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Subject: Re: [PATCH net] xsk: remove cheap_dma optimization
To:     Robin Murphy <robin.murphy@arm.com>,
        Christoph Hellwig <hch@lst.de>,
        Daniel Borkmann <daniel@iogearbox.net>
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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.