linux-kernel - [PATCH v2 2/2] iommu/amd: Use 'unsigned long' for domain->pt

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Date:   Fri, 26 Jun 2020 10:05:47 +0200
From:   Joerg Roedel <joro@...tes.org>
To:     Joerg Roedel <joro@...tes.org>
Cc:     iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
        Suravee Suthikulpanit <suravee.suthikulpanit@....com>,
        Qian Cai <cai@....pw>, Joerg Roedel <jroedel@...e.de>
Subject: [PATCH v2 2/2] iommu/amd: Use 'unsigned long' for domain->pt_root

From: Joerg Roedel <jroedel@...e.de>

Using atomic64_t can be quite expensive, so use unsigned long instead.
This is safe because the write becomes visible atomically.

Signed-off-by: Joerg Roedel <jroedel@...e.de>
---
 drivers/iommu/amd/amd_iommu_types.h |  2 +-
 drivers/iommu/amd/iommu.c           | 15 +++++++++++++--
 2 files changed, 14 insertions(+), 3 deletions(-)

diff --git a/drivers/iommu/amd/amd_iommu_types.h b/drivers/iommu/amd/amd_iommu_types.h
index 30a5d412255a..f6f102282dda 100644
--- a/drivers/iommu/amd/amd_iommu_types.h
+++ b/drivers/iommu/amd/amd_iommu_types.h
@@ -468,7 +468,7 @@ struct protection_domain {
 				       iommu core code */
 	spinlock_t lock;	/* mostly used to lock the page table*/
 	u16 id;			/* the domain id written to the device table */
-	atomic64_t pt_root;	/* pgtable root and pgtable mode */
+	unsigned long pt_root;	/* pgtable root and pgtable mode */
 	int glx;		/* Number of levels for GCR3 table */
 	u64 *gcr3_tbl;		/* Guest CR3 table */
 	unsigned long flags;	/* flags to find out type of domain */
diff --git a/drivers/iommu/amd/iommu.c b/drivers/iommu/amd/iommu.c
index 5286ddcfc2f9..aec585f47646 100644
--- a/drivers/iommu/amd/iommu.c
+++ b/drivers/iommu/amd/iommu.c
@@ -156,7 +156,12 @@ static struct protection_domain *to_pdomain(struct iommu_domain *dom)
 static void amd_iommu_domain_get_pgtable(struct protection_domain *domain,
 					 struct domain_pgtable *pgtable)
 {
-	u64 pt_root = atomic64_read(&domain->pt_root);
+	unsigned long pt_root;
+
+	/* Match the barrier in amd_iommu_domain_set_pt_root() */
+	smp_rmb();
+
+	pt_root = READ_ONCE(domain->pt_root);
 
 	pgtable->root = (u64 *)(pt_root & PAGE_MASK);
 	pgtable->mode = pt_root & 7; /* lowest 3 bits encode pgtable mode */
@@ -164,7 +169,13 @@ static void amd_iommu_domain_get_pgtable(struct protection_domain *domain,
 
 static void amd_iommu_domain_set_pt_root(struct protection_domain *domain, u64 root)
 {
-	atomic64_set(&domain->pt_root, root);
+	WRITE_ONCE(domain->pt_root, root);
+
+	/*
+	 * The new value needs to be gobally visible in case pt_root gets
+	 * cleared, so that the page-table can be safely freed.
+	 */
+	smp_wmb();
 }
 
 static void amd_iommu_domain_clr_pt_root(struct protection_domain *domain)
-- 
2.27.0