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

[Robin Murphy <robin.murphy@arm.com>]

On 2023-02-04 06:32, Baolu Lu wrote:
> 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.

The iotlb_gather helpers are really just that - little tools to help 
drivers with various common iotlb_gather accounting patterns. The 
decision whether to bother with that accounting at all should really 
come beforehand, and whether a driver supports flush queues is 
orthogonal to whether it uses any particular gather helper(s) or not, so 
I think the patch as-is is correct.

>> This patch adds a check to avoid IOVA gather page and IOTLB sync for
>> the lazy path.
>>
>> The performance difference on Sapphire Rapids 100Gb NIC is improved by
>> the following (as measured by iperf send):
> 
> Which test case have you done? Post the real data if you have any.
> 
>>
>> w/o this fix~48 Gbits/s. with this fix ~54 Gbits/s
>>
>> Cc: <stable@vger.kernel.org>
> 
> Again, add a Fixes tag so that people know how far this fix should be
> back ported.

Note that the overall issue probably dates back to the initial iommu-dma 
conversion, but if you think it's important enough to go back beyond 
5.15 when gather->queued was introduced, that'll need a different fix.

Cheers,
Robin.