[PATCH v3 6/8] 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(), vu_set_vnethdr(), 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 array and adjusts its length.

Signed-off-by: Laurent Vivier <lvivier@redhat.com>
---
 udp_vu.c | 110 +++++++++++++++++++++++++------------------------------
 1 file changed, 49 insertions(+), 61 deletions(-)

diff --git a/udp_vu.c b/udp_vu.c
index 8b0de312949c..7f6561f83505 100644
--- a/udp_vu.c
+++ b/udp_vu.c
@@ -33,9 +33,6 @@
 #include "udp_vu.h"
 #include "vu_common.h"
 
-static struct iovec     iov_vu		[VIRTQUEUE_MAX_SIZE];
-static struct vu_virtq_element	elem		[VIRTQUEUE_MAX_SIZE];
-
 /**
  * udp_vu_hdrlen() - Sum size of all headers, from UDP to virtio-net
  * @v6:		Set for IPv6 packet
@@ -58,78 +55,35 @@ 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 (in/out)
  * @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
  *
- * 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 iovec *iov,
-				size_t *cnt, unsigned *elem_used,
-				struct vu_virtq *vq, int s, bool v6)
+static ssize_t udp_vu_sock_recv(struct iovec *iov, size_t *cnt, int s, bool v6)
 {
-	const struct vu_dev *vdev = c->vdev;
-	struct msghdr msg  = { 0 };
+	struct iovec msg_iov[*cnt];
+	struct msghdr msg = { 0 };
 	struct iov_tail payload;
-	size_t hdrlen, iov_used;
-	unsigned elem_cnt;
-	unsigned i, j;
+	size_t hdrlen;
 	ssize_t dlen;
 
-	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");
-
-		*cnt = 0;
-		return 0;
-	}
-
 	/* compute L2 header length */
 	hdrlen = udp_vu_hdrlen(v6);
 
-	elem_cnt = vu_collect(vdev, vq, elem, ARRAY_SIZE(elem),
-			      iov, *cnt, &iov_used,
-			      IP_MAX_MTU + ETH_HLEN + VNET_HLEN, NULL);
-	if (elem_cnt == 0)
-		return -1;
-
-	ASSERT((size_t)elem_cnt == iov_used);	/* one iovec per element */
-
-	payload = IOV_TAIL(iov, iov_used, hdrlen);
+	payload = IOV_TAIL(iov, *cnt, 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, elem_cnt);
+	if (dlen < 0)
 		return -1;
-	}
-
-	*cnt = vu_pad(iov, iov_used, 0, dlen + hdrlen);
-
-	*elem_used = 0;
-	for (i = 0, j = 0; j < *cnt && i < elem_cnt; i++) {
-		if (j + elem[i].in_num > *cnt)
-			elem[i].in_num = *cnt - j;
-		j += elem[i].in_num;
-		(*elem_used)++;
-	}
 
-	vu_set_vnethdr(iov[0].iov_base, *elem_used);
-
-	/* release unused buffers */
-	vu_queue_rewind(vq, elem_cnt - *elem_used);
+	*cnt = vu_pad(iov, *cnt, 0, dlen + hdrlen);
 
 	return dlen;
 }
@@ -217,26 +171,60 @@ static void udp_vu_csum(const struct flowside *toside,
  */
 void udp_vu_sock_to_tap(const struct ctx *c, int s, int n, flow_sidx_t tosidx)
 {
+	static struct iovec     iov_vu		[VIRTQUEUE_MAX_SIZE];
+	static struct vu_virtq_element	elem	[VIRTQUEUE_MAX_SIZE];
 	const struct flowside *toside = flowside_at_sidx(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;
 
+	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++) {
-		unsigned elem_used;
+		unsigned elem_used, elem_cnt, j, k;
 		size_t iov_cnt;
 		ssize_t dlen;
 
-		iov_cnt = ARRAY_SIZE(iov_vu);
-		dlen = udp_vu_sock_recv(c, iov_vu, &iov_cnt, &elem_used, vq,
-					s, v6);
-		if (dlen < 0)
+		elem_cnt = vu_collect(vdev, vq, elem, ARRAY_SIZE(elem),
+				      iov_vu, ARRAY_SIZE(iov_vu), &iov_cnt,
+				      IP_MAX_MTU + ETH_HLEN + VNET_HLEN, NULL);
+		if (elem_cnt == 0)
+			break;
+
+		dlen = udp_vu_sock_recv(iov_vu, &iov_cnt, s, v6);
+		if (dlen < 0) {
+			vu_queue_rewind(vq, elem_cnt);
 			break;
+		}
+
+		elem_used = 0;
+		for (j = 0, k = 0; k < iov_cnt && j < elem_cnt; j++) {
+			if (k + elem[j].in_num > iov_cnt)
+				elem[j].in_num = iov_cnt - k;
+			k += elem[j].in_num;
+			elem_used++;
+		}
+
+		/* release unused buffers */
+		vu_queue_rewind(vq, elem_cnt - elem_used);
 
 		if (iov_cnt > 0) {
-			data = IOV_TAIL(iov_vu, iov_cnt, 0);
+			struct iov_tail data = IOV_TAIL(iov_vu, iov_cnt, 0);
+			vu_set_vnethdr(iov_vu[0].iov_base, elem_used);
 			udp_vu_prepare(c, &data, toside, dlen);
 			if (*c->pcap) {
 				udp_vu_csum(toside, &data);
-- 
2.53.0