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

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Date:   Thu, 25 Nov 2021 09:41:55 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Eric Dumazet' <eric.dumazet@...il.com>,
        "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>
CC:     netdev <netdev@...r.kernel.org>, Eric Dumazet <edumazet@...gle.com>
Subject: RE: [PATCH net-next 2/2] net: optimize skb_postpull_rcsum()

From: Eric Dumazet
> Sent: 24 November 2021 20:25
> 
> From: Eric Dumazet <edumazet@...gle.com>
> 
> 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.

	David
 
> 
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> ---
>  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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