netdev - Re: XDP performance regression due to CONFIG

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Date:   Fri, 13 Apr 2018 19:26:11 +0200
From:   Christoph Hellwig <hch@....de>
To:     Tushar Dave <tushar.n.dave@...cle.com>
Cc:     Christoph Hellwig <hch@....de>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        "xdp-newbies@...r.kernel.org" <xdp-newbies@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        David Woodhouse <dwmw2@...radead.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, 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.