virtio_console: avoid config access from irq

commit eeb8a7e8 upstream.

when multiport is off, virtio console invokes config access from irq
context, config access is blocking on s390.
Fix this up by scheduling work from config irq - similar to what we do
for multiport configs.
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Amit Shah <amit.shah@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
[bwh: Backported to 3.2:
 - Adjust context
 - Drop changes to virtcons_freeze()]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

drivers/char/virtio_console.c

@@ -124,6 +124,7 @@ struct ports_device {
 	 * notification
 	 */
 	struct work_struct control_work;
+	struct work_struct config_work;
 
 	struct list_head ports;
 
@@ -1556,10 +1557,21 @@ static void config_intr(struct virtio_device *vdev)
 
 	portdev = vdev->priv;
 
+	if (!use_multiport(portdev))
+		schedule_work(&portdev->config_work);
+}
+
+static void config_work_handler(struct work_struct *work)
+{
+	struct ports_device *portdev;
+
+	portdev = container_of(work, struct ports_device, control_work);
 	if (!use_multiport(portdev)) {
+		struct virtio_device *vdev;
 		struct port *port;
 		u16 rows, cols;
 
+		vdev = portdev->vdev;
 		vdev->config->get(vdev,
 				  offsetof(struct virtio_console_config, cols),
 				  &cols, sizeof(u16));
@@ -1731,6 +1743,8 @@ static int __devinit virtcons_probe(struct virtio_device *vdev)
 	spin_lock_init(&portdev->ports_lock);
 	INIT_LIST_HEAD(&portdev->ports);
 
+	INIT_WORK(&portdev->config_work, &config_work_handler);
+
 	if (multiport) {
 		unsigned int nr_added_bufs;
 
@@ -1806,6 +1820,8 @@ static void virtcons_remove(struct virtio_device *vdev)
 	/* Finish up work that's lined up */
 	if (use_multiport(portdev))
 		cancel_work_sync(&portdev->control_work);
+	else
+		cancel_work_sync(&portdev->config_work);
 
 	list_for_each_entry_safe(port, port2, &portdev->ports, list)
 		unplug_port(port);