[PATCH] vhost: vsock: don't send pkt when vq is not started

[Jia He <justin.he@xxxxxxx>]

Ning Bo reported an abnormal 2-second gap when booting Kata container [1].
The unconditional timeout is caused by VSOCK_DEFAULT_CONNECT_TIMEOUT of
connect at client side. The vhost vsock client tries to connect an
initlizing virtio vsock server.

The abnormal flow looks like:
host-userspace           vhost vsock                       guest vsock
==============           ===========                       ============
connect()     -------->  vhost_transport_send_pkt_work()   initializing
   |                     vq->private_data==NULL
   |                     will not be queued
   V
schedule_timeout(2s)
                         vhost_vsock_start()  <---------   device ready
                         set vq->private_data

wait for 2s and failed

connect() again          vq->private_data!=NULL          recv connecting pkt

1. host userspace sends a connect pkt, at that time, guest vsock is under
initializing, hence the vhost_vsock_start has not been called. So
vq->private_data==NULL, and the pkt is not been queued to send to guest.
2. then it sleeps for 2s
3. after guest vsock finishes initializing, vq->private_data is set.
4. When host userspace wakes up after 2s, send connecting pkt again,
everything is fine.

This fixes it by checking vq->private_data in vhost_transport_send_pkt,
and return at once if !vq->private_data. This makes user connect()
be returned with ECONNREFUSED.

After this patch, kata-runtime (with vsock enabled) boottime reduces from
3s to 1s on ThunderX2 arm64 server.

[1] https://github.com/kata-containers/runtime/issues/1917

Reported-by: Ning Bo <n.b@xxxxxxxx>
Signed-off-by: Jia He <justin.he@xxxxxxx>
---
 drivers/vhost/vsock.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/drivers/vhost/vsock.c b/drivers/vhost/vsock.c
index e36aaf9ba7bd..67474334dd88 100644
--- a/drivers/vhost/vsock.c
+++ b/drivers/vhost/vsock.c
@@ -241,6 +241,7 @@ vhost_transport_send_pkt(struct virtio_vsock_pkt *pkt)
 {
 	struct vhost_vsock *vsock;
 	int len = pkt->len;
+	struct vhost_virtqueue *vq;
 
 	rcu_read_lock();
 
@@ -252,6 +253,13 @@ vhost_transport_send_pkt(struct virtio_vsock_pkt *pkt)
 		return -ENODEV;
 	}
 
+	vq = &vsock->vqs[VSOCK_VQ_RX];
+	if (!vq->private_data) {
+		rcu_read_unlock();
+		virtio_transport_free_pkt(pkt);
+		return -ECONNREFUSED;
+	}
+
 	if (pkt->reply)
 		atomic_inc(&vsock->queued_replies);
 
-- 
2.17.1