[PATCH] iommu/io-pgtable-arm: Optimize partial walk flush for large scatter-gather list

Sai Prakash Ranjan
2021-06-09T14:53:42Z

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 (TLBIVA for
arm-smmu).

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 io_pgtable_tlb_flush_all() to invalidate the entire context
if size (pgsize) is greater than the granule size (4K, 16K, 64K). For this
example of 32MB scatter-gather list unmap, this results in just 16 ASID
based TLB invalidations or tlb_flush_all() callback (TLBIASID in case of
arm-smmu) as opposed to 8192 TLBIVAs thereby increasing the performance of
unmaps drastically.

Condition (size > granule size) is chosen for io_pgtable_tlb_flush_all()
because for any granule with supported pgsizes, we will have at least 512
TLB invalidations for which tlb_flush_all() is already recommended. For
example, take 4K granule with 2MB pgsize, this will result in 512 TLBIVA
in partial walk flush.

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 tlb_flush_all() as opposed to 16 tlb_flush_all().

Signed-off-by: Sai Prakash Ranjan <saiprakash.ranjan@codeaurora.org>
---
drivers/iommu/io-pgtable-arm.c | 7 +++++--
1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/drivers/iommu/io-pgtable-arm.c b/drivers/iommu/io-pgtable-arm.c
index 87def58e79b5..c3cb9add3179 100644
--- a/drivers/iommu/io-pgtable-arm.c
+++ b/drivers/iommu/io-pgtable-arm.c
@@ -589,8 +589,11 @@ static size_t __arm_lpae_unmap(struct arm_lpae_io_pgtable *data,

if (!iopte_leaf(pte, lvl, iop->fmt)) {
/* Also flush any partial walks */
- io_pgtable_tlb_flush_walk(iop, iova, size,
- ARM_LPAE_GRANULE(data));
+ if (size > ARM_LPAE_GRANULE(data))
+ io_pgtable_tlb_flush_all(iop);
+ else
+ io_pgtable_tlb_flush_walk(iop, iova, size,
+ ARM_LPAE_GRANULE(data));
ptep = iopte_deref(pte, data);
__arm_lpae_free_pgtable(data, lvl + 1, ptep);
} else if (iop->cfg.quirks & IO_PGTABLE_QUIRK_NON_STRICT) {
--
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