linux-kernel - RE: [PATCH 1/3] iommu/vt-d: Add rate limited information when PRQ overflows

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Message-ID: <MWHPR11MB188688A4C436B318AB0CDCB98CBD9@MWHPR11MB1886.namprd11.prod.outlook.com>
Date:   Mon, 25 Jan 2021 08:16:32 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     Lu Baolu <baolu.lu@...ux.intel.com>,
        Joerg Roedel <joro@...tes.org>, "Will Deacon" <will@...nel.org>
CC:     "Raj, Ashok" <ashok.raj@...el.com>,
        Jacob Pan <jacob.jun.pan@...ux.intel.com>,
        "Liu, Yi L" <yi.l.liu@...el.com>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH 1/3] iommu/vt-d: Add rate limited information when PRQ
 overflows

> From: Lu Baolu <baolu.lu@...ux.intel.com>
> Sent: Monday, January 25, 2021 2:29 PM
> 
> Hi Kevin,
> 
> On 2021/1/22 14:38, Tian, Kevin wrote:
> >> From: Lu Baolu <baolu.lu@...ux.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@...el.com>
> >> Signed-off-by: Lu Baolu <baolu.lu@...ux.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?
> 

Just a small comment. Is it useful to also print a warning in the true
overflow condition which has to wait for next interrupt to be cleared?

Thanks
Kevin