On Tue, 8 Sep 2020 09:59:46 -0700 Maran Wilson <maran.wil...@gmail.com> wrote:
> I'm trying to use the vfio-pci driver to pass-through two PCIe endpoint > devices into a VM. On the host, each of these PCIe endpoint devices is in > its own IOMMU group. From inside the VM, I would like to perform P2P DMA > operations. So basically, programming the DMA engine of one of the devices > to write directly to a BAR mapped region of the other device. > > > Is this something that is supported by the vfio driver, working with Qemu? > Are there any VM configuration gotchas I need to keep in mind for this > particular use-case? I'm on an AMD Rome server, FWIW. > > > This works on the host (when I'm not using VMs) with IOMMU disabled. And it > also works on the host with the IOMMU enabled as long as I add the > appropriate IOMMU mapping of the other device's BAR mapped address to the > appropriate IOMMU group. > > > But from what I can tell, when the endpoint devices are passed through to > the VM, it doesn't appear that any IOMMU mappings are created on the host > to translate gPA of the other endpoint's BAR mapped address. But of course > DMA to/from host DRAM does work in that same configuration, so I know IOMMU > mappings are being created to translate gPA of DRAM. As long as we can mmap the endpoint BAR (>= PAGE_SIZE) then the BAR GPA should be mapped through the IOMMU to enable p2p within the VM. You should be able to see this with tracing enabled in QEMU for vfio*. Thanks, Alex _______________________________________________ vfio-users mailing list vfio-users@redhat.com https://www.redhat.com/mailman/listinfo/vfio-users
