linux-kernel - [PATCH] dma-direct: avoid redundant memory sync for swiotlb

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Date:   Tue, 12 Apr 2022 19:38:05 +0800
From:   Chao Gao <chao.gao@...el.com>
To:     linux-kernel@...r.kernel.org, iommu@...ts.linux-foundation.org
Cc:     robin.murphy@....com, m.szyprowski@...sung.com, hch@....de,
        Chao Gao <chao.gao@...el.com>,
        Wang Zhaoyang1 <zhaoyang1.wang@...el.com>,
        Gao Liang <liang.gao@...el.com>,
        Kevin Tian <kevin.tian@...el.com>
Subject: [PATCH] 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%.

Reported-by: Wang Zhaoyang1 <zhaoyang1.wang@...el.com>
Reported-by: Gao Liang <liang.gao@...el.com>
Signed-off-by: Chao Gao <chao.gao@...el.com>
Reviewed-by: Kevin Tian <kevin.tian@...el.com>
---
 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