[Devel] [PATCH v3 1/1] vhost/vsock: suppress EHOSTUNREACH fast-fail during CPR pause

[Pavel Tikhomirov]

Looks good.

Reviewed-by: Pavel Tikhomirov <ptikhomirov at virtuozzo.com>

On 5/22/26 18:14, Denis V. Lunev wrote:
> QEMU's CPR live update issues VHOST_VSOCK_SET_RUNNING(0) on the source
> before VHOST_RESET_OWNER, nulling vq->private_data via
> vhost_vsock_drop_backends(). The fast-fail in vhost_transport_send_pkt()
> added by commit 4ff28534c799 ("ms/vhost/vsock: Refuse the connection
> immediately when guest isn't ready") then rejects every host send with
> -EHOSTUNREACH until the destination calls SET_RUNNING(1) -- the entire
> CPR window becomes a hard outage for host AF_VSOCK clients
> (VSTOR-131956).
> 
> Add a cpr_paused flag set inside vhost_vsock_drop_backends() when the
> backend was previously live, cleared by vhost_vsock_start(). When set,
> vhost_transport_send_pkt() queues the skb instead of fast-failing; the
> existing kick of send_pkt_work in vhost_vsock_start() drains it on
> resume. A device that has never run keeps cpr_paused == false and the
> boot-time fast-fail behaviour is preserved.
> 
> The flag is set before dropping backends so a concurrent sender on
> another CPU never observes (NULL backend, !paused).
> 
> https://virtuozzo.atlassian.net/browse/VSTOR-131956
> Signed-off-by: Denis V. Lunev <den at openvz.org>
> ---
>  drivers/vhost/vsock.c | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/vhost/vsock.c b/drivers/vhost/vsock.c
> index 0a518c3d1596..9d8083b065c3 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 cpr_paused;	/* between stop and next start; queues sends */
>  };
>  
>  static u32 vhost_transport_get_local_cid(void)
> @@ -295,7 +296,9 @@ vhost_transport_send_pkt(struct sk_buff *skb)
>  	 * 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.
>  	 */
> -	if (unlikely(!data_race(vhost_vq_get_backend(&vsock->vqs[VSOCK_VQ_RX])))) {
> +	/* cpr_paused: queue across CPR; else NULL backend means not ready. */
> +	if (unlikely(!data_race(vhost_vq_get_backend(&vsock->vqs[VSOCK_VQ_RX])) &&
> +		     !READ_ONCE(vsock->cpr_paused))) {
>  		rcu_read_unlock();
>  		kfree_skb(skb);
>  		return -EHOSTUNREACH;
> @@ -610,6 +613,8 @@ static int vhost_vsock_start(struct vhost_vsock *vsock)
>  		mutex_unlock(&vq->mutex);
>  	}
>  
> +	WRITE_ONCE(vsock->cpr_paused, false);
> +
>  	/* Some packets may have been queued before the device was started,
>  	 * let's kick the send worker to send them.
>  	 */
> @@ -641,6 +646,9 @@ static void vhost_vsock_drop_backends(struct vhost_vsock *vsock)
>  
>  	lockdep_assert_held(&vsock->dev.mutex);
>  
> +	if (vhost_vq_get_backend(&vsock->vqs[VSOCK_VQ_RX]))
> +		WRITE_ONCE(vsock->cpr_paused, true);
> +
>  	for (i = 0; i < ARRAY_SIZE(vsock->vqs); i++) {
>  		vq = &vsock->vqs[i];
>  

-- 
Best regards, Pavel Tikhomirov
Senior Software Developer, Virtuozzo.