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.w...@intel.com>
Reported-by: Gao Liang <liang....@intel.com>
Signed-off-by: Chao Gao <chao....@intel.com>
Reviewed-by: Kevin Tian <kevin.t...@intel.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
_______________________________________________
iommu mailing list
iommu@lists.linux-foundation.org
https://lists.linuxfoundation.org/mailman/listinfo/iommu
