netdev - RE: [PATCH net-next 2/2] net: optimize skb_postpull

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Date:   Fri, 3 Dec 2021 14:57:04 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Vladimir Oltean' <olteanv@...il.com>
CC:     Eric Dumazet <edumazet@...gle.com>,
        Eric Dumazet <eric.dumazet@...il.com>,
        "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        netdev <netdev@...r.kernel.org>
Subject: RE: [PATCH net-next 2/2] net: optimize skb_postpull_rcsum()

From: Vladimir Oltean
> Sent: 02 December 2021 21:40
...
> >
> > Try replacing both ~ with -.
> > So replace:
> > 		skb->csum = ~csum_partial(start, len, ~skb->csum);
> > with:
> > 		skb->csum = -csum_partial(start, len, -skb->csum);
> >
> > That should geneate ~0u instead 0 (if I've got my maths right).
> 
> Indeed, replacing both one's complement operations with two's complement
> seems to produce correct results (consistent with old code) in all cases
> that I am testing with (ICMP, TCP, UDP). Thanks!

You need to generate (or persuade Eric to generate) a patch.
I don't have the right source tree.

Any code that does ~csum_partial() is 'dubious' unless followed
by a check for 0.
The two's compliment negate save the conditional - provided the
offset of 1 can be added in earlier.

	David

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