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

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

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On Wed, Jan 07, 2026 at 09:08:09AM +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 <lvivier@redhat.com>

Reviewed-by: David Gibson <david@gibson.dropbear.id.au>

Although one possible nit noted..

> ---
> 
> Notes:
>     v2:
>       - move recvmsg() from udp_vu_sock_to_tap() to udp_vu_sock_recv()
> 
>  udp_vu.c | 32 ++++++++++++++++++++++----------
>  1 file changed, 22 insertions(+), 10 deletions(-)
> 
> diff --git a/udp_vu.c b/udp_vu.c
> index c30dcf97698f..3774d538a2d0 100644
> --- a/udp_vu.c
> +++ b/udp_vu.c
> @@ -65,7 +65,8 @@ static size_t udp_vu_hdrlen(bool v6)
>   * @v6:		Set for IPv6 connections
>   * @dlen:	Size of received data (output)
>   *
> - * Return: number of iov entries used to store the datagram
> + * Return: number of iov entries used to store the datagram, 0 if the datagram
> + *         was discarded because the virtqueue is not ready, -1 on error
>   */
>  static int udp_vu_sock_recv(const struct ctx *c, struct vu_virtq *vq, int s,
>  			    bool v6, ssize_t *dlen)
> @@ -77,6 +78,15 @@ static int udp_vu_sock_recv(const struct ctx *c, struct vu_virtq *vq, int s,
>  
>  	ASSERT(!c->no_udp);
>  
> +	if (!vu_queue_enabled(vq) || !vu_queue_started(vq)) {
> +		debug("Got UDP packet, but RX virtqueue not usable yet");
> +
> +		if (recvmsg(s, &msg, MSG_DONTWAIT) < 0)

You use MSG_DONTWAIT here, but you don't on the normal path.  I guess
it shouldn't make a difference, since we've come from epoll so we know
something is waiting for us.  But I think we want to make the paths
look as identical as we can from the point of view of the socket side,
and this makes it a bit less obvious.

> +			debug_perror("Failed to discard datagram");

This also isn't really accurate in the case of EAGAIN / EWOULDBLOCK.

> +
> +		return 0;
> +	}
> +
>  	/* compute L2 header length */
>  	hdrlen = udp_vu_hdrlen(v6);
>  
> @@ -87,7 +97,7 @@ static int udp_vu_sock_recv(const struct ctx *c, struct vu_virtq *vq, int s,
>  			     sizeof(struct virtio_net_hdr_mrg_rxbuf),
>  			     NULL);
>  	if (iov_cnt == 0)
> -		return 0;
> +		return -1;
>  
>  	/* reserve space for the headers */
>  	ASSERT(iov_vu[0].iov_len >= MAX(hdrlen, ETH_ZLEN));
> @@ -101,7 +111,7 @@ static int udp_vu_sock_recv(const struct ctx *c, struct vu_virtq *vq, int s,
>  	*dlen = recvmsg(s, &msg, 0);
>  	if (*dlen < 0) {
>  		vu_queue_rewind(vq, iov_cnt);
> -		return 0;
> +		return -1;
>  	}
>  
>  	/* restore the pointer to the headers address */
> @@ -216,15 +226,17 @@ void udp_vu_sock_to_tap(const struct ctx *c, int s, int n, flow_sidx_t tosidx)
>  		int iov_used;
>  
>  		iov_used = udp_vu_sock_recv(c, vq, s, v6, &dlen);
> -		if (iov_used <= 0)
> +		if (iov_used < 0)
>  			break;
>  
> -		udp_vu_prepare(c, toside, dlen);
> -		if (*c->pcap) {
> -			udp_vu_csum(toside, iov_used);
> -			pcap_iov(iov_vu, iov_used,
> -				 sizeof(struct virtio_net_hdr_mrg_rxbuf));
> +		if (iov_used > 0) {
> +			udp_vu_prepare(c, toside, dlen);
> +			if (*c->pcap) {
> +				udp_vu_csum(toside, iov_used);
> +				pcap_iov(iov_vu, iov_used,
> +					sizeof(struct virtio_net_hdr_mrg_rxbuf));
> +			}
> +			vu_flush(vdev, vq, elem, iov_used);
>  		}
> -		vu_flush(vdev, vq, elem, 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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