netdev - [PATCH net-next 1/2] gro: optimize skb_gro_postpull

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Date:   Wed, 24 Nov 2021 12:24:45 -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 1/2] gro: optimize skb_gro_postpull_rcsum()

From: Eric Dumazet <edumazet@...gle.com>

We can leverage third argument to csum_partial():

  X = csum_sub(X, csum_partial(start, len, 0));

  -->

  X = csum_add(X, ~csum_partial(start, len, 0));

  -->

  X = ~csum_partial(start, len, ~X);

This removes one add/adc pair and its dependency against the carry flag.

Signed-off-by: Eric Dumazet <edumazet@...gle.com>
---
 include/net/gro.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/include/net/gro.h b/include/net/gro.h
index 9c22a010369cb89f9511d78cc322be56170d7b20..b1139fca7c435ca0c353c4ed17628dd7f3df4401 100644
--- a/include/net/gro.h
+++ b/include/net/gro.h
@@ -173,8 +173,8 @@ static inline void skb_gro_postpull_rcsum(struct sk_buff *skb,
 					const void *start, unsigned int len)
 {
 	if (NAPI_GRO_CB(skb)->csum_valid)
-		NAPI_GRO_CB(skb)->csum = csum_sub(NAPI_GRO_CB(skb)->csum,
-						  csum_partial(start, len, 0));
+		NAPI_GRO_CB(skb)->csum = ~csum_partial(start, len,
+						       ~NAPI_GRO_CB(skb)->csum);
 }
 
 /* GRO checksum functions. These are logical equivalents of the normal
-- 
2.34.0.rc2.393.gf8c9666880-goog