[PATCH v1 11/26] iommu/vt-d: Cache max domain ID to avoid redundant calculation

Xu Yilun
2025-11-17T02:38:35Z

From: Lu Baolu <baolu.lu@linux.intel.com>

The cap_ndoms() helper calculates the maximum available domain ID from
the value of capability register, which can be inefficient if called
repeatedly. Cache the maximum supported domain ID in max_domain_id field
during initialization to avoid redundant calls to cap_ndoms() throughout
the IOMMU driver.

No functionality change.

Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
---
drivers/iommu/intel/iommu.h | 1 +
drivers/iommu/intel/dmar.c | 1 +
drivers/iommu/intel/iommu.c | 10 +++++-----
3 files changed, 7 insertions(+), 5 deletions(-)

diff --git a/drivers/iommu/intel/iommu.h b/drivers/iommu/intel/iommu.h
index 3056583d7f56..66c3aa549fd4 100644
--- a/drivers/iommu/intel/iommu.h
+++ b/drivers/iommu/intel/iommu.h
@@ -724,6 +724,7 @@ struct intel_iommu {
/* mutex to protect domain_ida */
struct mutex did_lock;
struct ida domain_ida; /* domain id allocator */
+ unsigned long max_domain_id;
unsigned long *copied_tables; /* bitmap of copied tables */
spinlock_t lock; /* protect context, domain ids */
struct root_entry *root_entry; /* virtual address */
diff --git a/drivers/iommu/intel/dmar.c b/drivers/iommu/intel/dmar.c
index ec975c73cfe6..a54934c0536f 100644
--- a/drivers/iommu/intel/dmar.c
+++ b/drivers/iommu/intel/dmar.c
@@ -1099,6 +1099,7 @@ static int alloc_iommu(struct dmar_drhd_unit *drhd)
spin_lock_init(&iommu->lock);
ida_init(&iommu->domain_ida);
mutex_init(&iommu->did_lock);
+ iommu->max_domain_id = cap_ndoms(iommu->cap);

ver = readl(iommu->reg + DMAR_VER_REG);
pr_info("%s: reg_base_addr %llx ver %d:%d cap %llx ecap %llx\n",
diff --git a/drivers/iommu/intel/iommu.c b/drivers/iommu/intel/iommu.c
index e236c7ec221f..848b300da63e 100644
--- a/drivers/iommu/intel/iommu.c
+++ b/drivers/iommu/intel/iommu.c
@@ -1356,7 +1356,7 @@ int domain_attach_iommu(struct dmar_domain *domain, struct intel_iommu *iommu)
}

num = ida_alloc_range(&iommu->domain_ida, IDA_START_DID,
- cap_ndoms(iommu->cap) - 1, GFP_KERNEL);
+ iommu->max_domain_id - 1, GFP_KERNEL);
if (num < 0) {
pr_err("%s: No free domain ids\n", iommu->name);
goto err_unlock;
@@ -1420,7 +1420,7 @@ static void copied_context_tear_down(struct intel_iommu *iommu,
did_old = context_domain_id(context);
context_clear_entry(context);

- if (did_old < cap_ndoms(iommu->cap)) {
+ if (did_old < iommu->max_domain_id) {
iommu->flush.flush_context(iommu, did_old,
PCI_DEVID(bus, devfn),
DMA_CCMD_MASK_NOBIT,
@@ -1986,7 +1986,7 @@ static int copy_context_table(struct intel_iommu *iommu,
continue;

did = context_domain_id(&ce);
- if (did >= 0 && did < cap_ndoms(iommu->cap))
+ if (did >= 0 && did < iommu->max_domain_id)
ida_alloc_range(&iommu->domain_ida, did, did, GFP_KERNEL);

set_context_copied(iommu, bus, devfn);
@@ -2902,7 +2902,7 @@ static ssize_t domains_supported_show(struct device *dev,
struct device_attribute *attr, char *buf)
{
struct intel_iommu *iommu = dev_to_intel_iommu(dev);
- return sysfs_emit(buf, "%ld\n", cap_ndoms(iommu->cap));
+ return sysfs_emit(buf, "%ld\n", iommu->max_domain_id);
}
static DEVICE_ATTR_RO(domains_supported);

@@ -2913,7 +2913,7 @@ static ssize_t domains_used_show(struct device *dev,
unsigned int count = 0;
int id;

- for (id = 0; id < cap_ndoms(iommu->cap); id++)
+ for (id = 0; id < iommu->max_domain_id; id++)
if (ida_exists(&iommu->domain_ida, id))
count++;

--
2.25.1