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

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Message-ID: <CABGOaVTzjJengG0e8AWFZE9ZG1245keuQHfRJ0zpoAMQrNmJ6g@mail.gmail.com>
Date:   Fri, 13 Mar 2020 12:06:34 +0530
From:   Yadu Kishore <kyk.segfault@...il.com>
To:     Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        David Miller <davem@...emloft.net>
Cc:     Eric Dumazet <eric.dumazet@...il.com>,
        Network Development <netdev@...r.kernel.org>,
        David Laight <David.Laight@...lab.com>
Subject: Re: [PATCH v2] net: Make skb_segment not to compute checksum if
 network controller supports checksumming

> Yes, given the discussion I have no objections. The change to
> skb_segment in v2 look fine.

I'm assuming that the changes in patch V2 are ok to be accepted and merged
to the kernel. Please let me know if there is anything else that is pending
from my side with respect to the patch.

Thanks,
Yadu Kishore

On Fri, Mar 6, 2020 at 10:42 PM David Laight <David.Laight@...lab.com> wrote:
>
> 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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