Re: [RFC PATCH 3/3] iommu/arm-smmu-v3: Add support for hitless replace of S1 domains

[Jason Gunthorpe]

On Fri, Oct 23, 2026 at 02:43:28PM +0100, Robin Murphy wrote:
> > @@ -3664,8 +3726,8 @@ static int arm_smmu_attach_dev(struct iommu_domain *domain, struct device *dev,
> >   		struct arm_smmu_cd target_cd;
> >   		arm_smmu_make_s1_cd(&target_cd, master, smmu_domain);
> > -		arm_smmu_write_cd_entry(master, IOMMU_NO_PASID, cdptr,
> > -					&target_cd);
> > +		arm_smmu_replace_cd_entry(master, IOMMU_NO_PASID, cdptr,
> > +					  &target_cd);
> 
> No, we cannot blindly do this for arbitrary ->attach_dev calls - the
> majority of S1->S1 transitions are going to be moving back and forth between
> the default domain and user domains, where there is zero expectation, let
> alone guarantee, that the translations would be equivalent and thus not
> liable to lead to TLB conflict aborts or amalgamation hell.

Heh, yeah this probably only works properly with BBML2 or something so
the temporary ASID doesn't explode.

Jason