From b3774dcec0995f6d31fa86bbfaeb74a5c6cda069 Mon Sep 17 00:00:00 2001 From: Jacob Pan Date: Tue, 2 Jul 2024 21:08:33 +0800 Subject: [PATCH] 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 Reviewed-by: Kevin Tian Reviewed-by: Yi Liu Link: https://lore.kernel.org/r/20240607173817.3914600-1-jacob.jun.pan@linux.intel.com Signed-off-by: Lu Baolu Link: https://lore.kernel.org/r/20240702130839.108139-2-baolu.lu@linux.intel.com Signed-off-by: Will Deacon Signed-off-by: Sasha Levin (cherry picked from commit 75a34515eb1be431819ec00cd09fe3a3eb369cdb) Signed-off-by: Harshit Mogalapalli Signed-off-by: Vegard Nossum --- 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 3720f24986ea..23e7e5901239 100644 --- a/drivers/iommu/dmar.c +++ b/drivers/iommu/dmar.c @@ -1289,7 +1289,7 @@ restart: */ 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