Messages in this thread | | | Date | Tue, 7 Feb 2023 14:42:06 +0800 | Subject | Re: [PATCH] iommu/vt-d: Avoid superfluous IOTLB tracking in lazy mode | From | Baolu Lu <> |
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On 2023/2/6 19:20, Robin Murphy wrote: > 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.
Okay, that's fine. Thanks for the explanation.
> >>> 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.
So perhaps
Cc: stable@vger.kernel.org # 5.15+
is enough?
Best regards, baolu
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