linux-kernel - [PATCH 2/3] iommu/vt-d: Use try_cmpxchg64() in intel_pasid_get

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Date: Wed, 22 May 2024 10:26:48 +0200
From: Uros Bizjak <ubizjak@...il.com>
To: iommu@...ts.linux.dev,
	linux-kernel@...r.kernel.org
Cc: Uros Bizjak <ubizjak@...il.com>,
	David Woodhouse <dwmw2@...radead.org>,
	Lu Baolu <baolu.lu@...ux.intel.com>,
	Joerg Roedel <joro@...tes.org>,
	Will Deacon <will@...nel.org>,
	Robin Murphy <robin.murphy@....com>
Subject: [PATCH 2/3] iommu/vt-d: Use try_cmpxchg64() in intel_pasid_get_entry()

Use try_cmpxchg64() instead of cmpxchg64 (*ptr, old, new) != old in
intel_pasid_get_entry().  cmpxchg returns success in ZF flag, so
this change saves a compare after cmpxchg (and related move
instruction in front of cmpxchg).

Signed-off-by: Uros Bizjak <ubizjak@...il.com>
Cc: David Woodhouse <dwmw2@...radead.org>
Cc: Lu Baolu <baolu.lu@...ux.intel.com>
Cc: Joerg Roedel <joro@...tes.org>
Cc: Will Deacon <will@...nel.org>
Cc: Robin Murphy <robin.murphy@....com>
---
 drivers/iommu/intel/pasid.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/drivers/iommu/intel/pasid.c b/drivers/iommu/intel/pasid.c
index abce19e2ad6f..9bf45bc4b967 100644
--- a/drivers/iommu/intel/pasid.c
+++ b/drivers/iommu/intel/pasid.c
@@ -146,6 +146,8 @@ static struct pasid_entry *intel_pasid_get_entry(struct device *dev, u32 pasid)
 retry:
 	entries = get_pasid_table_from_pde(&dir[dir_index]);
 	if (!entries) {
+		u64 tmp;
+
 		entries = iommu_alloc_page_node(info->iommu->node, GFP_ATOMIC);
 		if (!entries)
 			return NULL;
@@ -156,8 +158,9 @@ static struct pasid_entry *intel_pasid_get_entry(struct device *dev, u32 pasid)
 		 * clear. However, this entry might be populated by others
 		 * while we are preparing it. Use theirs with a retry.
 		 */
-		if (cmpxchg64(&dir[dir_index].val, 0ULL,
-			      (u64)virt_to_phys(entries) | PASID_PTE_PRESENT)) {
+		tmp = 0ULL;
+		if (!try_cmpxchg64(&dir[dir_index].val, &tmp,
+				   (u64)virt_to_phys(entries) | PASID_PTE_PRESENT)) {
 			iommu_free_page(entries);
 			goto retry;
 		}
-- 
2.45.1