Re: [PATCH v2] udp_vu: Discard datagrams when RX virtqueue is not usable

On Wed, 7 Jan 2026 09:08:09 +0100 Laurent Vivier wrote:

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.

To avoid that, we need to discard the data when the virtqueue is not ready. udp_buf_sock_to_tap() actually does the same as it reads data with udp_sock_recv() and if fd_tap is not initialized tap_send_frames() drops them.

Fixes: 28997fcb29b5 ("vhost-user: add vhost-user") Link: https://bugs.passt.top/show_bug.cgi?id=185 Signed-off-by: Laurent Vivier ---

Notes: v2: - move recvmsg() from udp_vu_sock_to_tap() to udp_vu_sock_recv()

Applied. -- Stefano