# Re: \[PATCH net-next 2/2\] net: optimize skb\_postpull\_rcsum()

- From: Eric Dumazet \<hidden\>
- Date: 2021-11-25 13:40:49 
- List: netdev
- Page: https://ratatoskr.run/netdev/2021/11/11217082
- Thread: https://ratatoskr.run/netdev/2021/11/11216985/t.md

```text
On Thu, Nov 25, 2021 at 1:41 AM David Laight [ref] wrote:
>
> From: Eric Dumazet
> > Sent: 24 November 2021 20:25
> >
> > From: Eric Dumazet <redacted>
> >
> > Remove one pair of add/adc instructions and their dependency
> > against carry flag.
> >
> > We can leverage third argument to csum_partial():
> >
> >   X = csum_block_sub(X, csum_partial(start, len, 0), 0);
> >
> >   -->
> >
> >   X = csum_block_add(X, ~csum_partial(start, len, 0), 0);
> >
> >   -->
> >
> >   X = ~csum_partial(start, len, ~X);
>
> That doesn't seem to refer to the change in this file.
>

It is describing the change.

The first step, of copying first the __skb_postpull_rcsum(skb, start,
len, 0) content
into __skb_postpull_rcsum() was kind of obvious.



>         David
>
> >
> > Signed-off-by: Eric Dumazet <redacted>
> > ---
> >  include/linux/skbuff.h | 6 +++++-
> >  1 file changed, 5 insertions(+), 1 deletion(-)
> >
> > diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
> > index eba256af64a577b458998845f2dc01a5ec80745a..eae4bd3237a41cc1b60b44c91fbfe21dfdd8f117 100644
> > --- a/include/linux/skbuff.h
> > +++ b/include/linux/skbuff.h
> > @@ -3485,7 +3485,11 @@ __skb_postpull_rcsum(struct sk_buff *skb, const void *start, unsigned int len,
> >  static inline void skb_postpull_rcsum(struct sk_buff *skb,
> >                                     const void *start, unsigned int len)
> >  {
> > -     __skb_postpull_rcsum(skb, start, len, 0);
> > +     if (skb->ip_summed == CHECKSUM_COMPLETE)
> > +             skb->csum = ~csum_partial(start, len, ~skb->csum);
> > +     else if (skb->ip_summed == CHECKSUM_PARTIAL &&
> > +              skb_checksum_start_offset(skb) < 0)
> > +             skb->ip_summed = CHECKSUM_NONE;
> >  }
> >
> >  static __always_inline void
> > --
> > 2.34.0.rc2.393.gf8c9666880-goog
>
> -
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> Registration No: 1397386 (Wales)
>
```