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Chao Gao authored
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:
Wang Zhaoyang1 <zhaoyang1.wang@intel.com> Reported-by:
Gao Liang <liang.gao@intel.com> Signed-off-by:
Chao Gao <chao.gao@intel.com> Reviewed-by:
Kevin Tian <kevin.tian@intel.com> Signed-off-by:
Christoph Hellwig <hch@lst.de>
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