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>

1 files changed, 15 insertions, 0 deletions

diff --git a/tap.c b/tap.c
index 558a734..d0dd72c 100644
--- a/tap.c
+++ b/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);
 
 /**