Re: all class init functions for all types in QEMU are called in select_machine(). Expected?

[Paolo Bonzini]

On 12/03/21 10:31, Claudio Fontana wrote:
> Hello Paolo and all,
>
> while debugging a class init ordering issue, I noticed that
>
> _all_ class init functions for all types registered in the QEMU QOM are called 
> in select_machine().
> Expected?
>
> In particular it happens here:
>
> static MachineClass *select_machine(void)
> {
>      GSList *machines = object_class_get_list(TYPE_MACHINE, false);
>
>
> object_class_get_list() ->
>    object_class_foreach() ->
>      g_hash_table_foreach() ->
>        object_class_foreach_tramp ->
>          type_initialize(type);
>
> Is this really desired? It looks suspect to me.

It is not a problem because class_init should be idempotent.  Changing 
QEMU to not do this would not be impossible, but most likely not worth 
the effort.  To do this, I think one would have to reimplement all of 
object_class_dynamic_cast to operate on TypeInfos (so for example walk 
all interfaces in the type info instead of using class->interfaces).

> If not here, where should be the right place, for example, for CPU class inits 
> to be called?

The first time they're used, upon a call to one of object_new, 
object_initialize, object_class_get_list or object_class_foreach.

> At the very least I would put a comment there around the beginning of 
> select_machine() saying:
>
> /* all types, all classes in QOM are initialized here, as a result of the 
> object_class_get_list call */

No, it's just a side effect that is not (or should not) be visible.

Paolo