[PATCH 4.19 52/96] iommu/vt-d: Handle volatile descriptor status read

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

4.19-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Jacob Pan <jacob.jun.pan@linux.intel.com>

[ Upstream commit b5e86a95541cea737394a1da967df4cd4d8f7182 ]

Queued invalidation wait descriptor status is volatile in that IOMMU
hardware writes the data upon completion.

Use READ_ONCE() to prevent compiler optimizations which ensures memory
reads every time. As a side effect, READ_ONCE() also enforces strict
types and may add an extra instruction. But it should not have negative
performance impact since we use cpu_relax anyway and the extra time(by
adding an instruction) may allow IOMMU HW request cacheline ownership
easier.

e.g. gcc 12.3
BEFORE:
	81 38 ad de 00 00       cmpl   $0x2,(%rax)

AFTER (with READ_ONCE())
    772f:       8b 00                   mov    (%rax),%eax
    7731:       3d ad de 00 00          cmp    $0x2,%eax
                                        //status data is 32 bit

Signed-off-by: Jacob Pan <jacob.jun.pan@linux.intel.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Yi Liu <yi.l.liu@intel.com>
Link: https://lore.kernel.org/r/20240607173817.3914600-1-jacob.jun.pan@linux.intel.com
Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
Link: https://lore.kernel.org/r/20240702130839.108139-2-baolu.lu@linux.intel.com
Signed-off-by: Will Deacon <will@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 drivers/iommu/dmar.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/iommu/dmar.c b/drivers/iommu/dmar.c
index 3ea851583724..865847546f8e 100644
--- a/drivers/iommu/dmar.c
+++ b/drivers/iommu/dmar.c
@@ -1292,7 +1292,7 @@ int qi_submit_sync(struct qi_desc *desc, struct intel_iommu *iommu)
 	 */
 	writel(qi->free_head << DMAR_IQ_SHIFT, iommu->reg + DMAR_IQT_REG);
 
-	while (qi->desc_status[wait_index] != QI_DONE) {
+	while (READ_ONCE(qi->desc_status[wait_index]) != QI_DONE) {
 		/*
 		 * We will leave the interrupts disabled, to prevent interrupt
 		 * context to queue another cmd while a cmd is already submitted
-- 
2.43.0