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 01:20:45PM -0700, Rob Clark wrote:
> On Wed, Mar 12, 2025 at 11:01 AM Robin Murphy <robin.murphy@arm.com> wrote:
> >
> > On 12/03/2025 5:23 pm, Rob Clark wrote:
> > > On Wed, Mar 12, 2025 at 9:47 AM Will Deacon <will@kernel.org> wrote:
> > >>
> > >> 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.
> > >
> > > Sure, but no one else is using this field in the fault-info.  So maybe
> > > the addition of a comment in the struct would be enough if it isn't
> > > going to cause an SError/etc to read it for S2 cb?
> >
> > Any worthwhile comment isn't going to be significantly shorter or
> > clearer than 1 extra line of "if (smmu_domain->stage ==
> > ARM_SMMU_DOMAIN_S1)"...
> 
> Just that smmu_domain isn't passed to
> arm_smmu_read_context_fault_info(), so it would add some extra churn
> on top of that one extra line

Churn is what we do best in this driver :)

Will