4459946e86
commit 9e02977bfad006af328add9434c8bffa40e053bb upstream.
When we looked into FIO performance with swiotlb enabled in VM, we found
swiotlb_bounce() is always called one more time than expected for each DMA
read request.
It turns out that the bounce buffer is copied to original DMA buffer twice
after the completion of a DMA request (one is done by in
dma_direct_sync_single_for_cpu(), the other by swiotlb_tbl_unmap_single()).
But the content in bounce buffer actually doesn't change between the two
rounds of copy. So, one round of copy is redundant.
Pass DMA_ATTR_SKIP_CPU_SYNC flag to swiotlb_tbl_unmap_single() to
skip the memory copy in it.
This fix increases FIO 64KB sequential read throughput in a guest with
swiotlb=force by 5.6%.
Fixes:
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|---|---|---|
| .. | ||
| Kconfig | ||
| Makefile | ||
| coherent.c | ||
| contiguous.c | ||
| debug.c | ||
| direct.c | ||
| dummy.c | ||
| mapping.c | ||
| remap.c | ||
| swiotlb.c | ||
| virt.c | ||