Commit 9e02977b authored by Chao Gao's avatar Chao Gao Committed by Christoph Hellwig
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dma-direct: avoid redundant memory sync for swiotlb 



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: 55897af6 ("dma-direct: merge swiotlb_dma_ops into the dma_direct code")
Reported-by: default avatarWang Zhaoyang1 <zhaoyang1.wang@intel.com>
Reported-by: default avatarGao Liang <liang.gao@intel.com>
Signed-off-by: default avatarChao Gao <chao.gao@intel.com>
Reviewed-by: default avatarKevin Tian <kevin.tian@intel.com>
Signed-off-by: default avatarChristoph Hellwig <hch@lst.de>
parent 4fe87e81

kernel/dma/direct.h

+2 −1
Original line number Diff line number Diff line
@@ -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 */