Re: [RFC PATCH v9 5/5] iommu/vt-d: don't loop for timeout ATS Invalidation request forever

[Ethan Zhao <haifeng.zhao@linux.intel.com>]

On 12/28/2023 4:38 PM, Tian, Kevin wrote:
>> From: Ethan Zhao <haifeng.zhao@linux.intel.com>
>> Sent: Thursday, December 28, 2023 8:17 AM
>>
>> When the ATS Invalidation request timeout happens, the qi_submit_sync()
>> will restart and loop for the invalidation request forever till it is
>> done, it will block another Invalidation thread such as the fq_timer
>> to issue invalidation request, cause the system lockup as following
>>
>> [exception RIP: native_queued_spin_lock_slowpath+92]
>>
>> RIP: ffffffffa9d1025c RSP: ffffb202f268cdc8 RFLAGS: 00000002
>>
>> RAX: 0000000000000101 RBX: ffffffffab36c2a0 RCX: 0000000000000000
>>
>> RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffffffffab36c2a0
>>
>> RBP: ffffffffab36c2a0 R8: 0000000000000001 R9: 0000000000000000
>>
>> R10: 0000000000000010 R11: 0000000000000018 R12: 0000000000000000
>>
>> R13: 0000000000000004 R14: ffff9e10d71b1c88 R15: ffff9e10d71b1980
>>
>> ORIG_RAX: ffffffffffffffff CS: 0010 SS: 0018
>>
>> #12 [ffffb202f268cdc8] native_queued_spin_lock_slowpath at
>> ffffffffa9d1025c
>>
>> #13 [ffffb202f268cdc8] do_raw_spin_lock at ffffffffa9d121f1
>>
>> #14 [ffffb202f268cdd8] _raw_spin_lock_irqsave at ffffffffaa51795b
>>
>> #15 [ffffb202f268cdf8] iommu_flush_dev_iotlb at ffffffffaa20df48
>>
>> #16 [ffffb202f268ce28] iommu_flush_iova at ffffffffaa20e182
>>
>> #17 [ffffb202f268ce60] iova_domain_flush at ffffffffaa220e27
>>
>> #18 [ffffb202f268ce70] fq_flush_timeout at ffffffffaa221c9d
>>
>> #19 [ffffb202f268cea8] call_timer_fn at ffffffffa9d46661
>>
>> #20 [ffffb202f268cf08] run_timer_softirq at ffffffffa9d47933
>>
>> #21 [ffffb202f268cf98] __softirqentry_text_start at ffffffffaa8000e0
>>
>> #22 [ffffb202f268cff0] asm_call_sysvec_on_stack at ffffffffaa60114f
>> --- ---
>> (the left part of exception see the hotplug case of ATS capable device)
>>
>> If one endpoint device just no response to the ATS Invalidation request,
>> but is not gone, it will bring down the whole system, to avoid such
>> case, don't try the timeout ATS Invalidation request forever.
>>
>> Signed-off-by: Ethan Zhao <haifeng.zhao@linux.intel.com>
>> ---
>>   drivers/iommu/intel/dmar.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/drivers/iommu/intel/dmar.c b/drivers/iommu/intel/dmar.c
>> index 76903a8bf963..206ab0b7294f 100644
>> --- a/drivers/iommu/intel/dmar.c
>> +++ b/drivers/iommu/intel/dmar.c
>> @@ -1457,7 +1457,7 @@ int qi_submit_sync(struct intel_iommu *iommu,
>> struct qi_desc *desc,
>>   	reclaim_free_desc(qi);
>>   	raw_spin_unlock_irqrestore(&qi->q_lock, flags);
>>
>> -	if (rc == -EAGAIN)
>> +	if (rc == -EAGAIN && type !=QI_DIOTLB_TYPE && type !=
>> QI_DEIOTLB_TYPE)
>>   		goto restart;
>>
> this change is moot.
>
> -EAGAIN is set only when hardware detects a ATS invalidation completion
> timeout in qi_check_fault(). so above just essentially kills the restart logic.

This change is intended to break the restar login when device-TLB

invalidation timeout happens, we don't know how long the ITE took

if the device is just no reponse.

>
> I'd wait for the maintainer of this driver to comment. this part doesn't
> look good but there might be some history reason so carefulness must
> be paid.

I would like to know the reason a hole is left here to hang the driver

forever.

Thanks,

Ethan