we're exporting dma- mapped adresses that are imported and somehow decoded by the guest virtio-gpu driver? Is something similar needed there? Also how would the guest side VF driver know that what is assumed to be a PF on the same device is actually a PF on the same device and not a completely different device with another driver? (In which case I assume it would like to export a system dma-buf)? Thanks, Thomas > > > > > Ideally using the same structure Qemu used to provide the offset to > > the vfio- > > pci driver, but not a must have. > > > > This way the driver for the GPU PF (XE) can leverage this > > interface, validates > > that the DMA-buf comes from a VF it feels responsible for and do > > the math to > > figure out in which parts of the VRAM needs to be accessed to > > scanout the > > picture. > Sounds good. This is definitely a viable path forward and it looks > like we are all > in agreement with this idea. > > I guess we can start exploring how to implement the private dmabuf > interface > mechanism right away. > > Thanks, > Vivek > > > > > This way this private vfio-pci interface can also be used by > > iommufd for > > example. > > > > Regards, > > Christian. > > > > > > > > Thanks, > > > Vivek > > > > > > > > > > > Regards, > > > > Christian. > > > > > > > > > > > > > > > What Simona agreed on is exactly what I proposed as well, > > > > > > that you > > > > > > get a private interface for exactly that use case. > > > > > > > > > > A "private" interface to exchange phys_addr_t between at > > > > > least > > > > > VFIO/KVM/iommufd - sure no complaint with that. > > > > > > > > > > Jason > > > >
