netdev - Re: Questions on XDP

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Date:   Sat, 18 Feb 2017 17:34:02 +0100
From:   Jesper Dangaard Brouer <brouer@...hat.com>
To:     John Fastabend <john.fastabend@...il.com>
Cc:     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>, brouer@...hat.com,
        David Miller <davem@...emloft.net>
Subject: Re: Questions on XDP

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().

This is also about controlling the cache-coherency state of the
struct-page cache-line.  (With two (or-more) packets per page,
the struct-page cache-line will be jumping around.) Controlling this is
essential when packets are transferred between CPUs. We need an
architecture were we can control this, please.

[1] https://prototype-kernel.readthedocs.io/en/latest/networking/XDP/design/requirements.html#page-per-packet

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer