Re: [RFC PATCH 00/11] iommu/riscv: Add hardware dirty tracking for second-stage domains

On Tue, Apr 28, 2026 at 09:13:48PM +0800, [email protected] wrote:
> From: Fangyu Yu <[email protected]>
> 
> The RISC-V IOMMU architecture defines an AMO_HWAD capability (Hardware
> Access/Dirty update) that allows the IOMMU to atomically set the A/D bits
> in second-stage PTEs on DMA access.  When DC.tc.GADE is asserted, the IOMMU
> autonomously sets D on the first write to a page mapped by an iohgatp
> domain.  This series wires that capability up to the iommufd dirty-tracking
> interface (IOMMU_HWPT_SET_DIRTY_TRACKING / IOMMU_HWPT_GET_DIRTY_BITMAP) and
> reports IOMMU_CAP_DIRTY_TRACKING.
> 
> Design notes
> ------------
> 
> * The feature is scoped to second-stage (iohgatp) domains only; these are
>   the domains created for KVM / VFIO device pass-through when userspace
>   allocates an HWPT with IOMMU_HWPT_ALLOC_NEST_PARENT or
>   IOMMU_HWPT_ALLOC_DIRTY_TRACKING.  First-stage (iosatp) domains are not
>   touched by this series.
> 
> * The page-table side plugs into the existing generic_pt dirty hook
>   framework (amdv1 / vtdss style).  RISC-V adds the three required PTE
>   ops – is_write_dirty / make_write_clean / make_write_dirty.
> 
> Testing
> -------
> 
> * Test on QEMU RISC-V, a virtio-net and an e1000e device was passed through
>   to an L2 guest via vfio-pci + iommufd.
> 
> * generic_pt KUnit: the existing test_dirty case now runs and passes for
>   the RISC-V 64-bit format.
> 
> Follow-up work
> --------------
> * Build a dedicated end-to-end test case that drives the full flow
>   (HWPT_ALLOC with DIRTY_TRACKING -> attach -> IOAS_MAP -> generate real
>   DMA -> SET_DIRTY_TRACKING -> GET_DIRTY_BITMAP -> verify bitmap against
>   expected IOVA footprint) so that the behaviour can be regression-tested
>   beyond the KUnit PTE-level coverage.
> 
> * If possible, rebase and retest on top of the updated "iommu irqbypass"
>   patchset.

Thanks for this series! I was starting to go down a similar road myself
in order to limit irqbypass to IOMMU_HWPT_ALLOC_NEST_PARENT domains since
I wasn't happy with other approaches, e.g. continuing to use s-stage, but
activating g-stage too with identity mappings since the MSI table can't be
activated otherwise. Or, simply using g-stage instead of s-stage in order
to get the MSI table enabled. In the end, I think the best is to require
nested for irqbypass and this series will provide a good base for that.

I'll rebase irqbypass on this series and test it out.

Thanks,
drew

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