globals: Allow global properties to be optional

Making some global properties optional will let us simplify
compat code when a given property works on most (but not all)
subclasses of a given type.

Device types will be able to opt out from optional compat
properties by simply not registering those properties.
Signed-off-by: NEduardo Habkost <ehabkost@redhat.com>
Reviewed-by: NCornelia Huck <cohuck@redhat.com>
Reviewed-by: NMarc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>

include/hw/qdev-core.h

@@ -250,6 +250,8 @@ struct PropertyInfo {
 /**
  * GlobalProperty:
  * @used: Set to true if property was used when initializing a device.
+ * @optional: If set to true, GlobalProperty will be skipped without errors
+ *            if the property doesn't exist.
  *
  * An error is fatal for non-hotplugged devices, when the global is applied.
  */
@@ -258,6 +260,7 @@ typedef struct GlobalProperty {
     const char *property;
     const char *value;
     bool used;
+    bool optional;
 } GlobalProperty;
 
 static inline void

qom/object.c

@@ -385,6 +385,9 @@ void object_apply_global_props(Object *obj, const GPtrArray *props, Error **errp
         if (object_dynamic_cast(obj, p->driver) == NULL) {
             continue;
         }
+        if (p->optional && !object_property_find(obj, p->property, NULL)) {
+            continue;
+        }
         p->used = true;
         object_property_parse(obj, p->value, p->property, &err);
         if (err != NULL) {