Messages in this thread | | | Date | Mon, 6 Feb 2023 11:20:44 +0000 | Subject | Re: [PATCH] iommu/vt-d: Avoid superfluous IOTLB tracking in lazy mode | From | Robin Murphy <> |
| |
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.
| |