RE: [PATCH] iommu/vt-d: Avoid superfluous IOTLB tracking in lazy mode

[Tian, Kevin]

> 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.