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

[Polina Vishneva <polina.vishneva@virtuozzo.com>]

On Mon, 2026-05-11 at 17:56 +0200, Stefano Garzarella wrote:
> 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?

It can, but it might be difficult to find a correct timeout.

And, generally, there's no way to distinguish "the guest hasn't yet initialized
the vq" from "the guest is up and running, but didn't reply to connect() in
time". That's exactly what this patch is attempting to fix.

>
> > 
> > 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() ?

Because it locks the mutex, which is slow and unacceptable in this hot path.

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

It's racy, but as described here in the comment and in the commit message,
any possible race outcome is covered by the subsequent checks.

> > {
> > +		rcu_read_unlock();
> > +		kfree_skb(skb);
> > +		return -ECONNREFUSED;
> 
> This is a generic send_pkt, is it okay to return ECONNREFUSED in any 
> case?

EHOSTUNREACH would probably be better.
All the current send_pkt functions only return ENODEV, but it has different
semantics: they mean that the local device isn't yet ready, while there we're
dealing with the opposite end not being ready.

Best regards, Polina.

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