net: local checksum offload for encapsulation
The arithmetic properties of the ones-complement checksum mean that a
correctly checksummed inner packet, including its checksum, has a ones
complement sum depending only on whatever value was used to initialise
the checksum field before checksumming (in the case of TCP and UDP,
this is the ones complement sum of the pseudo header, complemented).
Consequently, if we are going to offload the inner checksum with
CHECKSUM_PARTIAL, we can compute the outer checksum based only on the
packed data not covered by the inner checksum, and the initial value of
the inner checksum field.
Signed-off-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index 6ec86f1..cf906d1 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -3702,5 +3702,29 @@
return hdr_len + skb_gso_transport_seglen(skb);
}
+/* Local Checksum Offload.
+ * Compute outer checksum based on the assumption that the
+ * inner checksum will be offloaded later.
+ * Fill in outer checksum adjustment (e.g. with sum of outer
+ * pseudo-header) before calling.
+ * Also ensure that inner checksum is in linear data area.
+ */
+static inline __wsum lco_csum(struct sk_buff *skb)
+{
+ char *inner_csum_field;
+ __wsum csum;
+
+ /* Start with complement of inner checksum adjustment */
+ inner_csum_field = skb->data + skb_checksum_start_offset(skb) +
+ skb->csum_offset;
+ csum = ~csum_unfold(*(__force __sum16 *)inner_csum_field);
+ /* Add in checksum of our headers (incl. outer checksum
+ * adjustment filled in by caller)
+ */
+ csum = skb_checksum(skb, 0, skb_checksum_start_offset(skb), csum);
+ /* The result is the checksum from skb->data to end of packet */
+ return csum;
+}
+
#endif /* __KERNEL__ */
#endif /* _LINUX_SKBUFF_H */