[SRU][P][PATCH 1/1] iommu/vt-d: Optimize iotlb_sync_map for non-caching/non-RWBF modes

[Massimiliano Pellizzer]

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

BugLink: https://bugs.launchpad.net/bugs/2115738

The iotlb_sync_map iommu ops allows drivers to perform necessary cache
flushes when new mappings are established. For the Intel iommu driver,
this callback specifically serves two purposes:

- To flush caches when a second-stage page table is attached to a device
  whose iommu is operating in caching mode (CAP_REG.CM==1).
- To explicitly flush internal write buffers to ensure updates to memory-
  resident remapping structures are visible to hardware (CAP_REG.RWBF==1).

However, in scenarios where neither caching mode nor the RWBF flag is
active, the cache_tag_flush_range_np() helper, which is called in the
iotlb_sync_map path, effectively becomes a no-op.

Despite being a no-op, cache_tag_flush_range_np() involves iterating
through all cache tags of the iommu's attached to the domain, protected
by a spinlock. This unnecessary execution path introduces overhead,
leading to a measurable I/O performance regression. On systems with NVMes
under the same bridge, performance was observed to drop from approximately
~6150 MiB/s down to ~4985 MiB/s.

Introduce a flag in the dmar_domain structure. This flag will only be set
when iotlb_sync_map is required (i.e., when CM or RWBF is set). The
cache_tag_flush_range_np() is called only for domains where this flag is
set. This flag, once set, is immutable, given that there won't be mixed
configurations in real-world scenarios where some IOMMUs in a system
operate in caching mode while others do not. Theoretically, the
immutability of this flag does not impact functionality.

Reported-by: Ioanna Alifieraki <ioanna-maria.alifieraki at canonical.com>
Closes: https://bugs.launchpad.net/ubuntu/+source/linux/+bug/2115738
Link: https://lore.kernel.org/r/20250701171154.52435-1-ioanna-maria.alifieraki@canonical.com
Fixes: 129dab6e1286 ("iommu/vt-d: Use cache_tag_flush_range_np() in iotlb_sync_map")
Cc: stable at vger.kernel.org
Signed-off-by: Lu Baolu <baolu.lu at linux.intel.com>
Reviewed-by: Kevin Tian <kevin.tian at intel.com>
Link: https://lore.kernel.org/r/20250703031545.3378602-1-baolu.lu@linux.intel.com
Link: https://lore.kernel.org/r/20250714045028.958850-3-baolu.lu@linux.intel.com
Signed-off-by: Will Deacon <will at kernel.org>
(cherry picked from commit 12724ce3fe1a3d8f30d56e48b4f272d8860d1970 linux-next)
Signed-off-by: Massimiliano Pellizzer <massimiliano.pellizzer at canonical.com>
---
 drivers/iommu/intel/iommu.c | 19 ++++++++++++++++++-
 drivers/iommu/intel/iommu.h |  3 +++
 2 files changed, 21 insertions(+), 1 deletion(-)

diff --git a/drivers/iommu/intel/iommu.c b/drivers/iommu/intel/iommu.c
index 6ceed4b3091e..059aabfe6c88 100644
--- a/drivers/iommu/intel/iommu.c
+++ b/drivers/iommu/intel/iommu.c
@@ -1821,6 +1821,18 @@ static int domain_setup_first_level(struct intel_iommu *iommu,
 					  (pgd_t *)pgd, flags, old);
 }
 
+static bool domain_need_iotlb_sync_map(struct dmar_domain *domain,
+				       struct intel_iommu *iommu)
+{
+	if (cap_caching_mode(iommu->cap) && !domain->use_first_level)
+		return true;
+
+	if (rwbf_quirk || cap_rwbf(iommu->cap))
+		return true;
+
+	return false;
+}
+
 static int dmar_domain_attach_device(struct dmar_domain *domain,
 				     struct device *dev)
 {
@@ -1860,6 +1872,8 @@ static int dmar_domain_attach_device(struct dmar_domain *domain,
 	if (ret)
 		goto out_block_translation;
 
+	domain->iotlb_sync_map |= domain_need_iotlb_sync_map(domain, iommu);
+
 	return 0;
 
 out_block_translation:
@@ -4091,7 +4105,10 @@ static bool risky_device(struct pci_dev *pdev)
 static int intel_iommu_iotlb_sync_map(struct iommu_domain *domain,
 				      unsigned long iova, size_t size)
 {
-	cache_tag_flush_range_np(to_dmar_domain(domain), iova, iova + size - 1);
+	struct dmar_domain *dmar_domain = to_dmar_domain(domain);
+
+	if (dmar_domain->iotlb_sync_map)
+		cache_tag_flush_range_np(dmar_domain, iova, iova + size - 1);
 
 	return 0;
 }
diff --git a/drivers/iommu/intel/iommu.h b/drivers/iommu/intel/iommu.h
index 539066b85c23..e94e08804c56 100644
--- a/drivers/iommu/intel/iommu.h
+++ b/drivers/iommu/intel/iommu.h
@@ -615,6 +615,9 @@ struct dmar_domain {
 	u8 has_mappings:1;		/* Has mappings configured through
 					 * iommu_map() interface.
 					 */
+	u8 iotlb_sync_map:1;		/* Need to flush IOTLB cache or write
+					 * buffer when creating mappings.
+					 */
 
 	spinlock_t lock;		/* Protect device tracking lists */
 	struct list_head devices;	/* all devices' list */
-- 
2.48.1