Re: [RFC PATCH v1 10/38] iommufd/vdevice: Add TSM map ioctl

[Jason Gunthorpe <jgg@ziepe.ca>]

On Mon, Aug 04, 2025 at 08:02:23AM +0530, Alexey Kardashevskiy wrote:
> I ended up exporting the guestmemfd's kvm_gmem_get_folio() for gfn->pfn and its fd a bit differently in iommufd - "no extra referencing":
> https://github.com/AMDESE/linux-kvm/commit/f1ebd358327f026f413f8d3d64d46decfd6ab7f6

This patch needs to explain how the lifecylce works and why the IOMMU
can't UAF the memory. I think it cannot really work as shown without
more things like an invalidation callback.

IOW you need to define for how long the return result of
guest_memfd_get_pfn() is valid for.

> > I was kind of thinking it would be nice to have a guestmemfd mode that
> > was "pinned", meaning the memory is allocated and remains almost
> > always mapped into the TSM's page tables automatically. VFIO using
> > guests would set things this way.
> 
> Yeah while doing the above, I was wondering if I want to pass the fd
> type when DMA-mapping from an fd or "detect" it as I do in the above
> commit or have some iommufd_fdmap_ops in this fd saying "(no)
> pinning needed" (or make this a flag of IOMMU_IOAS_MAP_FILE).

It should be autodetected.

Since this is unique behavior for guestmemfd it is fine to start
there..

> btw in the AMD case, here it does not matter as much if it is
> private or shared, I map everything and let RMP and the VM deal with
> the permissions. Thanks,

I think ARM would like to do this as well.

Hence my suggestion that guestmemfd could just have unchanging 1G PFNs
and all shared/private changes have no impact on iommufd.

If so likely all this needs is an invalidation callback from
guestmemfd to iommufd to revoke on ftruncate.

Jason