netdev - Re: Questions on XDP

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Message-ID: <1487439675.1311.96.camel@edumazet-glaptop3.roam.corp.google.com>
Date:   Sat, 18 Feb 2017 09:41:15 -0800
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Jesper Dangaard Brouer <brouer@...hat.com>
Cc:     John Fastabend <john.fastabend@...il.com>,
        Alexander Duyck <alexander.duyck@...il.com>,
        Netdev <netdev@...r.kernel.org>,
        Tom Herbert <tom@...bertland.com>,
        Alexei Starovoitov <ast@...nel.org>,
        John Fastabend <john.r.fastabend@...el.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        David Miller <davem@...emloft.net>
Subject: Re: Questions on XDP

On Sat, 2017-02-18 at 17:34 +0100, Jesper Dangaard Brouer wrote:
> On Thu, 16 Feb 2017 14:36:41 -0800
> John Fastabend <john.fastabend@...il.com> wrote:
> 
> > On 17-02-16 12:41 PM, Alexander Duyck wrote:
> > > So I'm in the process of working on enabling XDP for the Intel NICs
> > > and I had a few questions so I just thought I would put them out here
> > > to try and get everything sorted before I paint myself into a corner.
> > >   
> > > So my first question is why does the documentation mention 1 frame per
> > > page for XDP?  
> 
> Yes, XDP defines upfront a memory model where there is only one packet
> per page[1], please respect that!
> 
> This is currently used/needed for fast-direct recycling of pages inside
> the driver for XDP_DROP and XDP_TX, _without_ performing any atomic
> refcnt operations on the page. E.g. see mlx4_en_rx_recycle().


XDP_DROP does not require having one page per frame.

(Look after my recent mlx4 patch series if you need to be convinced)

Only XDP_TX is.

This requirement makes XDP useless (very OOM likely) on arches with 64K
pages.