[net-next PATCH v3 3/6] udp: Do not pass checksum as a parameter to GSO segmentation

[Alexander Duyck <alexander.duyck@gmail.com>]

From: Alexander Duyck <alexander.h.duyck@intel.com>

This patch is meant to allow us to avoid having to recompute the checksum
from scratch and have it passed as a parameter.

Instead of taking that approach we can take advantage of the fact that the
length that was used to compute the existing checksum is included in the
UDP header.

Finally to avoid the need to invert the result we can just call csum16_add
and csum16_sub directly. By doing this we can avoid a number of
instructions in the loop that is handling segmentation.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
---
 include/net/udp.h      |    3 +--
 net/ipv4/udp_offload.c |   32 ++++++++++++++++++--------------
 net/ipv6/udp_offload.c |    7 +------
 3 files changed, 20 insertions(+), 22 deletions(-)

diff --git a/include/net/udp.h b/include/net/udp.h
index 8bd83b044ecd..9289b6425032 100644
--- a/include/net/udp.h
+++ b/include/net/udp.h
@@ -175,8 +175,7 @@ struct sk_buff **udp_gro_receive(struct sk_buff **head, struct sk_buff *skb,
 int udp_gro_complete(struct sk_buff *skb, int nhoff, udp_lookup_t lookup);
 
 struct sk_buff *__udp_gso_segment(struct sk_buff *gso_skb,
-				  netdev_features_t features,
-				  __sum16 check);
+				  netdev_features_t features);
 
 static inline struct udphdr *udp_gro_udphdr(struct sk_buff *skb)
 {
diff --git a/net/ipv4/udp_offload.c b/net/ipv4/udp_offload.c
index c1afcd2f1a76..92c182e99ddc 100644
--- a/net/ipv4/udp_offload.c
+++ b/net/ipv4/udp_offload.c
@@ -188,8 +188,7 @@ struct sk_buff *skb_udp_tunnel_segment(struct sk_buff *skb,
 EXPORT_SYMBOL(skb_udp_tunnel_segment);
 
 struct sk_buff *__udp_gso_segment(struct sk_buff *gso_skb,
-				  netdev_features_t features,
-				  __sum16 check)
+				  netdev_features_t features)
 {
 	struct sock *sk = gso_skb->sk;
 	unsigned int sum_truesize = 0;
@@ -197,6 +196,8 @@ struct sk_buff *__udp_gso_segment(struct sk_buff *gso_skb,
 	unsigned int hdrlen;
 	struct udphdr *uh;
 	unsigned int mss;
+	__sum16 check;
+	__be16 newlen;
 
 	mss = skb_shinfo(gso_skb)->gso_size;
 	if (gso_skb->len <= sizeof(*uh) + mss)
@@ -215,17 +216,25 @@ struct sk_buff *__udp_gso_segment(struct sk_buff *gso_skb,
 		return segs;
 	}
 
+	uh = udp_hdr(segs);
+
+	/* compute checksum adjustment based on old length versus new */
+	newlen = htons(sizeof(*uh) + mss);
+	check = csum16_add(csum16_sub(uh->check, uh->len), newlen);
+
 	for (seg = segs; seg; seg = seg->next) {
 		uh = udp_hdr(seg);
-		uh->len = htons(seg->len - hdrlen);
-		uh->check = check;
 
 		/* last packet can be partial gso_size */
-		if (!seg->next)
-			csum_replace2(&uh->check, htons(mss),
-				      htons(seg->len - hdrlen - sizeof(*uh)));
+		if (!seg->next) {
+			newlen = htons(seg->len - hdrlen);
+			check = csum16_add(csum16_sub(uh->check, uh->len),
+					   newlen);
+		}
+
+		uh->len = newlen;
+		uh->check = check;
 
-		uh->check = ~uh->check;
 		seg->destructor = sock_wfree;
 		seg->sk = sk;
 		sum_truesize += seg->truesize;
@@ -240,15 +249,10 @@ struct sk_buff *__udp_gso_segment(struct sk_buff *gso_skb,
 static struct sk_buff *__udp4_gso_segment(struct sk_buff *gso_skb,
 					  netdev_features_t features)
 {
-	const struct iphdr *iph = ip_hdr(gso_skb);
-	unsigned int mss = skb_shinfo(gso_skb)->gso_size;
-
 	if (!can_checksum_protocol(features, htons(ETH_P_IP)))
 		return ERR_PTR(-EIO);
 
-	return __udp_gso_segment(gso_skb, features,
-				 udp_v4_check(sizeof(struct udphdr) + mss,
-					      iph->saddr, iph->daddr, 0));
+	return __udp_gso_segment(gso_skb, features);
 }
 
 static struct sk_buff *udp4_ufo_fragment(struct sk_buff *skb,
diff --git a/net/ipv6/udp_offload.c b/net/ipv6/udp_offload.c
index dea03ec09715..61e34f1d2fa2 100644
--- a/net/ipv6/udp_offload.c
+++ b/net/ipv6/udp_offload.c
@@ -20,15 +20,10 @@
 static struct sk_buff *__udp6_gso_segment(struct sk_buff *gso_skb,
 					  netdev_features_t features)
 {
-	const struct ipv6hdr *ip6h = ipv6_hdr(gso_skb);
-	unsigned int mss = skb_shinfo(gso_skb)->gso_size;
-
 	if (!can_checksum_protocol(features, htons(ETH_P_IPV6)))
 		return ERR_PTR(-EIO);
 
-	return __udp_gso_segment(gso_skb, features,
-				 udp_v6_check(sizeof(struct udphdr) + mss,
-					      &ip6h->saddr, &ip6h->daddr, 0));
+	return __udp_gso_segment(gso_skb, features);
 }
 
 static struct sk_buff *udp6_ufo_fragment(struct sk_buff *skb,