netdev - Re: XDP performance regression due to CONFIG

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Message-ID: <20180412145123.GA7048@lst.de>
Date:   Thu, 12 Apr 2018 16:51:23 +0200
From:   Christoph Hellwig <hch@....de>
To:     Jesper Dangaard Brouer <brouer@...hat.com>
Cc:     "xdp-newbies@...r.kernel.org" <xdp-newbies@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Christoph Hellwig <hch@....de>,
        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 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..