On Tue, Sep 8, 2020 at 10:22 AM Alex Williamson <alex.william...@redhat.com> wrote:
> 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 > Thanks Alex. Does it require the endpoints having a common root port inside the VM? Or does that part not matter? If you happen to know the routine name or two in the driver and/or qemu that handles this, it would help me get my bearings sooner and allow me to instrument from the kernel side too to see why my experiment is not working. > should be able to see this with tracing enabled in QEMU for vfio*. > I will try that too, thanks! By the way, is this functionality present as far back as the 4.15 kernel? -Maran > Thanks, > > Alex > >
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