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

[Samiullah Khawaja]

On Fri, Sep 25, 2026 at 09:44:05PM +0000, David Matlack wrote:
> 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?

I tried with the IGB driver first, but I couldn't get the DMAs to work.
Its probably my setup, will recheck.

Sami