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. Thanks, Maran
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