Re: [PATCH v4 1/5] iommu/arm-smmu: Save additional information on context fault

[Will Deacon <will@kernel.org>]

On Wed, Mar 12, 2025 at 07:59:52AM -0700, Rob Clark wrote:
> On Wed, Mar 12, 2025 at 6:05 AM Will Deacon <will@kernel.org> wrote:
> > On Tue, Mar 11, 2025 at 06:36:38PM -0400, Connor Abbott wrote:
> > > On Tue, Mar 11, 2025 at 2:06 PM Will Deacon <will@kernel.org> wrote:
> > > > On Tue, Mar 04, 2025 at 11:56:47AM -0500, Connor Abbott wrote:
> > > > > diff --git a/drivers/iommu/arm/arm-smmu/arm-smmu.c b/drivers/iommu/arm/arm-smmu/arm-smmu.c
> > > > > index ade4684c14c9b2724a71e2457288dbfaf7562c83..a9213e0f1579d1e3be0bfba75eea1d5de23117de 100644
> > > > > --- a/drivers/iommu/arm/arm-smmu/arm-smmu.c
> > > > > +++ b/drivers/iommu/arm/arm-smmu/arm-smmu.c
> > > > > @@ -409,9 +409,12 @@ void arm_smmu_read_context_fault_info(struct arm_smmu_device *smmu, int idx,
> > > > >                                     struct arm_smmu_context_fault_info *cfi)
> > > > >  {
> > > > >       cfi->iova = arm_smmu_cb_readq(smmu, idx, ARM_SMMU_CB_FAR);
> > > > > +     cfi->ttbr0 = arm_smmu_cb_readq(smmu, idx, ARM_SMMU_CB_TTBR0);
> > > > >       cfi->fsr = arm_smmu_cb_read(smmu, idx, ARM_SMMU_CB_FSR);
> > > > > -     cfi->fsynr = arm_smmu_cb_read(smmu, idx, ARM_SMMU_CB_FSYNR0);
> > > > > +     cfi->fsynr0 = arm_smmu_cb_read(smmu, idx, ARM_SMMU_CB_FSYNR0);
> > > > > +     cfi->fsynr1 = arm_smmu_cb_read(smmu, idx, ARM_SMMU_CB_FSYNR1);
> > > >
> > > > We already have an implementation hook (->get_fault_info()) which the
> > > > qcom SMMU driver can override with qcom_adreno_smmu_get_fault_info().
> > > > That thing dumps these registers already so if we're moving that into
> > > > the core SMMU driver, let's get rid of the hook and move everybody over
> > > > rather than having it done in both places.
> > >
> > > As you probably saw, the next commit moves over
> > > qcom_adreno_smmu_get_fault_info() to use this. The current back door
> > > used by drm/msm to access these functions is specific to adreno_smmu
> > > and there isn't an equivalent interface to allow it to call a generic
> > > SMMU function so it isn't possible to move it entirely to the core. At
> > > least not without a bigger refactoring that isn't justified for this
> > > series that is just trying to fix things.
> >
> > Ok :(
> >
> > > > >       cfi->cbfrsynra = arm_smmu_gr1_read(smmu, ARM_SMMU_GR1_CBFRSYNRA(idx));
> > > > > +     cfi->contextidr = arm_smmu_cb_read(smmu, idx, ARM_SMMU_CB_CONTEXTIDR);
> > > >
> > > > I think the CONTEXTIDR register is stage-1 only, so we shouldn't dump
> > > > it for stage-2 domains.
> > > >
> > > Does it matter if we read the register though, as long as users are
> > > aware of this and don't use its value for anything?
> >
> > I think the contents are "UNKNOWN", so it could be hugely confusing even
> > if they just got logged someplace. Why is it difficult to avoid touching
> > it for stage-2?
> >
> Fwiw, we are only ever using stage-1

Sure, but this is in arm-smmu.c which is used by other people and supports
both stages.

Will