netdev - Re: XDP performance regression due to CONFIG

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Message-ID: <20180412145653.GA7172@lst.de>
Date:   Thu, 12 Apr 2018 16:56:53 +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 04:51:23PM +0200, Christoph Hellwig wrote:
> 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..

I guess that is because x86 selects it as the default as soon as
we have more than 4G memory. That should be solveable fairly easily
with the per-device dma ops, though.