qdev/qbus: add hidden device support

This adds support for hiding a device to the qbus and qdev APIs.  The
first user of this will be the virtio-net failover feature but the API
introduced with this patch could be used to implement other features as
well, for example hiding pci devices when a pci bus is powered off.

qdev_device_add() is modified to check for a failover_pair_id
argument in the option string. A DeviceListener callback
should_be_hidden() is added. It can be used by a standby device to
inform qdev that this device should not be added now. The standby device
handler can store the device options to plug the device in at a later
point in time.

One reason for hiding the device is that we don't want to expose both
devices to the guest kernel until the respective virtio feature bit
VIRTIO_NET_F_STANDBY was negotiated and we know that the devices will be
handled correctly by the guest.

More information on the kernel feature this is using:
 https://www.kernel.org/doc/html/latest/networking/net_failover.html

An example where the primary device is a vfio-pci device and the standby
device is a virtio-net device:

A device is hidden when it has an "failover_pair_id" option, e.g.

 -device virtio-net-pci,...,failover=on,...
 -device vfio-pci,...,failover_pair_id=net1,...

Signed-off-by: Jens Freimann <jfreimann@redhat.com>
Message-Id: <20191029114905.6856-2-jfreimann@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

1 files changed, 29 insertions, 0 deletions

diff --git a/include/hw/qdev-core.h b/include/hw/qdev-core.h
index aa123f88cb..710981af36 100644
--- a/include/hw/qdev-core.h
+++ b/include/hw/qdev-core.h
@@ -78,6 +78,19 @@ typedef void (*BusUnrealize)(BusState *bus, Error **errp);
  * respective parent types.
  *   </para>
  * </note>
+ *
+ * # Hiding a device #
+ * To hide a device, a DeviceListener function should_be_hidden() needs to
+ * be registered.
+ * It can be used to defer adding a device and therefore hide it from the
+ * guest. The handler registering to this DeviceListener can save the QOpts
+ * passed to it for re-using it later and must return that it wants the device
+ * to be/remain hidden or not. When the handler function decides the device
+ * shall not be hidden it will be added in qdev_device_add() and
+ * realized as any other device. Otherwise qdev_device_add() will return early
+ * without adding the device. The guest will not see a "hidden" device
+ * until it was marked don't hide and qdev_device_add called again.
+ *
  */
 typedef struct DeviceClass {
     /*< private >*/
@@ -154,6 +167,12 @@ struct DeviceState {
 struct DeviceListener {
     void (*realize)(DeviceListener *listener, DeviceState *dev);
     void (*unrealize)(DeviceListener *listener, DeviceState *dev);
+    /*
+     * This callback is called upon init of the DeviceState and allows to
+     * inform qdev that a device should be hidden, depending on the device
+     * opts, for example, to hide a standby device.
+     */
+    int (*should_be_hidden)(DeviceListener *listener, QemuOpts *device_opts);
     QTAILQ_ENTRY(DeviceListener) link;
 };
 
@@ -451,4 +470,14 @@ static inline bool qbus_is_hotpluggable(BusState *bus)
 void device_listener_register(DeviceListener *listener);
 void device_listener_unregister(DeviceListener *listener);
 
+/**
+ * @qdev_should_hide_device:
+ * @opts: QemuOpts as passed on cmdline.
+ *
+ * Check if a device should be added.
+ * When a device is added via qdev_device_add() this will be called,
+ * and return if the device should be added now or not.
+ */
+bool qdev_should_hide_device(QemuOpts *opts);
+
 #endif