Re: [BUG?] vfio/pci: VA alignment sensitivity of VFIO_IOMMU_MAP_DMA which target MMIO

[Alex Mastro <amastro@fb.com>]

Hi Jason,

By the way, we have been following progress on IOMMUFD, and would be interested
in dogfooding it for our use case when ready. The main blocker is IOMMUFD's
current lack of P2P support (IOMMU_IOAS_MAP fails when the VA range is backed
by MMIO).

dma-buf as a less ambiguous semantic for communicating this intent (rather than
the struggles of inferring what kind of memory is behind some VA range) makes a
lot of sense.

Based on tidbits we have gleaned, IOMMUFD P2P support intends to be built on
top of "Provide a new two step DMA mapping API" [1] and "vfio/pci: Allow MMIO
regions to be exported through dma-buf" [2].

Item [2] appears to have been picked up by "Host side (KVM/VFIO/IOMMUFD) support
for TDISP using TSM" [3].

Is the above understanding correct?

On top of this, there would need to be a new IOMMUFD uapi, or extension to
existing, which would accept an input dma-buf to map. Are there any patches in
progress which include this?

[1] https://lore.kernel.org/all/cover.1746424934.git.leon@kernel.org/
[2] https://lore.kernel.org/all/20250307052248.405803-1-vivek.kasireddy@intel.com
[3] https://lore.kernel.org/all/20250529053513.1592088-1-yilun.xu@linux.intel.com

Thanks,
Alex