[PATCH 3/3] tcp: Don't account for hash table size in tcp_hash()

[David Gibson <david@gibson.dropbear.id.au>]

Currently tcp_hash() returns the hash bucket for a value, that is the hash
modulo the size of the hash table.  Usually it's a bit more flexible to
have hash functions return a "raw" hash value and perform the modulus in
the callers.  That allows the same hash function to be used for multiple
tables of different sizes, or to re-use the hash for other purposes.

We don't do anything like that with tcp_hash() at present, but we have some
plans to do so.  Prepare for that by making tcp_hash() and tcp_conn_hash()
return raw hash values.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
---
 tcp.c | 23 ++++++++++-------------
 1 file changed, 10 insertions(+), 13 deletions(-)

diff --git a/tcp.c b/tcp.c
index 7e438b7..6b12feb 100644
--- a/tcp.c
+++ b/tcp.c
@@ -1159,18 +1159,15 @@ static int tcp_hash_match(const struct tcp_tap_conn *conn,
  * @eport:	Guest side endpoint port
  * @fport:	Guest side forwarding port
  *
- * Return: hash value, already modulo size of the hash table
+ * Return: hash value, needs to be adjusted for table size
  */
-static unsigned int tcp_hash(const struct ctx *c, const union inany_addr *faddr,
-			     in_port_t eport, in_port_t fport)
+static uint64_t tcp_hash(const struct ctx *c, const union inany_addr *faddr,
+			 in_port_t eport, in_port_t fport)
 {
 	struct siphash_state state = SIPHASH_INIT(c->hash_secret);
-	uint64_t hash;
 
 	inany_siphash_feed(&state, faddr);
-	hash = siphash_final(&state, 20, (uint64_t)eport << 16 | fport);
-
-	return (unsigned int)(hash % TCP_HASH_TABLE_SIZE);
+	return siphash_final(&state, 20, (uint64_t)eport << 16 | fport);
 }
 
 /**
@@ -1178,10 +1175,10 @@ static unsigned int tcp_hash(const struct ctx *c, const union inany_addr *faddr,
  * @c:		Execution context
  * @conn:	Connection
  *
- * Return: hash value, already modulo size of the hash table
+ * Return: hash value, needs to be adjusted for table size
  */
-static unsigned int tcp_conn_hash(const struct ctx *c,
-				  const struct tcp_tap_conn *conn)
+static uint64_t tcp_conn_hash(const struct ctx *c,
+			      const struct tcp_tap_conn *conn)
 {
 	return tcp_hash(c, &conn->faddr, conn->eport, conn->fport);
 }
@@ -1201,7 +1198,7 @@ static inline unsigned tcp_hash_probe(const struct ctx *c,
 	unsigned b;
 
 	/* Linear probing */
-	for (b = tcp_conn_hash(c, conn);
+	for (b = tcp_conn_hash(c, conn) % TCP_HASH_TABLE_SIZE;
 	     !flow_sidx_eq(tc_hash[b], FLOW_SIDX_NONE) &&
 		     !flow_sidx_eq(tc_hash[b], sidx);
 	     b = (b + 1) % TCP_HASH_TABLE_SIZE)
@@ -1243,7 +1240,7 @@ static void tcp_hash_remove(const struct ctx *c,
 	for (s = (b + 1) % TCP_HASH_TABLE_SIZE;
 	     (flow = flow_at_sidx(tc_hash[s]));
 	     s = (s + 1) % TCP_HASH_TABLE_SIZE) {
-		unsigned h = tcp_conn_hash(c, &flow->tcp);
+		unsigned h = tcp_conn_hash(c, &flow->tcp) % TCP_HASH_TABLE_SIZE;
 
 		if (in_mod_range(h, b, s, TCP_HASH_TABLE_SIZE)) {
 			/* tc_hash[s] can live in tc_hash[b]'s slot */
@@ -1299,7 +1296,7 @@ static struct tcp_tap_conn *tcp_hash_lookup(const struct ctx *c,
 
 	inany_from_af(&aany, af, faddr);
 
-	for (b = tcp_hash(c, &aany, eport, fport);
+	for (b = tcp_hash(c, &aany, eport, fport) % TCP_HASH_TABLE_SIZE;
 	     (flow = flow_at_sidx(tc_hash[b]))
 		     && !tcp_hash_match(&flow->tcp, &aany, eport, fport);
 	     b = (b + 1) % TCP_HASH_TABLE_SIZE)
-- 
2.43.0