linux-kernel - [PATCH] iommu/intel: Use try_cmpxchg64{,

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Date: Sun, 14 Apr 2024 18:23:11 +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] iommu/intel: Use try_cmpxchg64{,_local}() in iommu.c

Replace this pattern in iommu.c:

    cmpxchg64{,_local}(*ptr, 0, new) != 0

.. with the simpler and faster:

    !try_cmpxchg64{,_local}(*ptr, &tmp, new)

The x86 CMPXCHG instruction returns success in the ZF flag, so this change
saves a compare after the CMPXCHG.

No functional change intended.

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/iommu.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/drivers/iommu/intel/iommu.c b/drivers/iommu/intel/iommu.c
index 50eb9aed47cc..311073494f50 100644
--- a/drivers/iommu/intel/iommu.c
+++ b/drivers/iommu/intel/iommu.c
@@ -865,7 +865,7 @@ static struct dma_pte *pfn_to_dma_pte(struct dmar_domain *domain,
 			break;
 
 		if (!dma_pte_present(pte)) {
-			uint64_t pteval;
+			uint64_t pteval, tmp;
 
 			tmp_page = alloc_pgtable_page(domain->nid, gfp);
 
@@ -877,7 +877,8 @@ static struct dma_pte *pfn_to_dma_pte(struct dmar_domain *domain,
 			if (domain->use_first_level)
 				pteval |= DMA_FL_PTE_XD | DMA_FL_PTE_US | DMA_FL_PTE_ACCESS;
 
-			if (cmpxchg64(&pte->val, 0ULL, pteval))
+			tmp = 0ULL;
+			if (!try_cmpxchg64(&pte->val, &tmp, pteval))
 				/* Someone else set it while we were thinking; use theirs. */
 				free_pgtable_page(tmp_page);
 			else
@@ -2128,8 +2129,8 @@ __domain_mapping(struct dmar_domain *domain, unsigned long iov_pfn,
 		/* We don't need lock here, nobody else
 		 * touches the iova range
 		 */
-		tmp = cmpxchg64_local(&pte->val, 0ULL, pteval);
-		if (tmp) {
+		tmp = 0ULL;
+		if (!try_cmpxchg64_local(&pte->val, &tmp, pteval)) {
 			static int dumps = 5;
 			pr_crit("ERROR: DMA PTE for vPFN 0x%lx already set (to %llx not %llx)\n",
 				iov_pfn, tmp, (unsigned long long)pteval);
-- 
2.42.0