linux-kernel - Re: [RFC PATCH v10 5/5] iommu/vt-d: don't loop for timeout ATS Invalidation request forever

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Date: Thu, 11 Jan 2024 10:31:25 +0800
From: Baolu Lu <baolu.lu@...ux.intel.com>
To: Ethan Zhao <haifeng.zhao@...ux.intel.com>, kevin.tian@...el.com,
 bhelgaas@...gle.com, dwmw2@...radead.org, will@...nel.org,
 robin.murphy@....com, lukas@...ner.de
Cc: baolu.lu@...ux.intel.com, linux-pci@...r.kernel.org,
 iommu@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v10 5/5] iommu/vt-d: don't loop for timeout ATS
 Invalidation request forever

On 1/10/24 4:40 PM, Ethan Zhao wrote:
> 
> On 1/10/2024 1:28 PM, Baolu Lu wrote:
>> On 12/29/23 1:05 AM, Ethan Zhao wrote:
>>> 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
>>>
>>> (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@...ux.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 0a8d628a42ee..9edb4b44afca 100644
>>> --- a/drivers/iommu/intel/dmar.c
>>> +++ b/drivers/iommu/intel/dmar.c
>>> @@ -1453,7 +1453,7 @@ 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;
>>>         if (iotlb_start_ktime)
>>
>> Above is also unnecessary if qi_check_fault() returns -ETIMEDOUT,
>> instead of -EAGAIN. Or did I miss anything?
> 
> It is pro if we fold it into qi_check_fault(), the con is we have to add
> 
> more parameter to qi_check_fault(), no need check invalidation type
> 
> of QI_DIOTLB_TYPE&QI_DEIOTLB_TYPE in qi_check_fault() ?

No need to check the request type as multiple requests might be batched
together in a single call. This is also the reason why I asked you to
add a flag bit to this helper and make the intention explicit, say,

"This includes requests to interact with a PCI endpoint. The device may
  become unavailable at any time, so do not attempt to retry if ITE is
  detected and the device has gone away."

Best regards,
baolu