netdev - Re: [net-next PATCH v2 4/8] udp: Do not pass checksum as a parameter to GSO segmentation

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Date:   Sat, 5 May 2018 12:01:12 +0200
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Alexander Duyck <alexander.duyck@...il.com>
Cc:     Network Development <netdev@...r.kernel.org>,
        Willem de Bruijn <willemb@...gle.com>,
        David Miller <davem@...emloft.net>
Subject: Re: [net-next PATCH v2 4/8] udp: Do not pass checksum as a parameter
 to GSO segmentation

On Fri, May 4, 2018 at 8:30 PM, Alexander Duyck
<alexander.duyck@...il.com> wrote:
> From: Alexander Duyck <alexander.h.duyck@...el.com>
>
> This patch is meant to allow us to avoid having to recompute the checksum
> from scratch and have it passed as a parameter.
>
> Instead of taking that approach we can take advantage of the fact that the
> length that was used to compute the existing checksum is included in the
> UDP header. If we cancel that out by adding the value XOR with 0xFFFF we
> can then just add the new length in and fold that into the new result.
>
> I think this may be fixing a checksum bug in the original code as well
> since the checksum that was passed included the UDP header in the checksum
> computation, but then excluded it for the adjustment on the last frame. I
> believe this may have an effect on things in the cases where the two differ
> by bits that would result in things crossing the byte boundaries.

The replacement code, below, subtracts original payload size then adds
the new payload size. mss here excludes the udp header size.

>                 /* last packet can be partial gso_size */
> -               if (!seg->next)
> -                       csum_replace2(&uh->check, htons(mss),
> -                                     htons(seg->len - hdrlen - sizeof(*uh)));