[PATCH] udp_vu: Discard datagrams when RX virtqueue is not usable

[Laurent Vivier <lvivier@redhat.com>]

During vhost-user device initialization, UDP datagrams may arrive on
listening sockets before the guest has enabled the RX virtqueue.

When this happens, udp_vu_sock_recv() returns 0 without consuming
the datagram from the socket. The caller, udp_sock_fwd(), uses a
while loop with udp_peek_addr() to process pending datagrams. Since
the datagram remains in the socket buffer, udp_peek_addr() keeps
returning data available, causing a busy loop with 100% CPU usage.

Add an early check for virtqueue readiness in udp_vu_sock_to_tap(),
mirroring tcp_vu_data_from_sock(). When the queue is not enabled or
not started, explicitly discard the datagram with recvmsg() and
return. The caller will drain remaining datagrams through repeated
calls.

Fixes: 28997fcb29b5 ("vhost-user: add vhost-user")
Link: https://bugs.passt.top/show_bug.cgi?id=185
Signed-off-by: Laurent Vivier <lvivier@redhat.com>
---
 udp_vu.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/udp_vu.c b/udp_vu.c
index c30dcf97698f..67f4ee619bfb 100644
--- a/udp_vu.c
+++ b/udp_vu.c
@@ -211,6 +211,17 @@ void udp_vu_sock_to_tap(const struct ctx *c, int s, int n, flow_sidx_t tosidx)
 	struct vu_virtq *vq = &vdev->vq[VHOST_USER_RX_QUEUE];
 	int i;
 
+	if (!vu_queue_enabled(vq) || !vu_queue_started(vq)) {
+		struct msghdr msg = { 0 };
+
+		debug("Got UDP packet, but RX virtqueue not usable yet");
+
+		if (recvmsg(s, &msg, MSG_DONTWAIT) < 0)
+			debug_perror("Failed to discard datagram");
+
+		return;
+	}
+
 	for (i = 0; i < n; i++) {
 		ssize_t dlen;
 		int iov_used;
-- 
2.52.0