On 7/3/25 11:15, Lu Baolu wrote:
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.
Reported-by: Ioanna Alifierakiioanna-maria.alifieraki@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@vger.kernel.org Signed-off-by: Lu Baolubaolu.lu@linux.intel.com
drivers/iommu/intel/iommu.c | 19 ++++++++++++++++++- drivers/iommu/intel/iommu.h | 3 +++ 2 files changed, 21 insertions(+), 1 deletion(-)
Queued for linux-next.
-- baolu