Messages in this thread | | | Subject | Re: [PATCH net] xsk: remove cheap_dma optimization | From | Björn Töpel <> | Date | Wed, 1 Jul 2020 12:17:50 +0200 |
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On 2020-06-29 17:41, Robin Murphy wrote: > On 2020-06-28 18:16, Björn Töpel wrote: [...]> >> Somewhat related to the DMA API; It would have performance benefits for >> AF_XDP if the DMA range of the mapped memory was linear, i.e. by IOMMU >> utilization. I've started hacking a thing a little bit, but it would be >> nice if such API was part of the mapping core. >> >> Input: array of pages Output: array of dma addrs (and obviously dev, >> flags and such) >> >> For non-IOMMU len(array of pages) == len(array of dma addrs) >> For best-case IOMMU len(array of dma addrs) == 1 (large linear space) >> >> But that's for later. :-) > > FWIW you will typically get that behaviour from IOMMU-based > implementations of dma_map_sg() right now, although it's not strictly > guaranteed. If you can weather some additional setup cost of calling > sg_alloc_table_from_pages() plus walking the list after mapping to test > whether you did get a contiguous result, you could start taking > advantage of it as some of the dma-buf code in DRM and v4l2 does already > (although those cases actually treat it as a strict dependency rather > than an optimisation). > > I'm inclined to agree that if we're going to see more of these cases, a > new API call that did formally guarantee a DMA-contiguous mapping > (either via IOMMU or bounce buffering) or failure might indeed be handy. >
I forgot to reply to this one! My current hack is using the iommu code directly, similar to what vfio-pci does (hopefully not gutting the API this time ;-)).
Your approach sound much nicer, and easier. I'll try that out! Thanks a lot for the pointers, and I might be back with more questions.
Cheers, Björn
> Robin.
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