[PATCH v2 2/5] tcp_vu: Build headers on the stack and write them into the iovec

[Laurent Vivier <lvivier@redhat.com>]

tcp_vu_prepare() currently assumes the first iovec element provided by
the guest is large enough to hold all L2-L4 headers, and builds them
in place via pointer casts into iov[0].iov_base.  This assumption is
enforced by an assert().

Since the headers in the buffer are uninitialized anyway, we can just
as well build the Ethernet, IP, and TCP headers on the stack instead,
and write them into the iovec with IOV_PUSH_HEADER().  This mirrors the
approach already used in udp_vu_prepare(), and prepares for support of
elements with multiple iovecs.

Signed-off-by: Laurent Vivier <lvivier@redhat.com>
---
 tcp_vu.c | 60 ++++++++++++++++++++++++--------------------------------
 1 file changed, 26 insertions(+), 34 deletions(-)

diff --git a/tcp_vu.c b/tcp_vu.c
index 67b11e28118a..56d80317cc83 100644
--- a/tcp_vu.c
+++ b/tcp_vu.c
@@ -288,49 +288,41 @@ static void tcp_vu_prepare(const struct ctx *c, struct tcp_tap_conn *conn,
 	bool v6 = !(inany_v4(&toside->eaddr) && inany_v4(&toside->oaddr));
 	size_t hdrlen = tcp_vu_hdrlen(v6);
 	struct iov_tail payload = IOV_TAIL(iov, iov_cnt, hdrlen);
-	char *base = iov[0].iov_base;
-	struct ipv6hdr *ip6h = NULL;
-	struct iphdr *ip4h = NULL;
-	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);
+	struct ipv6hdr ip6h;
+	struct iphdr ip4h;
+	struct tcphdr th;
+	struct ethhdr eh;
 
-	eh = vu_eth(base);
-
-	memcpy(eh->h_dest, c->guest_mac, sizeof(eh->h_dest));
+	memcpy(eh.h_dest, c->guest_mac, sizeof(eh.h_dest));
 
 	/* 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);
-	} else {
-		eh->h_proto = htons(ETH_P_IPV6);
+	if (!v6)
+		ip4h = (struct iphdr)L2_BUF_IP4_INIT(IPPROTO_TCP);
+	else
+		ip6h = (struct ipv6hdr)L2_BUF_IP6_INIT(IPPROTO_TCP);
 
-		ip6h = vu_ip(base);
-		*ip6h = (struct ipv6hdr)L2_BUF_IP6_INIT(IPPROTO_TCP);
+	memset(&th, 0, sizeof(th));
+	th.doff = sizeof(th) / 4;
+	th.ack = 1;
+	th.psh = push;
 
-		th = vu_payloadv6(base);
-	}
+	tcp_fill_headers(c, conn, &eh, v6 ? NULL : &ip4h, v6 ? &ip6h : NULL, &th,
+			 &payload, *csum_flags, conn->seq_to_tap);
 
-	memset(th, 0, sizeof(*th));
-	th->doff = sizeof(*th) / 4;
-	th->ack = 1;
-	th->psh = push;
+	/* Preserve TCP_CSUM, overwrite IP4_CSUM as we set the checksum */
+	if (!v6)
+		*csum_flags = (*csum_flags & TCP_CSUM) | ip4h.check;
 
-	tcp_fill_headers(c, conn, eh, ip4h, ip6h, th, &payload,
-			 *csum_flags, conn->seq_to_tap);
+	/* write headers */
+	payload = IOV_TAIL(iov, iov_cnt, VNET_HLEN);
 
-	/* Preserve TCP_CSUM, overwrite IP4_CSUM as we set the checksum */
-	if (ip4h)
-		*csum_flags = (*csum_flags & TCP_CSUM) | ip4h->check;
+	IOV_PUSH_HEADER(&payload, eh);
+	if (!v6)
+		IOV_PUSH_HEADER(&payload, ip4h);
+	else
+		IOV_PUSH_HEADER(&payload, ip6h);
+	IOV_PUSH_HEADER(&payload, th);
 }
 
 /**
-- 
2.53.0