Messages in this thread | | | Subject | Re: [PATCH v2 1/3] iommu/arm-smmu: Add support for driver IOMMU fault handlers | From | Robin Murphy <> | Date | Tue, 26 Jan 2021 16:31:36 +0000 |
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On 2021-01-26 16:05, Rob Clark wrote: > On Tue, Jan 26, 2021 at 3:41 AM Robin Murphy <robin.murphy@arm.com> wrote: >> >> On 2021-01-25 21:51, Jordan Crouse wrote: >>> On Fri, Jan 22, 2021 at 12:53:17PM +0000, Robin Murphy wrote: >>>> On 2021-01-22 12:41, Will Deacon wrote: >>>>> On Tue, Nov 24, 2020 at 12:15:58PM -0700, Jordan Crouse wrote: >>>>>> Call report_iommu_fault() to allow upper-level drivers to register their >>>>>> own fault handlers. >>>>>> >>>>>> Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org> >>>>>> --- >>>>>> >>>>>> drivers/iommu/arm/arm-smmu/arm-smmu.c | 16 +++++++++++++--- >>>>>> 1 file changed, 13 insertions(+), 3 deletions(-) >>>>>> >>>>>> diff --git a/drivers/iommu/arm/arm-smmu/arm-smmu.c b/drivers/iommu/arm/arm-smmu/arm-smmu.c >>>>>> index 0f28a8614da3..7fd18bbda8f5 100644 >>>>>> --- a/drivers/iommu/arm/arm-smmu/arm-smmu.c >>>>>> +++ b/drivers/iommu/arm/arm-smmu/arm-smmu.c >>>>>> @@ -427,6 +427,7 @@ static irqreturn_t arm_smmu_context_fault(int irq, void *dev) >>>>>> struct arm_smmu_domain *smmu_domain = to_smmu_domain(domain); >>>>>> struct arm_smmu_device *smmu = smmu_domain->smmu; >>>>>> int idx = smmu_domain->cfg.cbndx; >>>>>> + int ret; >>>>>> fsr = arm_smmu_cb_read(smmu, idx, ARM_SMMU_CB_FSR); >>>>>> if (!(fsr & ARM_SMMU_FSR_FAULT)) >>>>>> @@ -436,11 +437,20 @@ static irqreturn_t arm_smmu_context_fault(int irq, void *dev) >>>>>> iova = arm_smmu_cb_readq(smmu, idx, ARM_SMMU_CB_FAR); >>>>>> cbfrsynra = arm_smmu_gr1_read(smmu, ARM_SMMU_GR1_CBFRSYNRA(idx)); >>>>>> - dev_err_ratelimited(smmu->dev, >>>>>> - "Unhandled context fault: fsr=0x%x, iova=0x%08lx, fsynr=0x%x, cbfrsynra=0x%x, cb=%d\n", >>>>>> + ret = report_iommu_fault(domain, dev, iova, >>>>>> + fsynr & ARM_SMMU_FSYNR0_WNR ? IOMMU_FAULT_WRITE : IOMMU_FAULT_READ); >>>>>> + >>>>>> + if (ret == -ENOSYS) >>>>>> + dev_err_ratelimited(smmu->dev, >>>>>> + "Unhandled context fault: fsr=0x%x, iova=0x%08lx, fsynr=0x%x, cbfrsynra=0x%x, cb=%d\n", >>>>>> fsr, iova, fsynr, cbfrsynra, idx); >>>>>> - arm_smmu_cb_write(smmu, idx, ARM_SMMU_CB_FSR, fsr); >>>>>> + /* >>>>>> + * If the iommu fault returns an error (except -ENOSYS) then assume that >>>>>> + * they will handle resuming on their own >>>>>> + */ >>>>>> + if (!ret || ret == -ENOSYS) >>>>>> + arm_smmu_cb_write(smmu, idx, ARM_SMMU_CB_FSR, fsr); >>>>> >>>>> Hmm, I don't grok this part. If the fault handler returned an error and >>>>> we don't clear the FSR, won't we just re-take the irq immediately? >>>> >>>> If we don't touch the FSR at all, yes. Even if we clear the fault indicator >>>> bits, the interrupt *might* remain asserted until a stalled transaction is >>>> actually resolved - that's that lovely IMP-DEF corner. >>>> >>>> Robin. >>>> >>> >>> This is for stall-on-fault. The idea is that if the developer chooses to do so >>> we would stall the GPU after a fault long enough to take a picture of it with >>> devcoredump and then release the FSR. Since we can't take the devcoredump from >>> the interrupt handler we schedule it in a worker and then return an error >>> to let the main handler know that we'll come back around clear the FSR later >>> when we are done. >> >> Sure, but clearing FSR is not writing to RESUME to resolve the stalled >> transaction(s). You can already snarf the FSR contents from your >> report_iommu_fault() handler if you want to, so either way I don't see >> what's gained by not clearing it as expected at the point where we've >> handled the *interrupt*, even if it will take longer to decide what to >> do with the underlying *fault* that it signalled. I'm particularly not >> keen on having unusual behaviour in the core interrupt handling which >> callers may unwittingly trigger, for the sake of one >> very-very-driver-specific flow having a slightly richer debugging >> experience. > > Tbf, "slightly" is an understatement.. it is a big enough improvement > that I've hacked up deferred resume several times to debug various > issues. ;-)
Oh, fear not, I fully appreciate that keeping the GPU stalled on a faulting transaction is a game-changer in itself ("almost like a real MMU!"). That comment was only aimed at whatever the perceived benefits are of deliberately not trying to clear the SMMU interrupt (even if it *would* stay clear). I have no issue with calling report_iommu_fault(), I'm just wary of doing anything weird with the result.
> (Which is always a bit of a PITA because of things moving around in > arm-smmu as well as the drm side of things.) > > But from my recollection, we can clear FSR immediately, all we need to > do is defer writing ARM_SMMU_CB_RESUME
Phew! Thanks for the reassurance :)
Robin.
> > BR, > -R > >> >> For actually *handling* faults, I thought we were going to need to hook >> up the new IOPF fault queue stuff anyway? >> >> Robin. >> >>> It is assumed that we'll have to turn off interrupts in our handler to allow >>> this to work. Its all very implementation specific, but then again we're >>> assuming that if you want to do this then you know what you are doing. >>> >>> In that spirit the error that skips the FSR should probably be something >>> specific instead of "all errors" - that way a well meaning handler that returns >>> a -EINVAL doesn't accidentally break itself. >>> >>> Jordan >>> >>>>> I think >>>>> it would be better to do this unconditionally, and print the "Unhandled >>>>> context fault" message for any non-zero value of ret. >>> >>>>> >>>>> Will >>>>> >>> >> _______________________________________________ >> iommu mailing list >> iommu@lists.linux-foundation.org >> https://lists.linuxfoundation.org/mailman/listinfo/iommu
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