tap, tcp: (Re-)abstract TAP specific header handling

Recent changes to the TCP code (reworking of the buffer handling) have
meant that it now (again) deals explicitly with the MODE_PASST specific
vnet_len field, instead of using the (partial) abstractions provided by the
tap layer.

The abstractions we had don't work for the new TCP structure, so make some
new ones that do: tap_hdr_iov() which constructs an iovec suitable for
containing (just) the TAP specific header and tap_hdr_update() which
updates it as necessary per-packet.

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

tap.h

@@ -16,6 +16,33 @@ struct tap_hdr {
 	uint32_t vnet_len;
 } __attribute__((packed));
 
+/**
+ * tap_hdr_iov() - struct iovec for a tap header
+ * @c:		Execution context
+ * @taph:	Pointer to tap specific header buffer
+ *
+ * Returns: A struct iovec covering the correct portion of @taph to use as the
+ *          tap specific header in the current configuration.
+ */
+static inline struct iovec tap_hdr_iov(const struct ctx *c,
+				       struct tap_hdr *thdr)
+{
+	return (struct iovec){
+		.iov_base = thdr,
+		.iov_len = c->mode == MODE_PASST ? sizeof(*thdr) : 0,
+	};
+}
+
+/**
+ * tap_hdr_update() - Update the tap specific header for a frame
+ * @taph:	Tap specific header buffer to update
+ * @l2len:	Frame length (including L2 headers)
+ */
+static inline void tap_hdr_update(struct tap_hdr *thdr, size_t l2len)
+{
+	thdr->vnet_len = htonl(l2len);
+}
+
 static inline size_t tap_hdr_len_(const struct ctx *c)
 {
 	if (c->mode == MODE_PASST)

tcp.c

@@ -452,7 +452,7 @@ struct tcp_flags_t {
 /* Ethernet header for IPv4 frames */
 static struct ethhdr		tcp4_eth_src;
 
-static uint32_t			tcp4_payload_vnet_len[TCP_FRAMES_MEM];
+static struct tap_hdr		tcp4_payload_tap_hdr[TCP_FRAMES_MEM];
 /* IPv4 headers */
 static struct iphdr		tcp4_payload_ip[TCP_FRAMES_MEM];
 /* TCP segments with payload for IPv4 frames */
@@ -463,7 +463,7 @@ static_assert(MSS4 <= sizeof(tcp4_payload[0].data), "MSS4 is greater than 65516"
 static struct tcp_buf_seq_update tcp4_seq_update[TCP_FRAMES_MEM];
 static unsigned int tcp4_payload_used;
 
-static uint32_t			tcp4_flags_vnet_len[TCP_FRAMES_MEM];
+static struct tap_hdr		tcp4_flags_tap_hdr[TCP_FRAMES_MEM];
 /* IPv4 headers for TCP segment without payload */
 static struct iphdr		tcp4_flags_ip[TCP_FRAMES_MEM];
 /* TCP segments without payload for IPv4 frames */
@@ -474,7 +474,7 @@ static unsigned int tcp4_flags_used;
 /* Ethernet header for IPv6 frames */
 static struct ethhdr		tcp6_eth_src;
 
-static uint32_t			tcp6_payload_vnet_len[TCP_FRAMES_MEM];
+static struct tap_hdr		tcp6_payload_tap_hdr[TCP_FRAMES_MEM];
 /* IPv6 headers */
 static struct ipv6hdr		tcp6_payload_ip[TCP_FRAMES_MEM];
 /* TCP headers and data for IPv6 frames */
@@ -485,7 +485,7 @@ static_assert(MSS6 <= sizeof(tcp6_payload[0].data), "MSS6 is greater than 65516"
 static struct tcp_buf_seq_update tcp6_seq_update[TCP_FRAMES_MEM];
 static unsigned int tcp6_payload_used;
 
-static uint32_t			tcp6_flags_vnet_len[TCP_FRAMES_MEM];
+static struct tap_hdr		tcp6_flags_tap_hdr[TCP_FRAMES_MEM];
 /* IPv6 headers for TCP segment without payload */
 static struct ipv6hdr		tcp6_flags_ip[TCP_FRAMES_MEM];
 /* TCP segment without payload for IPv6 frames */
@@ -499,14 +499,14 @@ static struct iovec	iov_sock		[TCP_FRAMES_MEM + 1];
 
 /*
  * enum tcp_iov_parts - I/O vector parts for one TCP frame
- * @TCP_IOV_VLEN	virtio net header
+ * @TCP_IOV_TAP		tap backend specific header
  * @TCP_IOV_ETH		Ethernet header
  * @TCP_IOV_IP		IP (v4/v6) header
  * @TCP_IOV_PAYLOAD	IP payload (TCP header + data)
  * @TCP_NUM_IOVS 	the number of entries in the iovec array
  */
 enum tcp_iov_parts {
-	TCP_IOV_VLEN	= 0,
+	TCP_IOV_TAP	= 0,
 	TCP_IOV_ETH	= 1,
 	TCP_IOV_IP	= 2,
 	TCP_IOV_PAYLOAD	= 3,
@@ -953,9 +953,7 @@ static void tcp_sock4_iov_init(const struct ctx *c)
 	for (i = 0; i < TCP_FRAMES_MEM; i++) {
 		iov = tcp4_l2_iov[i];
 
-		iov[TCP_IOV_VLEN].iov_base = &tcp4_payload_vnet_len[i];
+		iov[TCP_IOV_TAP] = tap_hdr_iov(c, &tcp4_payload_tap_hdr[i]);
-		iov[TCP_IOV_VLEN].iov_len = c->mode == MODE_PASTA ? 0 :
-				           sizeof(tcp4_payload_vnet_len[i]);
 		iov[TCP_IOV_ETH].iov_base = &tcp4_eth_src;
 		iov[TCP_IOV_ETH].iov_len = sizeof(tcp4_eth_src);
 		iov[TCP_IOV_IP].iov_base = &tcp4_payload_ip[i];
@@ -966,9 +964,7 @@ static void tcp_sock4_iov_init(const struct ctx *c)
 	for (i = 0; i < TCP_FRAMES_MEM; i++) {
 		iov = tcp4_l2_flags_iov[i];
 
-		iov[TCP_IOV_VLEN].iov_base = &tcp4_flags_vnet_len[i];
+		iov[TCP_IOV_TAP] = tap_hdr_iov(c, &tcp4_flags_tap_hdr[i]);
-		iov[TCP_IOV_VLEN].iov_len = c->mode == MODE_PASTA ? 0 :
-					     sizeof(tcp4_flags_vnet_len[i]);
 		iov[TCP_IOV_ETH].iov_base = &tcp4_eth_src;
 		iov[TCP_IOV_ETH].iov_len = sizeof(tcp4_eth_src);
 		iov[TCP_IOV_IP].iov_base = &tcp4_flags_ip[i];
@@ -1004,9 +1000,7 @@ static void tcp_sock6_iov_init(const struct ctx *c)
 	for (i = 0; i < TCP_FRAMES_MEM; i++) {
 		iov = tcp6_l2_iov[i];
 
-		iov[TCP_IOV_VLEN].iov_base = &tcp6_payload_vnet_len[i];
+		iov[TCP_IOV_TAP] = tap_hdr_iov(c, &tcp6_payload_tap_hdr[i]);
-		iov[TCP_IOV_VLEN].iov_len = c->mode == MODE_PASTA ? 0 :
-					   sizeof(tcp6_payload_vnet_len[i]);
 		iov[TCP_IOV_ETH].iov_base = &tcp6_eth_src;
 		iov[TCP_IOV_ETH].iov_len = sizeof(tcp6_eth_src);
 		iov[TCP_IOV_IP].iov_base = &tcp6_payload_ip[i];
@@ -1017,9 +1011,7 @@ static void tcp_sock6_iov_init(const struct ctx *c)
 	for (i = 0; i < TCP_FRAMES_MEM; i++) {
 		iov = tcp6_l2_flags_iov[i];
 
-		iov[TCP_IOV_VLEN].iov_base = &tcp6_flags_vnet_len[i];
+		iov[TCP_IOV_TAP] = tap_hdr_iov(c, &tcp6_flags_tap_hdr[i]);
-		iov[TCP_IOV_VLEN].iov_len = c->mode == MODE_PASTA ? 0 :
-					     sizeof(tcp6_flags_vnet_len[i]);
 		iov[TCP_IOV_ETH].iov_base = &tcp6_eth_src;
 		iov[TCP_IOV_ETH].iov_len = sizeof(tcp6_eth_src);
 		iov[TCP_IOV_IP].iov_base = &tcp6_flags_ip[i];
@@ -1658,7 +1650,7 @@ static int tcp_send_flag(struct ctx *c, struct tcp_tap_conn *conn, int flags)
 					conn->seq_to_tap);
 	iov[TCP_IOV_PAYLOAD].iov_len = l4len;
 
-	*(uint32_t *)iov[TCP_IOV_VLEN].iov_base = htonl(vnet_len + l4len);
+	tap_hdr_update(iov[TCP_IOV_TAP].iov_base, vnet_len + l4len);
 
 	if (th->ack) {
 		if (SEQ_GE(conn->seq_ack_to_tap, conn->seq_from_tap))
@@ -2141,7 +2133,6 @@ static void tcp_data_to_tap(const struct ctx *c, struct tcp_tap_conn *conn,
 {
 	uint32_t *seq_update = &conn->seq_to_tap;
 	struct iovec *iov;
-	uint32_t vnet_len;
 	size_t l4len;
 
 	if (CONN_V4(conn)) {
@@ -2159,8 +2150,8 @@ static void tcp_data_to_tap(const struct ctx *c, struct tcp_tap_conn *conn,
 		iov = tcp4_l2_iov[tcp4_payload_used++];
 		l4len = tcp_l2_buf_fill_headers(c, conn, iov, dlen, check, seq);
 		iov[TCP_IOV_PAYLOAD].iov_len = l4len;
-		vnet_len = sizeof(struct ethhdr) + sizeof(struct iphdr) + l4len;
+		tap_hdr_update(iov[TCP_IOV_TAP].iov_base, l4len
-		*(uint32_t *)iov[TCP_IOV_VLEN].iov_base = htonl(vnet_len);
+			       + sizeof(struct iphdr) + sizeof(struct ethhdr));
 		if (tcp4_payload_used > TCP_FRAMES_MEM - 1)
 			tcp_payload_flush(c);
 	} else if (CONN_V6(conn)) {
@@ -2170,9 +2161,8 @@ static void tcp_data_to_tap(const struct ctx *c, struct tcp_tap_conn *conn,
 		iov = tcp6_l2_iov[tcp6_payload_used++];
 		l4len = tcp_l2_buf_fill_headers(c, conn, iov, dlen, NULL, seq);
 		iov[TCP_IOV_PAYLOAD].iov_len = l4len;
-		vnet_len = sizeof(struct ethhdr) + sizeof(struct ipv6hdr)
+		tap_hdr_update(iov[TCP_IOV_TAP].iov_base, l4len
-			+ l4len;
+			       + sizeof(struct ipv6hdr) + sizeof(struct ethhdr));
-		*(uint32_t *)iov[TCP_IOV_VLEN].iov_base = htonl(vnet_len);
 		if (tcp6_payload_used > TCP_FRAMES_MEM - 1)
 			tcp_payload_flush(c);
 	}