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

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Date:   Fri, 3 Dec 2021 18:14:29 +0200
From:   Vladimir Oltean <olteanv@...il.com>
To:     David Laight <David.Laight@...LAB.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()

On Fri, Dec 03, 2021 at 02:57:04PM +0000, David Laight wrote:
> 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
> 
> -
> Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
> Registration No: 1397386 (Wales)
> 

Eric, could you please send a patch with this change?
If you want and if it helps, I can also help you reproduce this locally
using the dsa_loop mockup driver.