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

[David Gibson <david@gibson.dropbear.id.au>]

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On Mon, Jan 05, 2026 at 03:50:16PM +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.
> 
> 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>

My only concern here is that this will only consume a single datagram,
whereas it looks like the loop could consume multiple datagrams.  I'm
guessing something higher up will keep calling this until the queue is
empty, but I'm not certain.  Otherwise LGTM.

> ---
>  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
> 

-- 
David Gibson (he or they)	| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you, not the other way
				| around.
http://www.ozlabs.org/~dgibson

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