From: Eduardo Habkost <ehabkost@redhat.com>

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: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
---
 include/hw/qdev-core.h | 3 +++
 qom/object.c           | 3 +++
 2 files changed, 6 insertions(+)

diff --git a/include/hw/qdev-core.h b/include/hw/qdev-core.h
index 9614f76ae6..0a84c42756 100644
--- a/include/hw/qdev-core.h
+++ b/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
diff --git a/qom/object.c b/qom/object.c
index 4e5226ca12..b8c732063b 100644
--- a/qom/object.c
+++ b/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) {
-- 
MST