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

[Baolu Lu <baolu.lu@linux.intel.com>]

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 Alifieraki<ioanna-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 Baolu<baolu.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