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

[Sai Prakash Ranjan]

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@xxxxxxxxxxxxxx>
---
 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) {
-- 
QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a member
of Code Aurora Forum, hosted by The Linux Foundation