netdev - Re: XDP performance regression due to CONFIG

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Date:   Sat, 14 Apr 2018 21:29:26 +0200
From:   David Woodhouse <dwmw2@...radead.org>
To:     Christoph Hellwig <hch@....de>,
        Tushar Dave <tushar.n.dave@...cle.com>
Cc:     Jesper Dangaard Brouer <brouer@...hat.com>,
        "xdp-newbies@...r.kernel.org" <xdp-newbies@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        William Tu <u9012063@...il.com>,
        Björn Töpel <bjorn.topel@...el.com>,
        "Karlsson, Magnus" <magnus.karlsson@...el.com>,
        Alexander Duyck <alexander.duyck@...il.com>,
        Arnaldo Carvalho de Melo <acme@...hat.com>
Subject: Re: XDP performance regression due to CONFIG_RETPOLINE Spectre V2



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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