netdev - Re: [PATCH net-next] sfc: fix XDP-redirect in this driver

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Date:   Mon, 16 Mar 2020 11:35:15 +0100
From:   Jesper Dangaard Brouer <brouer@...hat.com>
To:     Edward Cree <ecree@...arflare.com>
Cc:     David Miller <davem@...emloft.net>, <netdev@...r.kernel.org>,
        <linux-net-drivers@...arflare.com>, <mhabets@...arflare.com>,
        <cmclachlan@...arflare.com>, <ilias.apalodimas@...aro.org>,
        <lorenzo@...nel.org>, <sameehj@...zon.com>, brouer@...hat.com
Subject: Re: [PATCH net-next] sfc: fix XDP-redirect in this driver

On Mon, 16 Mar 2020 10:10:01 +0000
Edward Cree <ecree@...arflare.com> wrote:

> On 16/03/2020 08:49, David Miller wrote:
> > Solarflare folks, please review.  
>
> This looks like a correct implementation of what it purports to do, so
> Acked-by: Edward Cree <ecree@...arflare.com>

Thanks for the review!

> It did take me some digging to understand _why_ it was needed though;
>  Jesper, is there any documentation of the tailroom requirement?  It
>  doesn't seem to be mentioned anywhere I could find.

I admit that is is poorly documented.  It is a requirement as both
cpumap and veth have a dependency when they process the redirected
packet.  We/I also need to update the doc on one page per packet
requirement, as it is (in-practice) no longer true.

I'm noticing these bugs, because I'm working on a patchset that add
tailroom extend, and also reserves this 'skb_shared_info' tailroom area.
The real goal is to later add XDP multi-buffer support, using this
'skb_shared_info' tailroom area, as desc here[2]

[2] https://github.com/xdp-project/xdp-project/blob/master/areas/core/xdp-multi-buffer01-design.org

> (Is there even any up-to-date doc of the XDP driver interface?  The
>  one at [1] looks a bit stale...)
> -Ed
> 
> [1]: https://prototype-kernel.readthedocs.io/en/latest/networking/XDP/design/requirements.html

This is my old and out-dated documentation. I didn't know people were
still referring to this.  I does score quite high on Google, so I
guess, that I really need to update this documentation.  (It was my
plan that this would be merged into kernel tree, but I can see it have
been bit-rotting for too long).

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