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 <[email protected]>
---
drivers/iommu/intel/dmar.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/drivers/iommu/intel/dmar.c b/drivers/iommu/intel/dmar.c
index 304e84949ca7..1c8d3141cb55 100644
--- a/drivers/iommu/intel/dmar.c
+++ b/drivers/iommu/intel/dmar.c
@@ -1446,7 +1446,7 @@ int qi_submit_sync(struct intel_iommu *iommu, struct qi_desc *desc,
*/
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
--
2.25.1