Currently for iommu_unmap() of large scatter-gather list with page size
elements, the majority of time is spent in flushing of partial walks in
__arm_lpae_unmap() which is a VA based TLB invalidation invalidating
page-by-page on iommus like arm-smmu-v2 (TLBIVA) which do not support
range based invalidations like on arm-smmu-v3.2.
For example: to unmap a 32MB scatter-gather list with page size elements
(8192 entries), there are 16->2MB buffer unmaps based on the pgsize (2MB
for 4K granule) and each of 2MB will further result in 512 TLBIVAs (2MB/4K)
resulting in a total of 8192 TLBIVAs (512*16) for 16->2MB causing a huge
overhead.
So instead use tlb_flush_all() callback (TLBIALL/TLBIASID) to invalidate
the entire context for partial walk flush on select few platforms where
cost of over-invalidation is less than unmap latency using the newly
introduced quirk IO_PGTABLE_QUIRK_TLB_INV_ALL. We also do this for
non-strict mode given its all about over-invalidation saving time on
individual unmaps and non-deterministic generally.
For this example of 32MB scatter-gather list unmap, this results in just
16 ASID based TLB invalidations (TLBIASIDs) as opposed to 8192 TLBIVAs
thereby increasing the performance of unmaps drastically.
Test on QTI SM8150 SoC for 10 iterations of iommu_{map_sg}/unmap:
(average over 10 iterations)
Before this optimization:
size iommu_map_sg iommu_unmap
4K 2.067 us 1.854 us
64K 9.598 us 8.802 us
1M 148.890 us 130.718 us
2M 305.864 us 67.291 us
12M 1793.604 us 390.838 us
16M 2386.848 us 518.187 us
24M 3563.296 us 775.989 us
32M 4747.171 us 1033.364 us
After this optimization:
size iommu_map_sg iommu_unmap
4K 1.723 us 1.765 us
64K 9.880 us 8.869 us
1M 155.364 us 135.223 us
2M 303.906 us 5.385 us
12M 1786.557 us 21.250 us
16M 2391.890 us 27.437 us
24M 3570.895 us 39.937 us
32M 4755.234 us 51.797 us
This is further reduced once the map/unmap_pages() support gets in which
will result in just 1 TLBIASID as compared to 16 TLBIASIDs.
Real world data also shows big difference in unmap performance as below:
There were reports of camera frame drops because of high overhead in
iommu unmap without this optimization because of frequent unmaps issued
by camera of about 100MB/s taking more than 100ms thereby causing frame
drops.
Changes in v2:
* Add a quirk to choose tlb_flush_all in partial walk flush
* Set the quirk for QTI SoC implementation
Sai Prakash Ranjan (3):
iommu/io-pgtable: Add a quirk to use tlb_flush_all() for partial walk
flush
iommu/io-pgtable: Optimize partial walk flush for large scatter-gather
list
iommu/arm-smmu-qcom: Set IO_PGTABLE_QUIRK_TLB_INV_ALL for QTI SoC impl
drivers/iommu/arm/arm-smmu/arm-smmu-qcom.c | 11 +++++++++++
drivers/iommu/io-pgtable-arm.c | 3 ++-
include/linux/io-pgtable.h | 11 +++++++++++
3 files changed, 24 insertions(+), 1 deletion(-)
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