linux-kernel - Re: [PATCH 1/1] iommu/vt-d: Optimize iotlb_sync

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Message-ID: <b95b6a35-e6b5-46df-9a5f-4cc7f4e823bb@linux.intel.com>
Date: Fri, 4 Jul 2025 09:25:51 +0800
From: Baolu Lu <baolu.lu@...ux.intel.com>
To: "Tian, Kevin" <kevin.tian@...el.com>, Joerg Roedel <joro@...tes.org>,
 Will Deacon <will@...nel.org>, Robin Murphy <robin.murphy@....com>,
 Ioanna Alifieraki <ioanna-maria.alifieraki@...onical.com>
Cc: "iommu@...ts.linux.dev" <iommu@...ts.linux.dev>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
 "stable@...r.kernel.org" <stable@...r.kernel.org>
Subject: Re: [PATCH 1/1] iommu/vt-d: Optimize iotlb_sync_map for
 non-caching/non-RWBF modes

On 7/3/25 15:16, Tian, Kevin wrote:
>> From: Lu Baolu<baolu.lu@...ux.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