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

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Message-Id: <20211124202446.2917972-3-eric.dumazet@gmail.com>
Date:   Wed, 24 Nov 2021 12:24:46 -0800
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>
Cc:     netdev <netdev@...r.kernel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Eric Dumazet <eric.dumazet@...il.com>
Subject: [PATCH net-next 2/2] net: optimize skb_postpull_rcsum()

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);

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