This is a note to let you know that I've just added the patch titled
iommu/vt-d: Handle volatile descriptor status read
to the 5.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
iommu-vt-d-handle-volatile-descriptor-status-read.patch
and it can be found in the queue-5.4 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
commit 3aa672eb89bb2a000c6649085242bedb95719805
Author: Jacob Pan <jacob.jun.pan@xxxxxxxxxxxxxxx>
Date: Tue Jul 2 21:08:33 2024 +0800
iommu/vt-d: Handle volatile descriptor status read
[ 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@xxxxxxxxxxxxxxx>
Reviewed-by: Kevin Tian <kevin.tian@xxxxxxxxx>
Reviewed-by: Yi Liu <yi.l.liu@xxxxxxxxx>
Link: https://lore.kernel.org/r/20240607173817.3914600-1-jacob.jun.pan@xxxxxxxxxxxxxxx
Signed-off-by: Lu Baolu <baolu.lu@xxxxxxxxxxxxxxx>
Link: https://lore.kernel.org/r/20240702130839.108139-2-baolu.lu@xxxxxxxxxxxxxxx
Signed-off-by: Will Deacon <will@xxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
diff --git a/drivers/iommu/dmar.c b/drivers/iommu/dmar.c
index 36900d65386f..a4805d17317d 100644
--- a/drivers/iommu/dmar.c
+++ b/drivers/iommu/dmar.c
@@ -1294,7 +1294,7 @@ int qi_submit_sync(struct qi_desc *desc, struct intel_iommu *iommu)
*/
writel(qi->free_head << 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
