Re: [Qemu-devel] [PATCH for-3.2 v4 25/28] machine: add compat-props interface

[Igor Mammedov <imammedo@redhat.com>]

On Thu, 29 Nov 2018 15:49:00 -0200
Eduardo Habkost <ehabkost@redhat.com> wrote:

> On Tue, Nov 27, 2018 at 01:27:58PM +0400, Marc-André Lureau wrote:
> > Let's make compatiblity properties an interface, so that objects other
> > than QDev can benefit from having machine compatiblity properties.
> > 
> > Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>  
> [...]
> > diff --git a/hw/core/qdev.c b/hw/core/qdev.c
> > index 3b31b2c025..b0ee05f837 100644
> > --- a/hw/core/qdev.c
> > +++ b/hw/core/qdev.c
> > @@ -970,28 +970,8 @@ static void device_initfn(Object *obj)
> >      QLIST_INIT(&dev->gpios);
> >  }
> >  
> > -static const GPtrArray *ac_compat_props;
> > -static const GPtrArray *mc_compat_props;
> > -
> > -void accel_register_compat_props(const GPtrArray *props)
> > -{
> > -    ac_compat_props = props;
> > -}
> > -
> > -void machine_register_compat_props(const GPtrArray *props)
> > -{
> > -    mc_compat_props = props;
> > -}
> > -
> >  static void device_post_init(Object *obj)
> >  {
> > -    if (ac_compat_props) {
> > -        object_apply_global_props(obj, ac_compat_props, &error_abort);
> > -    }
> > -    if (mc_compat_props) {
> > -        object_apply_global_props(obj, mc_compat_props, &error_abort);
> > -    }
> > -
> >      qdev_prop_set_globals(DEVICE(obj));
> >  }
> >  
> > @@ -1124,6 +1104,10 @@ static const TypeInfo device_type_info = {
> >      .class_init = device_class_init,
> >      .abstract = true,
> >      .class_size = sizeof(DeviceClass),
> > +    .interfaces = (InterfaceInfo[]) {
> > +        { TYPE_COMPAT_PROPS },
> > +        { }
> > +    }  
> 
> At first I thought TYPE_COMPAT_PROPS was a practical way to
> implement this feature, but now I'm worried: the ordering between
> compat_props_post_init() qdev_prop_set_globals() is very
> important (user-provided globals must always be set after compat
> props), and here the ordering is implicit and easy to break
> accidentally.
> 
> What if instead of a QOM interface we just provide a simple
> object_apply_compat_props() function?  e.g.:
> 
> qdev.c:
> 
>   static void device_post_init(Object *obj)
>   {
>       object_apply_compat_props(obj);
>       apply_user_provided_globals(obj);
>   }
> 
> object_interface.c:
> 
>   void user_creatable_complete(Object *obj, Error **errp)
>   {
>       object_apply_compat_props(obj);
>       ...
>       ucc->complete(...)
>   }
this would work as long as nothing is happening between
object_new() ... user_creatable_complete() but look at
current users of user_creatable_complete() so it's
fragile too.

Reasons for compat props interface are the same as for
instance_post_init/device_post_init.

the thing we can do here is getting rid of device_post_init
and making device override TYPE_COMPAT_PROPS::instance_post_init
to make explicit ordering like we do everywhere else:

diff --git a/hw/core/qdev.c b/hw/core/qdev.c
index 6b3cc55..46ad6f5 100644
--- a/hw/core/qdev.c
+++ b/hw/core/qdev.c
@@ -1033,11 +1033,19 @@ static void device_unparent(Object *obj)
     }
 }
 
+device_compat_props()
+{
+    dc->parent_compat_props()
+    apply_global_props()
+}
+
 static void device_class_init(ObjectClass *class, void *data)
 {
     DeviceClass *dc = DEVICE_CLASS(class);
 
     class->unparent = device_unparent;
+    dc->parent_compat_props = COMPAT_PROPS_GET_CLASS(class)->instance_post_init
+    COMPAT_PROPS_GET_CLASS(class)->instance_post_init = device_compat_props()
 
     /* by default all devices were considered as hotpluggable,
      * so with intent to check it in generic qdev_unplug() /

> Most people don't understand QOM interfaces and their
> initialization ordering rules.  Everybody understands C function
> calls.
> 
> > [...]  
>