tap: Add "tap headers" abstraction

Currently both the TCP and UDP code need to deal in various places with the
details of the L2 headers, and also the tap-specific "vnet_len" header.
This makes abstracting the tap interface to new backends (e.g. vhost-user
or tun) more difficult.

To improve this abstraction, create a new 'tap_hdr' structure which
represents both L2 (always Ethernet at the moment, but might be vary in
future) and any additional tap specific headers (such as the qemu socket's
vnet_len field).  Provide helper functions and macros to initialize, update
and use it.

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

tap.c

@@ -386,6 +386,21 @@ void tap_send_frames(struct ctx *c, const struct iovec *iov, size_t n)
 	pcap_multiple(iov, n, sizeof(uint32_t));
 }
 
+/**
+ * tap_update_mac() - Update tap L2 header with new Ethernet addresses
+ * @taph:	Tap headers to update
+ * @eth_d:	Ethernet destination address, NULL if unchanged
+ * @eth_s:	Ethernet source address, NULL if unchanged
+ */
+void tap_update_mac(struct tap_hdr *taph,
+		    const unsigned char *eth_d, const unsigned char *eth_s)
+{
+	if (eth_d)
+		memcpy(taph->eh.h_dest, eth_d, sizeof(taph->eh.h_dest));
+	if (eth_s)
+		memcpy(taph->eh.h_source, eth_s, sizeof(taph->eh.h_source));
+}
+
 PACKET_POOL_DECL(pool_l4, UIO_MAXIOV, pkt_buf);
 
 /**

tap.h

@@ -6,6 +6,55 @@
 #ifndef TAP_H
 #define TAP_H
 
+/**
+ * struct tap_hdr - L2 and tap specific headers
+ * @vnet_len:	Frame length (for qemu socket transport)
+ * @eh:		Ethernet header
+ */
+struct tap_hdr {
+	uint32_t vnet_len;
+	struct ethhdr eh;
+} __attribute__((packed));
+
+#define TAP_HDR_INIT(proto) { .eh.h_proto = htons_constant(proto) }
+
+static inline size_t tap_hdr_len_(const struct ctx *c)
+{
+	(void)c;
+	return sizeof(struct tap_hdr);
+}
+
+/**
+ * tap_iov_base() - Find start of tap frame
+ * @c:		Execution context
+ * @taph:	Pointer to L2 header buffer
+ *
+ * Returns: pointer to the start of tap frame - suitable for an
+ *          iov_base to be passed to tap_send_frames())
+ */
+static inline void *tap_iov_base(const struct ctx *c, struct tap_hdr *taph)
+{
+	return (char *)(taph + 1) - tap_hdr_len_(c);
+}
+
+/**
+ * tap_iov_len() - Finalize tap frame and return total length
+ * @c:		Execution context
+ * @taph:	Tap header to finalize
+ * @plen:	L2 payload length (excludes L2 and tap specific headers)
+ *
+ * Returns: length of the tap frame including L2 and tap specific
+ *          headers - suitable for an iov_len to be passed to
+ *          tap_send_frames()
+ */
+static inline size_t tap_iov_len(const struct ctx *c, struct tap_hdr *taph,
+				 size_t plen)
+{
+	if (c->mode == MODE_PASST)
+		taph->vnet_len = htonl(plen + sizeof(taph->eh));
+	return plen + tap_hdr_len_(c);
+}
+
 struct in_addr tap_ip4_daddr(const struct ctx *c);
 void tap_udp4_send(const struct ctx *c, struct in_addr src, in_port_t sport,
 		   struct in_addr dst, in_port_t dport,
@@ -23,6 +72,8 @@ void tap_icmp6_send(const struct ctx *c,
 		    void *in, size_t len);
 int tap_send(const struct ctx *c, const void *data, size_t len);
 void tap_send_frames(struct ctx *c, const struct iovec *iov, size_t n);
+void tap_update_mac(struct tap_hdr *taph,
+		    const unsigned char *eth_d, const unsigned char *eth_s);
 void tap_handler(struct ctx *c, int fd, uint32_t events,
 		 const struct timespec *now);
 void tap_sock_init(struct ctx *c);