netdev - RE: [PATCH v2] net: Make skb_segment not to compute checksum if network controller supports checksumming

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Date:   Fri, 6 Mar 2020 17:12:05 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Eric Dumazet' <eric.dumazet@...il.com>,
        'Willem de Bruijn' <willemdebruijn.kernel@...il.com>,
        Yadu Kishore <kyk.segfault@...il.com>
CC:     David Miller <davem@...emloft.net>,
        Network Development <netdev@...r.kernel.org>
Subject: RE: [PATCH v2] net: Make skb_segment not to compute checksum if
 network controller supports checksumming

From: Eric Dumazet
> Sent: 05 March 2020 17:20
> 
> On 3/5/20 9:00 AM, David Laight wrote:
> > From: Willem de Bruijn
> >> Sent: 05 March 2020 16:07
> > ..
> >> It seems do_csum is called because csum_partial_copy executes the
> >> two operations independently:
> >>
> >> __wsum
> >> csum_partial_copy(const void *src, void *dst, int len, __wsum sum)
> >> {
> >>         memcpy(dst, src, len);
> >>         return csum_partial(dst, len, sum);
> >> }
> >
> > And do_csum() is superbly horrid.
> > Not the least because it is 32bit on 64bit systems.
> 
> There are many versions, which one is discussed here ?
> 
> At least the current one seems to be 64bit optimized.
> 
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=5777eaed566a1d63e344d3dd
> 8f2b5e33be20643e

I was looking at the generic one in $(SRC)/lib/checksum.c.

FWIW I suspect the fastest code on pre sandy bridge 64bit intel cpus
(where adc is 2 clocks) is to do a normal 'add', shift the carries
into a 64bit register and do a software 'popcnt' every 512 bytes.
That may run at 8 bytes/clock + the popcnt.

	David

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