[PATCH v2 1/2] iommu/vt-d: Clear PRQ overflow only when PRQ is empty

[Lu Baolu]

It is incorrect to always clear PRO when it's set w/o first checking
whether the overflow condition has been cleared. Current code assumes
that if an overflow condition occurs it must have been cleared by earlier
loop. However since the code runs in a threaded context, the overflow
condition could occur even after setting the head to the tail under some
extreme condition. To be sane, we should read both head/tail again when
seeing a pending PRO and only clear PRO after all pending PRs have been
handled.

Suggested-by: Kevin Tian <kevin.tian@xxxxxxxxx>
Link: https://lore.kernel.org/linux-iommu/MWHPR11MB18862D2EA5BD432BF22D99A48CA09@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/
Signed-off-by: Lu Baolu <baolu.lu@xxxxxxxxxxxxxxx>
---
 drivers/iommu/intel/svm.c | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/drivers/iommu/intel/svm.c b/drivers/iommu/intel/svm.c
index 033b25886e57..d7c98c5fa4e7 100644
--- a/drivers/iommu/intel/svm.c
+++ b/drivers/iommu/intel/svm.c
@@ -1042,8 +1042,17 @@ 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)
-		writel(DMA_PRS_PRO, iommu->reg + DMAR_PRS_REG);
+	if (readl(iommu->reg + DMAR_PRS_REG) & DMA_PRS_PRO) {
+		pr_info_ratelimited("IOMMU: %s: PRQ overflow detected\n",
+				    iommu->name);
+		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) {
+			writel(DMA_PRS_PRO, iommu->reg + DMAR_PRS_REG);
+			pr_info_ratelimited("IOMMU: %s: PRQ overflow cleared",
+					    iommu->name);
+		}
+	}
 
 	if (!completion_done(&iommu->prq_complete))
 		complete(&iommu->prq_complete);
-- 
2.25.1