On Thu, 12 Apr 2018 16:56:53 +0200 Christoph Hellwig <hch@xxxxxx> wrote: > On Thu, Apr 12, 2018 at 04:51:23PM +0200, Christoph Hellwig wrote: > > On Thu, Apr 12, 2018 at 03:50:29PM +0200, Jesper Dangaard Brouer wrote: > > > --------------- > > > Implement support for keeping the DMA mapping through the XDP return > > > call, to remove RX map/unmap calls. Implement bulking for XDP > > > ndo_xdp_xmit and XDP return frame API. Bulking allows to perform DMA > > > bulking via scatter-gatter DMA calls, XDP TX need it for DMA > > > map+unmap. The driver RX DMA-sync (to CPU) per packet calls are harder > > > to mitigate (via bulk technique). Ask DMA maintainer for a common > > > case direct call for swiotlb DMA sync call ;-) > > > > Why do you even end up in swiotlb code? Once you bounce buffer your > > performance is toast anyway.. > > I guess that is because x86 selects it as the default as soon as > we have more than 4G memory. I were also confused why I ended up using SWIOTLB (SoftWare IO-TLB), that might explain it. And I'm not hitting the bounce-buffer case. How do I control which DMA engine I use? (So, I can play a little) > That should be solveable fairly easily with the per-device dma ops, > though. I didn't understand this part. I wanted to ask your opinion, on a hackish idea I have... Which is howto detect, if I can reuse the RX-DMA map address, for TX-DMA operation on another device (still/only calling sync_single_for_device). With XDP_REDIRECT we are redirecting between net_device's. Usually we keep the RX-DMA mapping as we recycle the page. On the redirect to TX-device (via ndo_xdp_xmit) we do a new DMA map+unmap for TX. The question is how to avoid this mapping(?). In some cases, with some DMA engines (or lack of) I guess the DMA address is actually the same as the RX-DMA mapping dma_addr_t already known, right? For those cases, would it be possible to just (re)use that address for TX? -- Best regards, Jesper Dangaard Brouer MSc.CS, Principal Kernel Engineer at Red Hat LinkedIn: http://www.linkedin.com/in/brouer
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