tcp: Re-use flow hash for initial sequence number generation

We generate TCP initial sequence numbers, when we need them, from a
hash of the source and destination addresses and ports, plus a
timestamp.  Moments later, we generate another hash of the same
information plus some more to insert the connection into the flow hash
table.

With some tweaks to the flow_hash_insert() interface and changing the
order we can re-use that hash table hash for the initial sequence
number, rather than calculating another one.  It won't generate
identical results, but that doesn't matter as long as the sequence
numbers are well scattered.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

1 files changed, 24 insertions, 6 deletions

diff --git a/flow.c b/flow.c
index b291f25..8eea861 100644
--- a/flow.c
+++ b/flow.c
@@ -455,16 +455,16 @@ static uint64_t flow_sidx_hash(const struct ctx *c, flow_sidx_t sidx)
 }
 
 /**
- * flow_hash_probe() - Find hash bucket for a flow
- * @c:		Execution context
+ * flow_hash_probe_() - Find hash bucket for a flow, given hash
+ * @hash:	Raw hash value for flow & side
  * @sidx:	Flow and side to find bucket for
  *
  * Return: If @sidx is in the hash table, its current bucket, otherwise a
  *         suitable free bucket for it.
  */
-static inline unsigned flow_hash_probe(const struct ctx *c, flow_sidx_t sidx)
+static inline unsigned flow_hash_probe_(uint64_t hash, flow_sidx_t sidx)
 {
-	unsigned b = flow_sidx_hash(c, sidx) % FLOW_HASH_SIZE;
+	unsigned b = hash % FLOW_HASH_SIZE;
 
 	/* Linear probing */
 	while (flow_sidx_valid(flow_hashtab[b]) &&
@@ -475,17 +475,35 @@ static inline unsigned flow_hash_probe(const struct ctx *c, flow_sidx_t sidx)
 }
 
 /**
+ * flow_hash_probe() - Find hash bucket for a flow
+ * @c:		Execution context
+ * @sidx:	Flow and side to find bucket for
+ *
+ * Return: If @sidx is in the hash table, its current bucket, otherwise a
+ *         suitable free bucket for it.
+ */
+static inline unsigned flow_hash_probe(const struct ctx *c, flow_sidx_t sidx)
+{
+	return flow_hash_probe_(flow_sidx_hash(c, sidx), sidx);
+}
+
+/**
  * flow_hash_insert() - Insert side of a flow into into hash table
  * @c:		Execution context
  * @sidx:	Flow & side index
+ *
+ * Return: raw (un-modded) hash value of side of flow
  */
-void flow_hash_insert(const struct ctx *c, flow_sidx_t sidx)
+uint64_t flow_hash_insert(const struct ctx *c, flow_sidx_t sidx)
 {
-	unsigned b = flow_hash_probe(c, sidx);
+	uint64_t hash = flow_sidx_hash(c, sidx);
+	unsigned b = flow_hash_probe_(hash, sidx);
 
 	flow_hashtab[b] = sidx;
 	flow_dbg(flow_at_sidx(sidx), "Side %u hash table insert: bucket: %u",
 		 sidx.sidei, b);
+
+	return hash;
 }
 
 /**