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

[Jason Gunthorpe <jgg@ziepe.ca>]

On Fri, Oct 23, 2026 at 01:10:56PM +0100, Robin Murphy wrote:
> On 25/09/2026 2:13 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.
> 
> Why bother transitioning through a 3rd ASID? The old and new pagetables must
> contain the same translations anyway, so why not just: switch ASID, switch
> TTB, invalidate old ASID?

The target ASID may be in use by another CD someplace else. Doing it
without the temporary could contaminate it with wrong translations
from the original TTB0.

From a driver perspective we don't know why the domain replace
operation was triggered. iommufd can trigger it for domains that are
not identical. Corrupting the target ASID when we don't know for sure
what is happening is not safe.

Obviously Samiullah's use case is hitless between the same translation
so it doesn't need the extra ASID, but the driver can't know this.

Samiullah please include this explanation in a comment..

Jason