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

[Laurent Vivier <lvivier@redhat.com>]

On 1/6/26 00:28, David Gibson wrote:
> 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.

I can move the change into udp_vu_sock_recv() (inside the datagram loop), it will be 
closer to what udp_buf_sock_to_tap() does: reads data with udp_sock_recv() and if fd_tap 
is not initialized tap_send_frames() drops them.

Thanks,
Laurent
>> ---
>>   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
>>
>