[PATCH v2 04/13] udp_vu: Use iov_tail to manage virtqueue buffers

[Laurent Vivier <lvivier@redhat.com>]

Replace direct iovec pointer arithmetic in UDP vhost-user handling with
iov_tail operations.

udp_vu_sock_recv() now takes an iov/cnt pair instead of using the
file-scoped iov_vu array, and returns the data length rather than the
iov count.  Internally it uses iov_drop_header() to skip past L2/L3/L4
headers before receiving, and iov_tail_clone() to build the recvmsg()
iovec, removing the manual pointer offset and restore pattern.

udp_vu_prepare() and udp_vu_csum() take a const struct iov_tail *
instead of referencing iov_vu directly, making data flow explicit.

udp_vu_csum() uses iov_drop_header() and IOV_REMOVE_HEADER() to locate
the UDP header and payload, replacing manual offset calculations via
vu_payloadv4()/vu_payloadv6().

Signed-off-by: Laurent Vivier <lvivier@redhat.com>
---
 udp_vu.c | 111 ++++++++++++++++++++++++++++---------------------------
 1 file changed, 57 insertions(+), 54 deletions(-)

diff --git a/udp_vu.c b/udp_vu.c
index 439f2cb399b7..a39254776099 100644
--- a/udp_vu.c
+++ b/udp_vu.c
@@ -59,21 +59,25 @@ static size_t udp_vu_hdrlen(bool v6)
 /**
  * udp_vu_sock_recv() - Receive datagrams from socket into vhost-user buffers
  * @c:		Execution context
+ * @iov:	IO vector for the frame (modified on output)
+ * @cnt:	Number of IO vector entries (in/out)
  * @vq:		virtqueue to use to receive data
  * @s:		Socket to receive from
  * @v6:		Set for IPv6 connections
- * @dlen:	Size of received data (output)
  *
- * Return: number of iov entries used to store the datagram, 0 if the datagram
+ * Return: size of received data, 0 if the datagram
  *         was discarded because the virtqueue is not ready, -1 on error
  */
-static int udp_vu_sock_recv(const struct ctx *c, struct vu_virtq *vq, int s,
-			    bool v6, ssize_t *dlen)
+static ssize_t udp_vu_sock_recv(const struct ctx *c, struct iovec *iov,
+				size_t *cnt, struct vu_virtq *vq, int s,
+				bool v6)
 {
 	const struct vu_dev *vdev = c->vdev;
 	struct msghdr msg  = { 0 };
-	int iov_cnt, iov_used;
+	struct iov_tail payload;
 	size_t hdrlen;
+	ssize_t dlen;
+	int iov_cnt;
 
 	ASSERT(!c->no_udp);
 
@@ -83,78 +87,74 @@ static int udp_vu_sock_recv(const struct ctx *c, struct vu_virtq *vq, int s,
 		if (recvmsg(s, &msg, MSG_DONTWAIT) < 0)
 			debug_perror("Failed to discard datagram");
 
+		*cnt = 0;
 		return 0;
 	}
 
 	/* compute L2 header length */
 	hdrlen = udp_vu_hdrlen(v6);
 
-	vu_init_elem(elem, iov_vu, ARRAY_SIZE(elem));
+	vu_init_elem(elem, iov, *cnt);
 
 	iov_cnt = vu_collect(vdev, vq, elem, ARRAY_SIZE(elem),
 			     IP_MAX_MTU + ETH_HLEN + VNET_HLEN, NULL);
 	if (iov_cnt == 0)
 		return -1;
 
-	/* reserve space for the headers */
-	ASSERT(iov_vu[0].iov_len >= MAX(hdrlen, ETH_ZLEN + VNET_HLEN));
+	payload = IOV_TAIL(iov, iov_cnt, hdrlen);
 
-	iov_vu[0].iov_base = (char *)iov_vu[0].iov_base + hdrlen;
-	iov_vu[0].iov_len -= hdrlen;
+	struct iovec msg_iov[payload.cnt];
+	msg.msg_iov = msg_iov;
+	msg.msg_iovlen = iov_tail_clone(msg.msg_iov, payload.cnt, &payload);
 
 	/* read data from the socket */
-	msg.msg_iov = iov_vu;
-	msg.msg_iovlen = iov_cnt;
-
-	*dlen = recvmsg(s, &msg, 0);
-	if (*dlen < 0) {
+	dlen = recvmsg(s, &msg, 0);
+	if (dlen < 0) {
 		vu_queue_rewind(vq, iov_cnt);
 		return -1;
 	}
 
-	/* restore the pointer to the headers address */
-	iov_vu[0].iov_base = (char *)iov_vu[0].iov_base - hdrlen;
-	iov_vu[0].iov_len += hdrlen;
-
 	/* Pad short frames to ETH_ZLEN */
-	if (ETH_ZLEN + VNET_HLEN > *dlen + hdrlen) {
-		iov_memset(iov_vu, iov_cnt, *dlen + hdrlen, 0,
-			   ETH_ZLEN + VNET_HLEN - (*dlen + hdrlen));
+	if (ETH_ZLEN + VNET_HLEN > dlen + hdrlen) {
+		iov_memset(iov, iov_cnt, dlen + hdrlen, 0,
+			   ETH_ZLEN + VNET_HLEN - (dlen + hdrlen));
 	}
-	iov_used = iov_truncate(iov_vu, iov_cnt, *dlen + hdrlen);
+	*cnt = iov_truncate(iov, iov_cnt, dlen + hdrlen);
 
-	vu_set_vnethdr(iov_vu[0].iov_base, iov_used);
+	vu_set_vnethdr(iov[0].iov_base, *cnt);
 
 	/* release unused buffers */
-	vu_queue_rewind(vq, iov_cnt - iov_used);
+	vu_queue_rewind(vq, iov_cnt - *cnt);
 
-	return iov_used;
+	return dlen;
 }
 
 /**
  * udp_vu_prepare() - Prepare the packet header
  * @c:		Execution context
+ * @data:	IO vector tail for the frame
  * @toside:	Address information for one side of the flow
  * @dlen:	Packet data length
  *
  * Return: Layer-4 length
  */
-static size_t udp_vu_prepare(const struct ctx *c,
+static size_t udp_vu_prepare(const struct ctx *c, const struct iov_tail *data,
 			     const struct flowside *toside, ssize_t dlen)
 {
+	const struct iovec *iov = data->iov;
 	struct ethhdr *eh;
 	size_t l4len;
 
 	/* ethernet header */
-	eh = vu_eth(iov_vu[0].iov_base);
+	eh = vu_eth(iov[0].iov_base);
 
 	memcpy(eh->h_dest, c->guest_mac, sizeof(eh->h_dest));
 	memcpy(eh->h_source, c->our_tap_mac, sizeof(eh->h_source));
 
 	/* initialize header */
 	if (inany_v4(&toside->eaddr) && inany_v4(&toside->oaddr)) {
-		struct iphdr *iph = vu_ip(iov_vu[0].iov_base);
-		struct udp_payload_t *bp = vu_payloadv4(iov_vu[0].iov_base);
+		struct iphdr *iph = vu_ip(iov[0].iov_base);
+		struct udp_payload_t *bp = vu_payloadv4(iov[0].iov_base);
 
 		eh->h_proto = htons(ETH_P_IP);
 
@@ -162,8 +162,8 @@ static size_t udp_vu_prepare(const struct ctx *c,
 
 		l4len = udp_update_hdr4(iph, bp, toside, dlen, true);
 	} else {
-		struct ipv6hdr *ip6h = vu_ip(iov_vu[0].iov_base);
-		struct udp_payload_t *bp = vu_payloadv6(iov_vu[0].iov_base);
+		struct ipv6hdr *ip6h = vu_ip(iov[0].iov_base);
+		struct udp_payload_t *bp = vu_payloadv6(iov[0].iov_base);
 
 		eh->h_proto = htons(ETH_P_IPV6);
 
@@ -178,25 +178,25 @@ static size_t udp_vu_prepare(const struct ctx *c,
 /**
  * udp_vu_csum() - Calculate and set checksum for a UDP packet
  * @toside:	Address information for one side of the flow
- * @iov_used:	Number of used iov_vu items
+ * @data:	IO vector tail for the frame
  */
-static void udp_vu_csum(const struct flowside *toside, int iov_used)
+static void udp_vu_csum(const struct flowside *toside,
+			const struct iov_tail *data)
 {
 	const struct in_addr *src4 = inany_v4(&toside->oaddr);
 	const struct in_addr *dst4 = inany_v4(&toside->eaddr);
-	char *base = iov_vu[0].iov_base;
-	struct udp_payload_t *bp;
-	struct iov_tail data;
+	struct iov_tail payload = *data;
+	struct udphdr *uh, uh_storage;
+	bool ipv4 = src4 && dst4;
 
-	if (src4 && dst4) {
-		bp = vu_payloadv4(base);
-		data = IOV_TAIL(iov_vu, iov_used, (char *)&bp->data - base);
-		csum_udp4(&bp->uh, *src4, *dst4, &data);
-	} else {
-		bp = vu_payloadv6(base);
-		data = IOV_TAIL(iov_vu, iov_used, (char *)&bp->data - base);
-		csum_udp6(&bp->uh, &toside->oaddr.a6, &toside->eaddr.a6, &data);
-	}
+	iov_drop_header(&payload,
+			udp_vu_hdrlen(!ipv4) - sizeof(struct udphdr));
+	uh = IOV_REMOVE_HEADER(&payload, uh_storage);
+
+	if (ipv4)
+		csum_udp4(uh, *src4, *dst4, &payload);
+	else
+		csum_udp6(uh, &toside->oaddr.a6, &toside->eaddr.a6, &payload);
 }
 
 /**
@@ -212,23 +212,26 @@ void udp_vu_sock_to_tap(const struct ctx *c, int s, int n, flow_sidx_t tosidx)
 	bool v6 = !(inany_v4(&toside->eaddr) && inany_v4(&toside->oaddr));
 	struct vu_dev *vdev = c->vdev;
 	struct vu_virtq *vq = &vdev->vq[VHOST_USER_RX_QUEUE];
+	struct iov_tail data;
 	int i;
 
 	for (i = 0; i < n; i++) {
+		size_t iov_cnt;
 		ssize_t dlen;
-		int iov_used;
 
-		iov_used = udp_vu_sock_recv(c, vq, s, v6, &dlen);
-		if (iov_used < 0)
+		iov_cnt = VIRTQUEUE_MAX_SIZE;
+		dlen = udp_vu_sock_recv(c, iov_vu, &iov_cnt, vq, s, v6);
+		if (dlen < 0)
 			break;
 
-		if (iov_used > 0) {
-			udp_vu_prepare(c, toside, dlen);
+		if (iov_cnt > 0) {
+			data = IOV_TAIL(iov_vu, iov_cnt, 0);
+			udp_vu_prepare(c, &data, toside, dlen);
 			if (*c->pcap) {
-				udp_vu_csum(toside, iov_used);
-				pcap_iov(iov_vu, iov_used, VNET_HLEN);
+				udp_vu_csum(toside, &data);
+				pcap_iov(data.iov, data.cnt, VNET_HLEN);
 			}
-			vu_flush(vdev, vq, elem, iov_used);
+			vu_flush(vdev, vq, elem, data.cnt);
 		}
 	}
 }
-- 
2.53.0