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

Alexey Kardashevskiy
2025-08-04T02:32:34Z

On 28/7/25 19:47, Jason Gunthorpe wrote:
> On Mon, Jul 28, 2025 at 07:21:47PM +0530, Aneesh Kumar K.V (Arm) wrote:
>> With passthrough devices, we need to make sure private memory is
>> allocated and assigned to the secure guest before we can issue the DMA.
>> For ARM RMM, we only need to map and the secure SMMU management is
>> internal to RMM. For shared IPA, vfio/iommufd DMA MAP/UNMAP interface
>> does the equivalent
>
> I'm not really sure what this is about? It is about getting KVM to pin
> all the memory and commit it to the RMM so it can be used for DMA?
>
> But it looks really strange to have an iommufd ioctl that just calls a
> KVM function. Feeling this should be a KVM function, or a guestmfd
> behavior??

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

It is a new iommufd->kvm dependency though.

> 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).

The "detection" is (mapping_inaccessible(mapping) && mapping_unevictable(mapping)), works for now.

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,

>
> Jason

--
Alexey