[PATCH 0/5] Optimize iommu_map_sg() performance

Isaac J. Manjarres
2021-01-09T01:51:31Z

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(-)