Re: [RFC PATCH 0/7] vfio: selftests: Add peer-to-peer DMA selftest

["David Matlack" <dmatlack@google.com>]

On Thu Nov 12, 2026 at 12:35 PM PST, Alex Mastro wrote:
> Hey David,
>
> On Wed, Nov 11, 2026 at 07:24:04PM +0000, David Matlack wrote:
>  
>>     Is requiring userspace to re-issue IOMMU_IOAS_MAP_FILE the expected
>>     uAPI behavior? VFIO_DEVICE_RESET (and runtime PM transitions) on the
>>     target device also bracket the reset with
>>     vfio_pci_dma_buf_move(vdev, true) and vfio_pci_dma_buf_move(vdev,
>>     false), tearing down any active IOMMU_IOAS_MAP_FILE mapping of that
>>     device's BAR even though PCI_COMMAND_MEMORY remains enabled when
>>     VFIO_DEVICE_RESET returns.
>
> There was some discussion about this in the thread below. My understanding of
> Jason's point is that the current behavior, and differences between what
> revoking and move-capable importers see in regards to vfio-exported dma-buf is
> somewhat incidental and not necessarily desirable.

Thanks, I will read through that thread. It seemed like a surprising
uAPI.

>
> https://lore.kernel.org/all/arMBqyI4XNfD+q1W@devgpu015.cco6.facebook.com/
>
>>   - Physical target devices and P2P topology checks: mem_bar_descs[]
>>     currently only matches QEMU's pci-testdev, and
>>     vfio_pci_has_p2p_path() does not yet check PCI topology or ACS to
>>     verify that P2P traffic can flow between two physical endpoints.
>> 
>>     Suggestions on physical target BARs to add to mem_bar_descs[] and on
>>     checking P2P reachability from userspace are welcome.
>
> FWIW, you can also exercise the "P2P" path with just a single device if you
> IOMMU map a device's BAR into it's own IOAS, and have the device DMA itself. For
> eventually validating aspects of Live Update, maybe you don't necessarily care
> if an actual peer device is being DMA'd?

The way I designed the P2P selftest support requires 2 things:

 (A) initiator: A device that has a VFIO selftests driver.
 (B) target: A device that can provide a memory BAR (vfio_pci_find_mem_bar()).

So far I haven't found a single device that can do both. But you are
right that theoretically a single device could satisfy both
requirements.

I can relax vfio_pci_has_p2p_path() to allow initiator == target.