> From: Baolu Lu <baolu.lu@xxxxxxxxxxxxxxx>
> Sent: Tuesday, February 7, 2023 2:46 PM
>
> On 2023/2/6 11:48, Tian, Kevin wrote:
> >> From: Baolu Lu <baolu.lu@xxxxxxxxxxxxxxx>
> >> Sent: Saturday, February 4, 2023 2:32 PM
> >>
> >> On 2023/2/4 7:04, Jacob Pan wrote:
> >>> Intel IOMMU driver implements IOTLB flush queue with domain selective
> >>> or PASID selective invalidations. In this case there's no need to track
> >>> IOVA page range and sync IOTLBs, which may cause significant
> >> performance
> >>> hit.
> >>
> >> [Add cc Robin]
> >>
> >> If I understand this patch correctly, this might be caused by below
> >> helper:
> >>
> >> /**
> >> * iommu_iotlb_gather_add_page - Gather for page-based TLB
> invalidation
> >> * @domain: IOMMU domain to be invalidated
> >> * @gather: TLB gather data
> >> * @iova: start of page to invalidate
> >> * @size: size of page to invalidate
> >> *
> >> * Helper for IOMMU drivers to build invalidation commands based on
> >> individual
> >> * pages, or with page size/table level hints which cannot be gathered
> >> if they
> >> * differ.
> >> */
> >> static inline void iommu_iotlb_gather_add_page(struct iommu_domain
> >> *domain,
> >> struct
> >> iommu_iotlb_gather *gather,
> >> unsigned long iova,
> >> size_t size)
> >> {
> >> /*
> >> * If the new page is disjoint from the current range or is
> >> mapped at
> >> * a different granularity, then sync the TLB so that the gather
> >> * structure can be rewritten.
> >> */
> >> if ((gather->pgsize && gather->pgsize != size) ||
> >> iommu_iotlb_gather_is_disjoint(gather, iova, size))
> >> iommu_iotlb_sync(domain, gather);
> >>
> >> gather->pgsize = size;
> >> iommu_iotlb_gather_add_range(gather, iova, size);
> >> }
> >>
> >> As the comments for iommu_iotlb_gather_is_disjoint() says,
> >>
> >> "...For many IOMMUs, flushing the IOMMU in this case is better
> >> than merging the two, which might lead to unnecessary invalidations.
> >> ..."
> >>
> >> So, perhaps the right fix for this performance issue is to add
> >>
> >> if (!gather->queued)
> >>
> >> in iommu_iotlb_gather_add_page() or iommu_iotlb_gather_is_disjoint()?
> >> It should benefit other arch's as well.
> >>
> >
> > There are only two callers of this helper: intel and arm-smmu-v3.
> >
> > Looks other drivers just implements direct flush via
> io_pgtable_tlb_add_page().
> >
> > and their unmap callback typically does:
> >
> > if (!iommu_iotlb_gather_queued(gather))
> > io_pgtable_tlb_add_page();
> >
> > from this angle it's same policy as Jacob's does, i.e. if it's already
> > queued then no need to further call optimization for direct flush.
>
> Perhaps we can use iommu_iotlb_gather_queued() to replace direct
> gather->queued check in this patch as well?
>
yes
