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

[Stefano Brivio <sbrivio@redhat.com>]

On Wed,  7 Jan 2026 09:08:09 +0100
Laurent Vivier <lvivier@redhat.com> 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 <lvivier@redhat.com>
> ---
> 
> Notes:
>     v2:
>       - move recvmsg() from udp_vu_sock_to_tap() to udp_vu_sock_recv()

Applied.

-- 
Stefano