linux-kernel - [PATCH AUTOSEL 4.9 049/107] iommu/arm-smmu-v3: Use explicit mb() when moving cons pointer

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Date:   Mon, 28 Jan 2019 11:18:49 -0500
From:   Sasha Levin <sashal@...nel.org>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:     Will Deacon <will.deacon@....com>,
        Robin Murphy <robin.murphy@....com>,
        Sasha Levin <sashal@...nel.org>,
        iommu@...ts.linux-foundation.org
Subject: [PATCH AUTOSEL 4.9 049/107] iommu/arm-smmu-v3: Use explicit mb() when moving cons pointer

From: Will Deacon <will.deacon@....com>

[ Upstream commit a868e8530441286342f90c1fd9c5f24de3aa2880 ]

After removing an entry from a queue (e.g. reading an event in
arm_smmu_evtq_thread()) it is necessary to advance the MMIO consumer
pointer to free the queue slot back to the SMMU. A memory barrier is
required here so that all reads targetting the queue entry have
completed before the consumer pointer is updated.

The implementation of queue_inc_cons() relies on a writel() to complete
the previous reads, but this is incorrect because writel() is only
guaranteed to complete prior writes. This patch replaces the call to
writel() with an mb(); writel_relaxed() sequence, which gives us the
read->write ordering which we require.

Cc: Robin Murphy <robin.murphy@....com>
Signed-off-by: Will Deacon <will.deacon@....com>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 drivers/iommu/arm-smmu-v3.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/drivers/iommu/arm-smmu-v3.c b/drivers/iommu/arm-smmu-v3.c
index ff4be1174ff0..7bd98585d78d 100644
--- a/drivers/iommu/arm-smmu-v3.c
+++ b/drivers/iommu/arm-smmu-v3.c
@@ -697,7 +697,13 @@ static void queue_inc_cons(struct arm_smmu_queue *q)
 	u32 cons = (Q_WRP(q, q->cons) | Q_IDX(q, q->cons)) + 1;

 	q->cons = Q_OVF(q, q->cons) | Q_WRP(q, cons) | Q_IDX(q, cons);
-	writel(q->cons, q->cons_reg);
+
+	/*
+	 * Ensure that all CPU accesses (reads and writes) to the queue
+	 * are complete before we update the cons pointer.
+	 */
+	mb();
+	writel_relaxed(q->cons, q->cons_reg);
 }

 static int queue_sync_prod(struct arm_smmu_queue *q)
-- 
2.19.1