Re: [PATCH 00/12] Consolidate domain cache invalidation

[Baolu Lu]

On 4/8/24 11:03 AM, Tian, Kevin wrote:
> > From: Baolu Lu <baolu.lu@xxxxxxxxxxxxxxx>
> > Sent: Sunday, April 7, 2024 3:28 PM
> >
> > On 3/28/24 3:59 PM, Tian, Kevin wrote:
> > > > From: Lu Baolu<baolu.lu@xxxxxxxxxxxxxxx>
> > > > Sent: Monday, March 25, 2024 10:17 AM
> > > >
> > > > The IOMMU hardware cache needs to be invalidated whenever the
> > > > mappings
> > > > in the domain are changed. Currently, domain cache invalidation is
> > > > scattered across different places, causing several issues:
> > > >
> > > > - IOMMU IOTLB Invalidation: This is done by iterating through the domain
> > > >     IDs of each domain using the following code:
> > > >
> > > >           xa_for_each(&dmar_domain->iommu_array, i, info)
> > > >                   iommu_flush_iotlb_psi(info->iommu, dmar_domain,
> > > >                                         start_pfn, nrpages,
> > > >                                         list_empty(&gather->freelist), 0);
> > > >
> > > >     This code could theoretically cause a use-after-free problem because
> > > >     there's no lock to protect the "info" pointer within the loop.
> > > >
> > > > - Inconsistent Invalidation Methods: Different domain types implement
> > > >     their own cache invalidation methods, making the code difficult to
> > > >     maintain. For example, the DMA domain, SVA domain, and nested
> > domain
> > > >     have similar cache invalidation code scattered across different files.
> > > >
> > > > - SVA Domain Inconsistency: The SVA domain implementation uses a
> > > >     completely different data structure to track attached devices compared
> > > >     to other domains. This creates unnecessary differences and, even
> > > >     worse, leads to duplicate IOTLB invalidation when an SVA domain is
> > > >     attached to devices belonging to different IOMMU domains.
> > > can you elaborate how duplicated invalidations are caused?
> >
> > Yes, sure.
> >
> > Current Intel SVA implementation keeps the bond between mm and a PASID
> > of a device in a list of intel_svm_dev. In the mm notifier callback, it
> > iterates all intel_svam_dev in the list and invalidates the IOTLB and
> > device TLB sequentially.
> >
> > If multiple devices belong to a single IOMMU, the IOTLB will be flushed
> > multiple times. However, since these devices share the same domain ID
> > and PASID, a single IOTLB cache invalidation is sufficient. The
> > additional flushes are redundant and negatively impact performance.
>
> yes it's redundant. But what does "devices belonging to different
> IOMMU domains" in the original context try to convey? From above
> explanation it sounds irrelevant...

My typo. :-) Sorry for the confusion.

I should say,

"... attached to devices belonging to a same IOMMU ..."

Best regards,
baolu