linux-kernel - Re: [PATCH v2 1/1] iommu/vt-d: Remove caching mode check before device TLB flush

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Message-ID: <57d2fe8c-b438-42ae-a1ff-0ba0ee226c37@linux.intel.com>
Date: Thu, 11 Apr 2024 21:48:41 +0800
From: Baolu Lu <baolu.lu@...ux.intel.com>
To: Robin Murphy <robin.murphy@....com>, iommu@...ts.linux.dev
Cc: baolu.lu@...ux.intel.com, Kevin Tian <kevin.tian@...el.com>,
 Yi Liu <yi.l.liu@...el.com>, Jacob Pan <jacob.jun.pan@...ux.intel.com>,
 Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/1] iommu/vt-d: Remove caching mode check before
 device TLB flush

On 2024/4/11 21:13, Robin Murphy wrote:
> On 10/04/2024 6:58 am, Lu Baolu wrote:
>> The Caching Mode (CM) of the Intel IOMMU indicates if the hardware
>> implementation caches not-present or erroneous translation-structure
>> entries except the first-stage translation. The caching mode is
>> irrelevant to the device TLB , therefore there is no need to check
>> it before a device TLB invalidation operation.
>>
>> iommu_flush_iotlb_psi() is called in map and unmap paths. The caching
>> mode check before device TLB invalidation will cause device TLB
>> invalidation always issued if IOMMU is not running in caching mode.
>> This is wrong and causes unnecessary performance overhead.
>>
>> The removal of caching mode check in intel_flush_iotlb_all() doesn't
>> impact anything no matter the IOMMU is working in caching mode or not.
>> Commit <29b32839725f> ("iommu/vt-d: Do not use flush-queue when
>> caching-mode is on") has already disabled flush-queue for caching mode,
>> hence caching mode will never call intel_flush_iotlb_all().
> 
> Well, technically it might still, at domain creation via 
> iommu_create_device_direct_mappings(), but domain->has_iotlb_device 
> should definitely be false at that point 🙂

Oh! I overlooked that path. :-)

Yes. iommu_create_device_direct_mappings() is called before setting the
domain to device for intel iommu driver, hence in practice the
domain->has_iotlb_device is always false.

Best regards,
baolu