Re: [RFC PATCH 0/3] iommu/arm-smmu-v3: Add support for hitless replace of CD entries

[David Matlack <dmatlack@google.com>]

On 2026-09-25 01:13 AM, Samiullah Khawaja wrote:
> This patch series adds support for replacing IOMMU domains with S1 page
> tables hitlessly, that is without disrupting the ongoing DMAs. This is
> one of the dependencies of the Liveupdate IOMMU support for Arm SMMUv3.
> 
> S1 domains are programmed by CD entries in the CD table. During a new
> domain attach these are updated using arm_smmu_write_cd_entry(). Since
> ASID and TTB0 are in different 64-bit words, arm_smmu_write_cd_entry()
> considers it a non-hitless update and sets V=0. This means such an
> update is disruptive and would generate translation faults if there are
> ongoing DMAs.
> 
> Replace the CD entries by using an unused temporary ASID in the
> following sequence to allow hitless replacement of CD entry,
> 
> - Update the CD entry with temporary ASID.
> - Update the CD entry with the target TTB0.
> - Invalidate the temporary ASID.
> - Update the CD entry with target ASID.
> - Invalidate and release the temporary ASID.
> 
> The series adds a vfio selftest that triggers a continuous DMA and does
> the hwpt replace while the DMA is ongoing. Since domain replacement is a
> race with the translation requests, the test does it multiple times to
> capture the small window where IOMMU faults can occur.
> 
> Without this change we can observe the SMMU faults in the kernel logs
> and with it the SMMU faults do not occur.
> 
> This is only tested on qemu with the emulated SMMUv3 and an out of tree
> NVMe vfio selftest driver, not on real hardware. Following are not
> covered and can be done later,

Out of curiousity, does the in-tree VFIO selftests driver for IGB not
work for this testing scenario?