[PATCH rc v5 1/4] iommu/arm-smmu-v3: Add update_safe bits to fix STE update sequence

[Will Deacon]

On Wed, Jan 07, 2026 at 08:36:46PM -0400, Jason Gunthorpe wrote:
> On Wed, Jan 07, 2026 at 09:20:06PM +0000, Will Deacon wrote:
> > >  drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.h   |  2 ++
> > >  .../iommu/arm/arm-smmu-v3/arm-smmu-v3-test.c  | 18 ++++++++++---
> > >  drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c   | 27 ++++++++++++++-----
> > >  3 files changed, 37 insertions(+), 10 deletions(-)
> > 
> > Hmm. So this appears to ignore the safe bits entirely, whereas the
> > rationale for the change is that going from {MEV,EATS} disabled to
> > enabled is safe (which I agree with). 
> 
> The argument was it doesn't matter for either direction be it disabled
> to enabled or vice versa, see my reply to Mustfa in the v4 posting:
> 
> https://lore.kernel.org/all/20251218180129.GA254720@nvidia.com/

It would be good to include some of that rationale in the comment and
commit message for patch 3, as at the moment it only talks about the
change in one direction.

I'm also still not convinced that this is generally safe, even if it
works within what Linux currently does. For example, if somebody tries
to disable S2S and enable ATS at the same time, couldn't you transiently
get an illegal STE?

Will