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Date: Wed, 28 Jun 2017 11:18:11 +0200
From: Daniel Borkmann <daniel@iogearbox.net>
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Subject: Re: UDP checksum calculation
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To: "Panneman, J.J. (Jeffrey)" <jeffrey.panneman@tno.nl>
Cc: "xdp-newbies@vger.kernel.org" <xdp-newbies@vger.kernel.org>

On 06/28/2017 10:46 AM, Panneman, J.J. (Jeffrey) wrote:
> Hi All,
>
> I posted this question in the Cillium Slack too, but I figured posting it here might help other people too.
> I checked out the Cillium source(from what I see makes use of the provided tc-helpers: https://github.com/cilium/cilium/blob/master/bpf/lib/csum.h), but is there an appropriate way to do checksum (re)calculation for UDP with XDP ? Since it does not really like long loops that much and the payload possibly being quite long. I don't really see a way to loop over the payload to recalculate the checksum, would anyone perhaps have a code snippet for this ?

Depends on the use-case, are you rewriting also UDP payload
in XDP? If not, why is [1] not sufficient resp. a variant of
that which calcs the diff? csum_diff() could be an option, but
why not doing it in BPF itself?

   [1] http://lingrok.org/xref/linux-net-next/tools/testing/selftests/bpf/test_xdp.c#141