On Tue, Apr 12, 2022 at 07:38:05PM +0800, Chao Gao wrote:
>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%.
>
Sorry. A fixes tag is missing:
Fixes: 55897af63091 ("dma-direct: merge swiotlb_dma_ops into the dma_direct
code")
>Reported-by: Wang Zhaoyang1 <[email protected]>
>Reported-by: Gao Liang <[email protected]>
>Signed-off-by: Chao Gao <[email protected]>
>Reviewed-by: Kevin Tian <[email protected]>
>---
> kernel/dma/direct.h | 3 ++-
> 1 file changed, 2 insertions(+), 1 deletion(-)
>
>diff --git a/kernel/dma/direct.h b/kernel/dma/direct.h
>index 4632b0f4f72e..8a6cd53dbe8c 100644
>--- a/kernel/dma/direct.h
>+++ b/kernel/dma/direct.h
>@@ -114,6 +114,7 @@ static inline void dma_direct_unmap_page(struct device
>*dev, dma_addr_t addr,
> dma_direct_sync_single_for_cpu(dev, addr, size, dir);
>
> if (unlikely(is_swiotlb_buffer(dev, phys)))
>- swiotlb_tbl_unmap_single(dev, phys, size, dir, attrs);
>+ swiotlb_tbl_unmap_single(dev, phys, size, dir,
>+ attrs | DMA_ATTR_SKIP_CPU_SYNC);
> }
> #endif /* _KERNEL_DMA_DIRECT_H */
>--
>2.25.1
>
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