Re: [PATCH v3 04/10] udp_vu: Move virtqueue management from udp_vu_sock_recv() to its caller

11 May 2026


      On 2026-04-16 11:57, Laurent Vivier wrote:
...
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 
Reviewed-by: David Gibson 
Reviewed-by: Jon Maloy 

see below.
...
---
  udp_vu.c | 98 +++++++++++++++++++++++++++++---------------------------
  1 file changed, 50 insertions(+), 48 deletions(-)
diff --git a/udp_vu.c b/udp_vu.c
index f8629af58ab5..bd9fd5abb971 100644
--- a/udp_vu.c
[...]
+
+	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_cnt, elem_used;
unsigned int is standard in our code, I think.
/jon
...
+		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)
+		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;
+
+		assert((size_t)elem_cnt == iov_cnt);	/* one iovec per element */
+
+		dlen = udp_vu_sock_recv(s, v6, &iov_cnt);
+		if (dlen < 0) {
+			vu_queue_rewind(vq, iov_cnt);
      	break;
+		}
+
+		elem_used = iov_cnt; /* one iovec per element */
+
+		/* release unused buffers */
+		vu_queue_rewind(vq, elem_cnt - elem_used);
-		if (iov_used > 0) {
+		if (iov_cnt > 0) {
      	udp_vu_prepare(c, toside, dlen);
      	if (*c->pcap) {
-				udp_vu_csum(toside, iov_used);
-				pcap_iov(iov_vu, iov_used, VNET_HLEN);
+				udp_vu_csum(toside, iov_cnt);
+				pcap_iov(iov_vu, iov_cnt, VNET_HLEN);
      	}
-			vu_flush(vdev, vq, elem, iov_used);
+			vu_flush(vdev, vq, elem, iov_cnt);
      	vu_queue_notify(vdev, vq);
      }
  }