[Devel] [PATCH VZ10] vhost/vsock: only refuse connection when guest has never been ready

[Denis V. Lunev]

On 5/21/26 23:31, Andrey Drobyshev wrote:
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> Commit 4ff28534c799 ("ms/vhost/vsock: Refuse the connection immediately
> when guest isn't ready") added a check which immediately returns
> EHOSTUNREACH when the guest isn't ready yet.  Namely, we check that guest
> hasn't enabled the RX vq yet, i.e. virtio-vsock has never beed enabled.
>
> However, the check also affects the transient state when backend is
> temporarily set to NULL during VHOST_VSOCK_SET_RUNNING(0).  Notably,
> this is the case with qemu-update operation, during which we perform
> VHOST_RESET_OWNER.  In this case sendmsg()/connect() on otherwise healthy
> connection gets EHOSTUNREACH.
>
> Gate the fast-fail on a sticky started_once bit set in
> vhost_vsock_start() and never cleared.  Once the guest has brought
> up virtio-vsock at least once, a NULL backend means a transient stop
> window and the packet must be queued for vhost_vsock_start() to drain
> on re-attach.
>
> Fixes: 4ff28534c799 ("ms/vhost/vsock: Refuse the connection immediately when guest isn't ready")
> https://virtuozzo.atlassian.net/browse/VSTOR-131956
> Signed-off-by: Andrey Drobyshev <andrey.drobyshev at virtuozzo.com>
> ---
>  drivers/vhost/vsock.c | 21 +++++++++++----------
>  1 file changed, 11 insertions(+), 10 deletions(-)
>
> diff --git a/drivers/vhost/vsock.c b/drivers/vhost/vsock.c
> index 0a518c3d1596..f39354db630e 100644
> --- a/drivers/vhost/vsock.c
> +++ b/drivers/vhost/vsock.c
> @@ -57,6 +57,7 @@ struct vhost_vsock {
>  
>  	u32 guest_cid;
>  	bool seqpacket_allow;
> +	bool started_once;	/* latched in vhost_vsock_start(); never cleared */
>  };
>  
>  static u32 vhost_transport_get_local_cid(void)
> @@ -285,17 +286,14 @@ vhost_transport_send_pkt(struct sk_buff *skb)
>  		return -ENODEV;
>  	}
>  
> -	/* Fast-fail if the guest hasn't enabled the RX vq yet. Queuing the packet
> -	 * and making the caller wait is pointless: even if the guest manages to init
> -	 * within the timeout, it'll immediately reply with RST, because there's no
> -	 * listener on the port yet.
> -	 *
> -	 * vhost_vq_get_backend() without vq->mutex is acceptable here: locking
> -	 * the mutex would be too expensive in this hot path, and we already have
> -	 * all the outcomes covered: if the backend becomes NULL right after the check,
> -	 * vhost_transport_do_send_pkt() will check it under the mutex anyway.
> +	/*
> +	 * Fast-fail only when the guest has never enabled virtio-vsock.
> +	 * Once it has, a NULL backend means a transient SET_RUNNING(0)
> +	 * window (e.g. VHOST_RESET_OWNER); the packet must be
> +	 * queued for vhost_vsock_start() to drain on re-attach.
>  	 */
> -	if (unlikely(!data_race(vhost_vq_get_backend(&vsock->vqs[VSOCK_VQ_RX])))) {
> +	if (unlikely(!READ_ONCE(vsock->started_once)) &&
> +	    !data_race(vhost_vq_get_backend(&vsock->vqs[VSOCK_VQ_RX]))) {
>  		rcu_read_unlock();
>  		kfree_skb(skb);
>  		return -EHOSTUNREACH;
> @@ -615,6 +613,9 @@ static int vhost_vsock_start(struct vhost_vsock *vsock)
>  	 */
>  	vhost_vq_work_queue(&vsock->vqs[VSOCK_VQ_RX], &vsock->send_pkt_work);
>  
> +	/* See vhost_transport_send_pkt(); never cleared. */
> +	WRITE_ONCE(vsock->started_once, true);
> +
>  	mutex_unlock(&vsock->dev.mutex);
>  	return 0;
>  
what about guest reset + stuck in BIOS?

Den