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

[Tian, Kevin]

> From: Lu Baolu <baolu.lu@xxxxxxxxxxxxxxx>
> 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@xxxxxxxxxxxxxxx>

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