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

[Baolu Lu]

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