Re: [PATCH v2 1/3] iommu/arm-smmu: Add support for driver IOMMU fault handlers

[Robin Murphy <robin.murphy@arm.com>]

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.

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
>>>
>