From: Robin Murphy <robin.murphy@xxxxxxx> Date: Fri, 26 Jan 2024 15:48:54 +0000 > On 26/01/2024 1:54 pm, Alexander Lobakin wrote: >> From: Eric Dumazet <edumazet@xxxxxxxxxx> >> >> Quite often, NIC devices do not need dma_sync operations on x86_64 >> at least. >> Indeed, when dev_is_dma_coherent(dev) is true and >> dev_use_swiotlb(dev) is false, iommu_dma_sync_single_for_cpu() >> and friends do nothing. >> >> However, indirectly calling them when CONFIG_RETPOLINE=y consumes about >> 10% of cycles on a cpu receiving packets from softirq at ~100Gbit rate. >> Even if/when CONFIG_RETPOLINE is not set, there is a cost of about 3%. >> >> Add dev->skip_dma_sync boolean which is set during the device >> initialization depending on the setup: dev_is_dma_coherent() for direct >> DMA, !(sync_single_for_device || sync_single_for_cpu) or positive result >> from the new callback, dma_map_ops::can_skip_sync for non-NULL DMA ops. >> Then later, if/when swiotlb is used for the first time, the flag >> is turned off, from swiotlb_tbl_map_single(). > > I think you could probably just promote the dma_uses_io_tlb flag from > SWIOTLB_DYNAMIC to a general SWIOTLB thing to serve this purpose now. Nice catch! > > Similarly I don't think a new op is necessary now that we have > dma_map_ops.flags. A simple static flag to indicate that sync may be> skipped under the same conditions as implied for dma-direct - i.e. > dev_is_dma_coherent(dev) && !dev->dma_use_io_tlb - seems like it ought > to suffice. In my initial implementation, I used a new dma_map_ops flag, but then I realized different DMA ops may require or not require syncing under different conditions, not only dev_is_dma_coherent(). Or am I wrong and they would always be the same? > > Thanks, > Robin. Thanks, Olek
