On Fri, 2018-04-13 at 19:26 +0200, Christoph Hellwig wrote: > On Fri, Apr 13, 2018 at 10:12:41AM -0700, Tushar Dave wrote: > > I guess there is nothing we need to do! > > > > On x86, in case of no intel iommu or iommu is disabled, you end up in > > swiotlb for DMA API calls when system has 4G memory. > > However, AFAICT, for 64bit DMA capable devices swiotlb DMA APIs do not > > use bounce buffer until and unless you have swiotlb=force specified in > > kernel commandline. > > Sure. But that means very sync_*_to_device and sync_*_to_cpu now > involves an indirect call to do exactly nothing, which in the workload > Jesper is looking at is causing a huge performance degradation due to > retpolines. We should look at using the if (dma_ops == swiotlb_dma_ops) swiotlb_map_page() else dma_ops->map_page() trick for this. Perhaps with alternatives so that when an Intel or AMD IOMMU is detected, it's *that* which is checked for as the special case.
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