KVM: s390: Gracefully handle busy conditions on ccw_device_start

In rare cases a virtio command might try to issue a ccw before a former
ccw was answered with a tsch. This will cause CC=2 (busy). Lets just
retry in that case.
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

drivers/s390/kvm/virtio_ccw.c

@@ -132,11 +132,14 @@ static int ccw_io_helper(struct virtio_ccw_device *vcdev,
 	unsigned long flags;
 	int flag = intparm & VIRTIO_CCW_INTPARM_MASK;
 
+	do {
 		spin_lock_irqsave(get_ccwdev_lock(vcdev->cdev), flags);
 		ret = ccw_device_start(vcdev->cdev, ccw, intparm, 0, 0);
 		if (!ret)
 			vcdev->curr_io |= flag;
 		spin_unlock_irqrestore(get_ccwdev_lock(vcdev->cdev), flags);
+		cpu_relax();
+	} while (ret == -EBUSY);
 	wait_event(vcdev->wait_q, doing_io(vcdev, flag) == 0);
 	return ret ? ret : vcdev->err;
 }