linux-kernel - [PATCH 02/18] iommu/vt-d: Use try_cmpxchg64{,

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Date: Wed, 24 Apr 2024 15:16:28 +0800
From: Lu Baolu <baolu.lu@...ux.intel.com>
To: Joerg Roedel <joro@...tes.org>
Cc: Jason Gunthorpe <jgg@...pe.ca>,
	Jingqi Liu <Jingqi.liu@...el.com>,
	Dimitri Sivanich <sivanich@....com>,
	Uros Bizjak <ubizjak@...il.com>,
	Colin Ian King <colin.i.king@...il.com>,
	iommu@...ts.linux.dev,
	linux-kernel@...r.kernel.org
Subject: [PATCH 02/18] iommu/vt-d: Use try_cmpxchg64{,_local}() in iommu.c

From: Uros Bizjak <ubizjak@...il.com>

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>
Reviewed-by: Jason Gunthorpe <jgg@...dia.com>
Link: https://lore.kernel.org/r/20240414162454.49584-1-ubizjak@gmail.com
Signed-off-by: Lu Baolu <baolu.lu@...ux.intel.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 a7ecd90303dc..4a2afe89b464 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);
-- 
2.34.1