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 15:16, Tian, Kevin wrote:
>> From: Lu Baolu<baolu.lu@linux.intel.com>
>> Sent: Thursday, July 3, 2025 11:16 AM
>>
>> 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.
> so for the same bridge case two NVMe disks likely are in the same
> iommu group sharing a domain. Then there is contention on the
> spinlock from two parallel threads on two disks. when disks come
> from different bridges they are attached to different domains hence
> no contention.
> 
> is it a correct description for the difference between same vs.
> different bridge?

Yes. I have the same understanding.

Thanks,
baolu