Re: [PATCH] vhost/vsock: Refuse the connection immediately when guest isn't ready

[Stefano Garzarella <sgarzare@redhat.com>]

On Mon, May 11, 2026 at 04:56:10PM +0200, Polina Vishneva wrote:
>From: "Denis V. Lunev" <den@openvz.org>
>
>When the host initiates an AF_VSOCK connect() to a guest that has not
>yet loaded the virtio-vsock transport (i.e. still booting), the caller
>blocks for VSOCK_DEFAULT_CONNECT_TIMEOUT (2 seconds), because
>vhost_transport_do_send_pkt() silently exits when
>vhost_vq_get_backend(vq) returns NULL.

Can SO_VM_SOCKETS_CONNECT_TIMEOUT helps on this?

>
>If the guest doesn't start listening within this timeout, connect()
>returns ETIMEDOUT.
>
>This delay is usually pointless and it doesn't well align with our
>behavior at other initialization stages: for example, if a connection is
>attempted when the guest driver is already loaded, but when nothing is
>listening yet, it returns ECONNRESET immediately without any wait.
>
>Fix this by checking the RX virtqueue backend in
>vhost_transport_send_pkt() before queuing. If the backend is NULL,
>return -ECONNREFUSED immediately.
>
>Signed-off-by: Denis V. Lunev <den@openvz.org>
>Co-developed-by: Polina Vishneva <polina.vishneva@virtuozzo.com>
>Signed-off-by: Polina Vishneva <polina.vishneva@virtuozzo.com>
>---
> drivers/vhost/vsock.c | 10 ++++++++++
> 1 file changed, 10 insertions(+)
>
>diff --git a/drivers/vhost/vsock.c b/drivers/vhost/vsock.c
>index 1d8ec6bed53e..a3f218292c3a 100644
>--- a/drivers/vhost/vsock.c
>+++ b/drivers/vhost/vsock.c
>@@ -302,6 +302,16 @@ vhost_transport_send_pkt(struct sk_buff *skb, struct net *net)
> 		return -ENODEV;
> 	}
>
>+	/* Fast-fail if the guest hasn't enabled the RX vq yet. Reading
>+	 * private_data without vq->mutex is deliberate: even if the backend becomes
>+	 * NULL right after that check, do_send_pkt() checks it under the mutex.
>+	 */
>+	if (!data_race(READ_ONCE(vsock->vqs[VSOCK_VQ_RX].private_data))) 

Why not using vhost_vq_get_backend() ?

Also is READ_ONCE() okay without WRITE_ONCE() where it is set ?

>{
>+		rcu_read_unlock();
>+		kfree_skb(skb);
>+		return -ECONNREFUSED;

This is a generic send_pkt, is it okay to return ECONNREFUSED in any 
case?

Thanks,
Stefano

>+	}
>+
> 	if (virtio_vsock_skb_reply(skb))
> 		atomic_inc(&vsock->queued_replies);
>
>
>base-commit: 8ab992f815d6736b5c7a6f5fd7bfe7bc106bb3dc
>-- 
>2.53.0
>