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

Joerg Roedel
2020-06-26T08:08:36Z

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@suse.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