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