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

[Chao Gao]

On Tue, Apr 12, 2022 at 02:33:05PM +0100, Robin Murphy wrote:
>On 12/04/2022 12:38 pm, 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.
>
>It's still a little suboptimal and non-obvious to call into SWIOTLB twice
>though - even better might be for SWIOTLB to call arch_sync_dma_for_cpu() at
>the appropriate place internally,

Hi Robin,

dma_direct_sync_single_for_cpu() also calls arch_sync_dma_for_cpu_all()
and arch_dma_mark_clean() in some cases. if SWIOTLB does sync internally,
should these two functions be called by SWIOTLB?

Personally, it might be better if swiotlb can just focus on bounce buffer
alloc/free. Adding more DMA coherence logic into swiotlb will make it
a little complicated.

How about an open-coded version of dma_direct_sync_single_for_cpu
in dma_direct_unmap_page with swiotlb_sync_single_for_cpu replaced by
swiotlb_tbl_unmap_single?