The iommu_map_sg() code currently iterates through the given scatter-gather list, and in the worst case, invokes iommu_map() for each element in the scatter-gather list, which calls into the IOMMU driver through an indirect call. For an IOMMU driver that uses a format supported by the io-pgtable code, the IOMMU driver will then call into the io-pgtable code to map the chunk.
Jumping between the IOMMU core code, the IOMMU driver, and the io-pgtable code and back for each element in a scatter-gather list is not efficient. Instead, add a map_sg() hook in both the IOMMU driver ops and the io-pgtable ops. iommu_map_sg() can then call into the IOMMU driver's map_sg() hook with the entire scatter-gather list, which can call into the io-pgtable map_sg() hook, which can process the entire scatter-gather list, signficantly reducing the number of indirect calls, and jumps between these layers, boosting performance. On a system that uses the ARM SMMU driver, and the ARM LPAE format, the current implementation of iommu_map_sg() yields the following latencies for mapping scatter-gather lists of various sizes. These latencies are calculated by repeating the mapping operation 10 times: size iommu_map_sg latency 4K 0.624 us 64K 9.468 us 1M 122.557 us 2M 239.807 us 12M 1435.979 us 24M 2884.968 us 32M 3832.979 us On the same system, the proposed modifications yield the following results: size iommu_map_sg latency 4K 3.645 us 64K 4.198 us 1M 11.010 us 2M 17.125 us 12M 82.416 us 24M 158.677 us 32M 210.468 us The procedure for collecting the iommu_map_sg latencies is the same in both experiments. Clearly, reducing the jumps between the different layers in the IOMMU code offers a signficant performance boost in iommu_map_sg() latency. Thanks, Isaac Isaac J. Manjarres (5): iommu/io-pgtable: Introduce map_sg() as a page table op iommu/io-pgtable-arm: Hook up map_sg() iommu/io-pgtable-arm-v7s: Hook up map_sg() iommu: Introduce map_sg() as an IOMMU op for IOMMU drivers iommu/arm-smmu: Hook up map_sg() drivers/iommu/arm/arm-smmu/arm-smmu.c | 19 ++++++++ drivers/iommu/io-pgtable-arm-v7s.c | 90 +++++++++++++++++++++++++++++++++++ drivers/iommu/io-pgtable-arm.c | 86 +++++++++++++++++++++++++++++++++ drivers/iommu/iommu.c | 25 ++++++++-- include/linux/io-pgtable.h | 6 +++ include/linux/iommu.h | 13 +++++ 6 files changed, 234 insertions(+), 5 deletions(-) -- The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum, a Linux Foundation Collaborative Project _______________________________________________ iommu mailing list iommu@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/iommu