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SubjectRe: [PATCH 1/3] iommu/vt-d: Add rate limited information when PRQ overflows
Date
Hi Kevin,

On 2021/1/22 14:38, Tian, Kevin wrote:
>> From: Lu Baolu <baolu.lu@linux.intel.com>
>> Sent: Thursday, January 21, 2021 9:45 AM
>>
>> So that the uses could get chances to know what happened.
>>
>> Suggested-by: Ashok Raj <ashok.raj@intel.com>
>> Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
>> ---
>> drivers/iommu/intel/svm.c | 10 ++++++++--
>> 1 file changed, 8 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/iommu/intel/svm.c b/drivers/iommu/intel/svm.c
>> index 033b25886e57..f49fe715477b 100644
>> --- a/drivers/iommu/intel/svm.c
>> +++ b/drivers/iommu/intel/svm.c
>> @@ -895,6 +895,7 @@ static irqreturn_t prq_event_thread(int irq, void *d)
>> struct intel_iommu *iommu = d;
>> struct intel_svm *svm = NULL;
>> int head, tail, handled = 0;
>> + struct page_req_dsc *req;
>>
>> /* Clear PPR bit before reading head/tail registers, to
>> * ensure that we get a new interrupt if needed. */
>> @@ -904,7 +905,6 @@ static irqreturn_t prq_event_thread(int irq, void *d)
>> head = dmar_readq(iommu->reg + DMAR_PQH_REG) &
>> PRQ_RING_MASK;
>> while (head != tail) {
>> struct vm_area_struct *vma;
>> - struct page_req_dsc *req;
>> struct qi_desc resp;
>> int result;
>> vm_fault_t ret;
>> @@ -1042,8 +1042,14 @@ static irqreturn_t prq_event_thread(int irq, void
>> *d)
>> * Clear the page request overflow bit and wake up all threads that
>> * are waiting for the completion of this handling.
>> */
>> - if (readl(iommu->reg + DMAR_PRS_REG) & DMA_PRS_PRO)
>> + if (readl(iommu->reg + DMAR_PRS_REG) & DMA_PRS_PRO) {
>> + head = dmar_readq(iommu->reg + DMAR_PQH_REG) &
>> PRQ_RING_MASK;
>> + req = &iommu->prq[head / sizeof(*req)];
>> + pr_warn_ratelimited("IOMMU: %s: Page request overflow:
>> HEAD: %08llx %08llx",
>> + iommu->name, ((unsigned long long
>> *)req)[0],
>> + ((unsigned long long *)req)[1]);
>> writel(DMA_PRS_PRO, iommu->reg + DMAR_PRS_REG);
>> + }
>>
>
> Not about rate limiting but I think we may have a problem in above
> logic. It is incorrect to always clear PRO when it's set w/o first checking
> whether the overflow condition has been cleared. This code assumes
> that if an overflow condition occurs it must have been cleared by earlier
> loop when hitting this check. However since this code runs in a threaded
> context, the overflow condition could occur even after you reset the head
> to the tail (under some extreme condition). To be sane I think we'd better
> read both head/tail again after seeing a pending PRO here and only clear
> PRO when it becomes a false indicator based on latest head/tail.
>

Yes, agreed. We can check the head and tail and clear the overflow bit
until the queue is empty. The finial code looks like:

/*
* Clear the page request overflow bit and wake up all threads that
* are waiting for the completion of this handling.
*/
if (readl(iommu->reg + DMAR_PRS_REG) & DMA_PRS_PRO) {
head = dmar_readq(iommu->reg + DMAR_PQH_REG) &
PRQ_RING_MASK;
tail = dmar_readq(iommu->reg + DMAR_PQT_REG) &
PRQ_RING_MASK;
if (head == tail) {
req = &iommu->prq[head / sizeof(*req)];
pr_warn_ratelimited("IOMMU: %s: Page request
overflow cleared: HEAD: %08llx %08llx",
iommu->name, ((unsigned
long long *)req)[0],
((unsigned long long
*)req)[1]);
writel(DMA_PRS_PRO, iommu->reg + DMAR_PRS_REG);
}
}

Thought?

> Thanks
> Kevin
>

Best regards,
baolu

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