[PATCH 04/12] udp_vu: Move virtqueue management from udp_vu_sock_recv() to its caller

[Laurent Vivier <lvivier@redhat.com>]

udp_vu_sock_recv() currently mixes two concerns: receiving data from the
socket and managing virtqueue buffers (collecting, rewinding, releasing).
This makes the function harder to reason about and couples socket I/O
with virtqueue state.

Move all virtqueue operations, vu_collect(), vu_init_elem(),
vu_queue_rewind(), and the queue-readiness check, into
udp_vu_sock_to_tap(), which is the only caller.  This turns
udp_vu_sock_recv() into a pure socket receive function that simply reads
into the provided iov_tail and adjusts its length.

Signed-off-by: Laurent Vivier <lvivier@redhat.com>
---
 udp_vu.c | 79 ++++++++++++++++++++++++++------------------------------
 1 file changed, 37 insertions(+), 42 deletions(-)

diff --git a/udp_vu.c b/udp_vu.c
index 8f4d0aedac10..aefcab0b86c2 100644
--- a/udp_vu.c
+++ b/udp_vu.c
@@ -58,75 +58,41 @@ static size_t udp_vu_hdrlen(bool v6)
 
 /**
  * udp_vu_sock_recv() - Receive datagrams from socket into vhost-user buffers
- * @c:		Execution context
  * @data:	IO vector tail for the frame (modified on output)
- * @vq:		virtqueue to use to receive data
  * @s:		Socket to receive from
  * @v6:		Set for IPv6 connections
  *
- * Return: size of received data, 0 if the datagram
- *         was discarded because the virtqueue is not ready, -1 on error
+ * Return: size of received data, -1 on error
  */
-static ssize_t udp_vu_sock_recv(const struct ctx *c, struct iov_tail *data,
-				struct vu_virtq *vq, int s, bool v6)
+static ssize_t udp_vu_sock_recv(struct iov_tail *data, int s, bool v6)
 {
-	const struct vu_dev *vdev = c->vdev;
-	struct msghdr msg  = { 0 };
+	struct iovec msg_iov[data->cnt];
+	struct msghdr msg = { 0 };
 	struct iov_tail payload;
 	size_t hdrlen;
 	ssize_t dlen;
-	int iov_cnt;
-
-	ASSERT(!c->no_udp);
-
-	if (!vu_queue_enabled(vq) || !vu_queue_started(vq)) {
-		debug("Got UDP packet, but RX virtqueue not usable yet");
-
-		if (recvmsg(s, &msg, MSG_DONTWAIT) < 0)
-			debug_perror("Failed to discard datagram");
-
-		data->cnt = 0;
-		return 0;
-	}
 
 	/* compute L2 header length */
 	hdrlen = udp_vu_hdrlen(v6);
 
-	vu_init_elem(elem, (struct iovec *)data->iov, data->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;
-
-	data->cnt = iov_cnt;
-
 	/* reserve space for the headers */
 	ASSERT(iov_tail_size(data) >= MAX(hdrlen, ETH_ZLEN + VNET_HLEN));
 
 	payload = *data;
 	iov_drop_header(&payload, 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 */
 	dlen = recvmsg(s, &msg, 0);
-	if (dlen < 0) {
-		vu_queue_rewind(vq, iov_cnt);
+	if (dlen < 0)
 		return -1;
-	}
 
 	iov_tail_truncate(data, MAX(dlen + hdrlen, ETH_ZLEN + VNET_HLEN));
 	iov_tail_zero_end(data, dlen + hdrlen);
 	iov_tail_truncate(data, dlen + hdrlen);
 
-	vu_set_vnethdr(vdev, data->iov[0].iov_base, data->cnt);
-
-	/* release unused buffers */
-	vu_queue_rewind(vq, iov_cnt - data->cnt);
-
 	return dlen;
 }
 
@@ -216,16 +182,45 @@ void udp_vu_sock_to_tap(const struct ctx *c, int s, int n, flow_sidx_t tosidx)
 	struct iov_tail data;
 	int i;
 
+	ASSERT(!c->no_udp);
+
+	if (!vu_queue_enabled(vq) || !vu_queue_started(vq)) {
+		struct msghdr msg = { 0 };
+
+		debug("Got UDP packet, but RX virtqueue not usable yet");
+
+		for (i = 0; i < n; i++) {
+			if (recvmsg(s, &msg, MSG_DONTWAIT) < 0)
+				debug_perror("Failed to discard datagram");
+		}
+
+		return;
+	}
+
 	for (i = 0; i < n; i++) {
 		ssize_t dlen;
+		int elem_cnt;
 
-		data = IOV_TAIL(iov_vu, VIRTQUEUE_MAX_SIZE, 0);
+		vu_init_elem(elem, iov_vu, ARRAY_SIZE(elem));
 
-		dlen = udp_vu_sock_recv(c, &data, vq, s, v6);
-		if (dlen < 0)
+		elem_cnt = vu_collect(vdev, vq, elem, ARRAY_SIZE(elem),
+				      IP_MAX_MTU + ETH_HLEN + VNET_HLEN, NULL);
+		if (elem_cnt == 0)
 			break;
 
+		data = IOV_TAIL(iov_vu, elem_cnt, 0);
+
+		dlen = udp_vu_sock_recv(&data, s, v6);
+		if (dlen < 0) {
+			vu_queue_rewind(vq, elem_cnt);
+			continue;
+		}
+
+		/* release unused buffers */
+		vu_queue_rewind(vq, elem_cnt - data.cnt);
+
 		if (data.cnt > 0) {
+			vu_set_vnethdr(vdev, data.iov[0].iov_base, data.cnt);
 			udp_vu_prepare(c, &data, toside, dlen);
 			if (*c->pcap) {
 				udp_vu_csum(toside, &data);
-- 
2.53.0