tcp: Don't compute total bytes in a message until we need it

tcp[46]_l2_buf_bytes keep track of the total number of bytes we have
queued to send to the tap interface.  tcp_l2_buf_flush_passt() uses this
to determine if sendmsg() has sent all the data we requested, or whether
we need to resend a trailing portion.

However, the logic for finding where we're up to in the case of a short
sendmsg() can equally well tell whether we've had one at all, without
knowing the total number in advance.  This does require an extra loop after
each sendmsg(), but it's doing simple arithmetic on values we've already
been accessing, and it leads to overall simpler code.

tcp[46]_l2_flags_buf_bytes were being calculated, but never used for
anything, so simply remove them.

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

1 files changed, 18 insertions, 35 deletions

diff --git a/tcp.c b/tcp.c
index 43f6427..f74ed00 100644
--- a/tcp.c
+++ b/tcp.c
@@ -476,7 +476,6 @@ static struct tcp4_l2_buf_t {
 tcp4_l2_buf[TCP_FRAMES_MEM];
 
 static unsigned int tcp4_l2_buf_used;
-static size_t tcp4_l2_buf_bytes;
 
 /**
  * tcp6_l2_buf_t - Pre-cooked IPv6 packet buffers for tap connections
@@ -507,7 +506,6 @@ struct tcp6_l2_buf_t {
 tcp6_l2_buf[TCP_FRAMES_MEM];
 
 static unsigned int tcp6_l2_buf_used;
-static size_t tcp6_l2_buf_bytes;
 
 /* recvmsg()/sendmsg() data for tap */
 static char 		tcp_buf_discard		[MAX_WINDOW];
@@ -555,7 +553,6 @@ static struct tcp4_l2_flags_buf_t {
 tcp4_l2_flags_buf[TCP_FRAMES_MEM];
 
 static unsigned int tcp4_l2_flags_buf_used;
-static size_t tcp4_l2_flags_buf_bytes;
 
 /**
  * tcp6_l2_flags_buf_t - IPv6 packet buffers for segments without data (flags)
@@ -585,7 +582,6 @@ static struct tcp6_l2_flags_buf_t {
 tcp6_l2_flags_buf[TCP_FRAMES_MEM];
 
 static unsigned int tcp6_l2_flags_buf_used;
-static size_t tcp6_l2_flags_buf_bytes;
 
 /* TCP connections */
 union tcp_conn tc[TCP_MAX_CONNS];
@@ -1422,29 +1418,30 @@ static int tcp_l2_buf_write_one(struct ctx *c, const struct iovec *iov)
  * tcp_l2_buf_flush_passt() - Send a message on the passt tap interface
  * @c:		Execution context
  * @mh:		Message header that was partially sent by sendmsg()
- * @buf_bytes:	Total number of bytes to send
  */
-static void tcp_l2_buf_flush_passt(const struct ctx *c,
-				   const struct msghdr *mh, size_t buf_bytes)
+static void tcp_l2_buf_flush_passt(const struct ctx *c, const struct msghdr *mh)
 {
-	size_t end = 0, missing, sent;
+	size_t end = 0, missing;
 	struct iovec *iov;
 	unsigned int i;
-	ssize_t n;
+	ssize_t sent;
 	char *p;
 
-	n = sendmsg(c->fd_tap, mh, MSG_NOSIGNAL | MSG_DONTWAIT);
-	if (n < 0 || ((sent = (size_t)n) == buf_bytes))
+	sent = sendmsg(c->fd_tap, mh, MSG_NOSIGNAL | MSG_DONTWAIT);
+	if (sent < 0)
 		return;
 
 	/* Ensure a complete last message on partial sendmsg() */
 	for (i = 0, iov = mh->msg_iov; i < mh->msg_iovlen; i++, iov++) {
 		end += iov->iov_len;
-		if (end >= sent)
+		if (end >= (size_t)sent)
 			break;
 	}
 
 	missing = end - sent;
+	if (!missing)
+		return;
+
 	p = (char *)iov->iov_base + iov->iov_len - missing;
 	if (send(c->fd_tap, p, missing, MSG_NOSIGNAL))
 		debug("TCP: failed to flush %lu missing bytes to tap", missing);
@@ -1455,16 +1452,15 @@ static void tcp_l2_buf_flush_passt(const struct ctx *c,
  * @c:		Execution context
  * @mh:		Message header pointing to buffers, msg_iovlen not set
  * @buf_used:	Pointer to count of used buffers, set to 0 on return
- * @buf_bytes:	Pointer to count of buffer bytes, set to 0 on return
  */
 static void tcp_l2_buf_flush(struct ctx *c, struct msghdr *mh,
-			     unsigned int *buf_used, size_t *buf_bytes)
+			     unsigned int *buf_used)
 {
 	if (!(mh->msg_iovlen = *buf_used))
 		return;
 
 	if (c->mode == MODE_PASST) {
-		tcp_l2_buf_flush_passt(c, mh, *buf_bytes);
+		tcp_l2_buf_flush_passt(c, mh);
 	} else {
 		size_t i;
 
@@ -1475,7 +1471,7 @@ static void tcp_l2_buf_flush(struct ctx *c, struct msghdr *mh,
 				i--;
 		}
 	}
-	*buf_used = *buf_bytes = 0;
+	*buf_used = 0;
 
 	pcap_multiple(mh->msg_iov, mh->msg_iovlen, sizeof(uint32_t));
 }
@@ -1488,17 +1484,14 @@ static void tcp_l2_flags_buf_flush(struct ctx *c)
 {
 	struct msghdr mh = { 0 };
 	unsigned int *buf_used;
-	size_t *buf_bytes;
 
 	mh.msg_iov	= tcp6_l2_flags_iov;
 	buf_used	= &tcp6_l2_flags_buf_used;
-	buf_bytes	= &tcp6_l2_flags_buf_bytes;
-	tcp_l2_buf_flush(c, &mh, buf_used, buf_bytes);
+	tcp_l2_buf_flush(c, &mh, buf_used);
 
 	mh.msg_iov	= tcp4_l2_flags_iov;
 	buf_used	= &tcp4_l2_flags_buf_used;
-	buf_bytes	= &tcp4_l2_flags_buf_bytes;
-	tcp_l2_buf_flush(c, &mh, buf_used, buf_bytes);
+	tcp_l2_buf_flush(c, &mh, buf_used);
 }
 
 /**
@@ -1509,17 +1502,14 @@ static void tcp_l2_data_buf_flush(struct ctx *c)
 {
 	struct msghdr mh = { 0 };
 	unsigned int *buf_used;
-	size_t *buf_bytes;
 
 	mh.msg_iov = tcp6_l2_iov;
 	buf_used	= &tcp6_l2_buf_used;
-	buf_bytes	= &tcp6_l2_buf_bytes;
-	tcp_l2_buf_flush(c, &mh, buf_used, buf_bytes);
+	tcp_l2_buf_flush(c, &mh, buf_used);
 
 	mh.msg_iov = tcp4_l2_iov;
 	buf_used	= &tcp4_l2_buf_used;
-	buf_bytes	= &tcp4_l2_buf_bytes;
-	tcp_l2_buf_flush(c, &mh, buf_used, buf_bytes);
+	tcp_l2_buf_flush(c, &mh, buf_used);
 }
 
 /**
@@ -1833,11 +1823,6 @@ static int tcp_send_flag(struct ctx *c, struct tcp_tap_conn *conn, int flags)
 					  NULL, conn->seq_to_tap);
 	iov->iov_len = eth_len + sizeof(uint32_t);
 
-	if (CONN_V4(conn))
-		tcp4_l2_flags_buf_bytes += iov->iov_len;
-	else
-		tcp6_l2_flags_buf_bytes += iov->iov_len;
-
 	if (th->ack)
 		conn_flag(c, conn, ~ACK_TO_TAP_DUE);
 
@@ -1853,7 +1838,6 @@ static int tcp_send_flag(struct ctx *c, struct tcp_tap_conn *conn, int flags)
 			memcpy(b4 + 1, b4, sizeof(*b4));
 			(iov + 1)->iov_len = iov->iov_len;
 			tcp4_l2_flags_buf_used++;
-			tcp4_l2_flags_buf_bytes += iov->iov_len;
 		}
 
 		if (tcp4_l2_flags_buf_used > ARRAY_SIZE(tcp4_l2_flags_buf) - 2)
@@ -1863,7 +1847,6 @@ static int tcp_send_flag(struct ctx *c, struct tcp_tap_conn *conn, int flags)
 			memcpy(b6 + 1, b6, sizeof(*b6));
 			(iov + 1)->iov_len = iov->iov_len;
 			tcp6_l2_flags_buf_used++;
-			tcp6_l2_flags_buf_bytes += iov->iov_len;
 		}
 
 		if (tcp6_l2_flags_buf_used > ARRAY_SIZE(tcp6_l2_flags_buf) - 2)
@@ -2207,7 +2190,7 @@ static void tcp_data_to_tap(struct ctx *c, struct tcp_tap_conn *conn,
 		len = tcp_l2_buf_fill_headers(c, conn, b, plen, check, seq);
 
 		iov = tcp4_l2_iov + tcp4_l2_buf_used++;
-		tcp4_l2_buf_bytes += iov->iov_len = len + sizeof(b->vnet_len);
+		iov->iov_len = len + sizeof(b->vnet_len);
 		if (tcp4_l2_buf_used > ARRAY_SIZE(tcp4_l2_buf) - 1)
 			tcp_l2_data_buf_flush(c);
 	} else if (CONN_V6(conn)) {
@@ -2216,7 +2199,7 @@ static void tcp_data_to_tap(struct ctx *c, struct tcp_tap_conn *conn,
 		len = tcp_l2_buf_fill_headers(c, conn, b, plen, NULL, seq);
 
 		iov = tcp6_l2_iov + tcp6_l2_buf_used++;
-		tcp6_l2_buf_bytes += iov->iov_len = len + sizeof(b->vnet_len);
+		iov->iov_len = len + sizeof(b->vnet_len);
 		if (tcp6_l2_buf_used > ARRAY_SIZE(tcp6_l2_buf) - 1)
 			tcp_l2_data_buf_flush(c);
 	}