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

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Message-ID: <BN9PR11MB5276AEA3AA7ED2D945745D388C062@BN9PR11MB5276.namprd11.prod.outlook.com>
Date: Wed, 10 Apr 2024 06:14:24 +0000
From: "Tian, Kevin" <kevin.tian@...el.com>
To: Lu Baolu <baolu.lu@...ux.intel.com>, "iommu@...ts.linux.dev"
	<iommu@...ts.linux.dev>
CC: "Liu, Yi L" <yi.l.liu@...el.com>, Jacob Pan
	<jacob.jun.pan@...ux.intel.com>, Joerg Roedel <joro@...tes.org>, Will Deacon
	<will@...nel.org>, Robin Murphy <robin.murphy@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH v2 1/1] iommu/vt-d: Remove caching mode check before
 device TLB flush

> From: Lu Baolu <baolu.lu@...ux.intel.com>
> Sent: Wednesday, April 10, 2024 1:58 PM
> 
> 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().
> 
> Fixes: bf92df30df90 ("intel-iommu: Only avoid flushing device IOTLB for
> domain ID 0 in caching mode")
> Signed-off-by: Lu Baolu <baolu.lu@...ux.intel.com>

Reviewed-by: Kevin Tian <kevin.tian@...el.com>