[PATCH 3/7] tcp_vu: Use iov_tail helpers to build headers in tcp_vu_prepare()

[Laurent Vivier <lvivier@redhat.com>]

Replace direct pointer arithmetic using vu_eth(), vu_ip(),
vu_payloadv4() and vu_payloadv6() with the iov_tail abstraction
to build Ethernet, IP and TCP headers in tcp_vu_prepare().

This removes the assumption that all headers reside in a single
contiguous iov entry.

Signed-off-by: Laurent Vivier <lvivier@redhat.com>
---
 tcp_vu.c | 61 ++++++++++++++++++++++++++------------------------------
 1 file changed, 28 insertions(+), 33 deletions(-)

diff --git a/tcp_vu.c b/tcp_vu.c
index c6206b7a689c..a39f6ea018e9 100644
--- a/tcp_vu.c
+++ b/tcp_vu.c
@@ -222,6 +222,7 @@ static ssize_t tcp_vu_sock_recv(const struct ctx *c, struct vu_virtq *vq,
 		/* reserve space for headers in iov */
 		iov = &elem[elem_cnt].in_sg[0];
 		assert(iov->iov_len >= hdrlen);
+
 		iov->iov_base = (char *)iov->iov_base + hdrlen;
 		iov->iov_len -= hdrlen;
 		head[(*head_cnt)++] = elem_cnt;
@@ -285,51 +286,45 @@ static void tcp_vu_prepare(const struct ctx *c, struct tcp_tap_conn *conn,
 			   int *check, bool no_tcp_csum, bool push)
 {
 	const struct flowside *toside = TAPFLOW(conn);
-	bool v6 = !(inany_v4(&toside->eaddr) && inany_v4(&toside->oaddr));
-	size_t hdrlen = tcp_vu_hdrlen(v6);
-	char *base = iov[0].iov_base;
-	struct ipv6hdr *ip6h = NULL;
-	struct iphdr *ip4h = NULL;
+	bool ipv4 = inany_v4(&toside->eaddr) && inany_v4(&toside->oaddr);
 	struct iov_tail payload;
-	struct tcphdr *th;
-	struct ethhdr *eh;
-
-	/* we guess the first iovec provided by the guest can embed
-	 * all the headers needed by L2 frame, including any padding
-	 */
-	assert(iov[0].iov_len >= hdrlen);
-
-	eh = vu_eth(base);
 
-	memcpy(eh->h_dest, c->guest_mac, sizeof(eh->h_dest));
+	payload = IOV_TAIL(iov, iov_cnt, VNET_HLEN);
+	with_header(struct ethhdr, eh, &payload)
+		memcpy(eh->h_dest, c->guest_mac, sizeof(eh->h_dest));
+	IOV_DROP_HEADER(&payload, struct ethhdr);
 
 	/* initialize header */
 
-	if (!v6) {
-		eh->h_proto = htons(ETH_P_IP);
-
-		ip4h = vu_ip(base);
-		*ip4h = (struct iphdr)L2_BUF_IP4_INIT(IPPROTO_TCP);
-		th = vu_payloadv4(base);
+	if (ipv4) {
+		with_header(struct iphdr, ip4h, &payload)
+			*ip4h = (struct iphdr)L2_BUF_IP4_INIT(IPPROTO_TCP);
+		IOV_DROP_HEADER(&payload, struct iphdr);
 	} else {
-		eh->h_proto = htons(ETH_P_IPV6);
-
-		ip6h = vu_ip(base);
-		*ip6h = (struct ipv6hdr)L2_BUF_IP6_INIT(IPPROTO_TCP);
-
-		th = vu_payloadv6(base);
+		with_header(struct ipv6hdr, ip6h, &payload)
+			*ip6h = (struct ipv6hdr)L2_BUF_IP6_INIT(IPPROTO_TCP);
+		IOV_DROP_HEADER(&payload, struct ipv6hdr);
 	}
 
-	memset(th, 0, sizeof(*th));
-	th->doff = sizeof(*th) / 4;
-	th->ack = 1;
-	th->psh = push;
+	with_header(struct tcphdr, th, &payload) {
+		memset(th, 0, sizeof(*th));
+		th->doff = sizeof(*th) / 4;
+		th->ack = 1;
+		th->psh = push;
+	}
 
 	payload = IOV_TAIL(iov, iov_cnt, VNET_HLEN);
-	tcp_fill_headers(c, conn, !v6, &payload, *check, conn->seq_to_tap,
+	tcp_fill_headers(c, conn, ipv4, &payload, *check, conn->seq_to_tap,
 			 no_tcp_csum);
-	if (ip4h)
+	if (ipv4) {
+		struct iphdr ip4h_storage;
+		const struct iphdr *ip4h;
+
+		IOV_DROP_HEADER(&payload, struct ethhdr);
+		ip4h = IOV_PEEK_HEADER(&payload, ip4h_storage);
+
 		*check = ip4h->check;
+	}
 }
 
 /**
-- 
2.53.0