> 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.
