Re: Integrating QOM into QAPI

[Markus Armbruster]

Daniel P. Berrangé <address@hidden> writes:

> On Tue, Jan 21, 2020 at 02:36:17PM +0100, Markus Armbruster wrote:
>> Marc-André Lureau <address@hidden> writes:
>> 
>> > Hi
>> >
>> > On Tue, Jan 21, 2020 at 3:32 PM Stefan Hajnoczi <address@hidden> wrote:
>> >>
>> >> On Tue, Jan 21, 2020 at 06:42:47AM +0100, Markus Armbruster wrote:
>> >> > Stefan Hajnoczi <address@hidden> writes:
>> >> >
>> >> > > On Wed, Jan 15, 2020 at 01:15:17PM +0100, Markus Armbruster wrote:
>> >> > >> Christophe de Dinechin <address@hidden> writes:
>> >> > >> >> On 15 Jan 2020, at 10:20, Markus Armbruster <address@hidden> 
>> >> > >> >> wrote:
>> >> > >> * qemuMonitorJSONSetIOThread() uses it to control iothread's 
>> >> > >> properties
>> >> > >>   poll-max-ns, poll-grow, poll-shrink.  Their use with -object is
>> >> > >>   documented (in qemu-options.hx), their use with qom-set is not.
>> >> > >
>> >> > > I'm happy to use a different interface.
>> >> > >
>> >> > > Writing a boilerplate "iothread-set-poll-params" QMP command in C 
>> >> > > would
>> >> > > be a step backwards.
>> >> >
>> >> > No argument.
>> >> >
>> >> > > Maybe the QAPI code generator could map something like this:
>> >> > >
>> >> > >   { 'command': 'iothread-set-poll-params',
>> >> > >     'data': {
>> >> > >         'id': 'str',
>> >> > >     '*max-ns': 'uint64',
>> >> > >     '*grow': 'uint64',
>> >> > >     '*shrink': 'uint64'
>> >> > >     },
>> >> > >     'map-to-qom-set': 'IOThread'
>> >> > >   }
>> >> > >
>> >> > > And turn it into QOM accessors on the IOThread object.
>> >> >
>> >> > I think a generic "set this configuration to that value" command is just
>> >> > fine.  qom-set fails on several counts, though:
>> >> >
>> >> > * Tolerable: qom-set is not actually generic, it applies only to QOM.
>> >> >
>> >> > * qom-set lets you set tons of stuff that is not meant to be changed at
>> >> >   run time.  If it breaks your guest, you get to keep the pieces.
>> >> >
>> >> > * There is virtually no documentation on what can be set to what values,
>> >> >   and their semantics.
>> >> >
>> >> > In its current state, QOM is a user interface superfund site.
>> >>
>> >> Thoughts about a solution:
>> >>
>> >> Static QOM properties should be declared via QAPI instead of
>> >> imperatively via QOM APIs.  That way they are introspectable and type
>> >> information is present in the schema.
>> >>
>> >> The QAPI code generator could emit a function that is callable from
>> >> .class_init().  This eliminates the need to manually call
>> >> object_class_property_add().
>> 
>> We need to make up our minds what exactly we want generated.  Then we
>> can design the QAPI language, and code up the generator.
>> 
>> Skeleton QOM type, to help with the discussion:
>> 
>>     #define TYPE_FOO "foo"
>> 
>>     #define FOO(obj) OBJECT_CHECK(Foo, (obj), TYPE_FOO)
>>     #define FOO_CLASS(klass) \
>>         OBJECT_CLASS_CHECK(FooClass, (klass), TYPE_FOO)
>>     #define FOO_GET_CLASS(obj) \
>>         OBJECT_GET_CLASS(FooClass, (obj), TYPE_FOO)
>> 
>>     typedef FooClass {
>>         ParentClass parent_class;
>>         ... // hand-written per-class state
>>     }
>> 
>>     struct Chardev {
>>         ParentObject parent_obj;
>>         ... // hand-written instance (per-object) state
>>     };
>> 
>>     static const TypeInfo char_type_info = {
>>         .name = TYPE_FOO,
>>         .parent = TYPE_OBJECT,
>>         .instance_size = sizeof(Foo),
>>         .instance_init = ...,                   // methods to initialize
>>         .instance_post_init = ...,              // and finalize instances,
>>         .instance_finalize = ...,               // all optional
>>         .abstract = ...,                        // true or false (d'oh)
>>         .class_size = sizeof(FooClass),
>>         .class_init = ...,                      // methods to initialize
>>         .class_base_init = ...,                 // classes, optional
>>         .class_data = ...,                      // extra argument for them
>>         .interfaces = ...
>>     };
>> 
>> There's substantial boilerplate, with plenty of hand-written code in the
>> gaps.  What of the boilerplate do we plan to generate?  How do we plan
>> to fill the gaps, if any?
>
> FWIW, even without a QOM generator, we can do waaaaaaay better on the
> amount of boilerplate needed for QOM without very much work. It just
> needs a few convenience macros writing.
>
> QOM is not GObject, but is heavily inspired by it and so looking at
> GObject gives us a design pattern we can aim to match in terms of
> amount of boilerplate.
>
> What we do manually with TypeInfo struct there has essentially always
> been done by a 1 line macro in GObject:
>
>   G_DEFINE_TYPE(virIdentity, vir_identity, G_TYPE_OBJECT)
>
> If implementing interfaces, there's 1 extra line needed per interface
> to associate them.
>
>   
> https://developer.gnome.org/gobject/stable/gobject-Type-Information.html#G-DEFINE-TYPE:CAPS
>   
>
> And what we do in the header file to add the 4 or more FOO_XXX macros,
> and the class struct and the object struct has recently been turned
> into a 2-liner:
>
>   #define VIR_TYPE_IDENTITY vir_identity_get_type()
>   G_DECLARE_FINAL_TYPE(virIdentity, vir_identity, VIR, IDENTITY, GObject);
>
>   
> https://developer.gnome.org/gobject/stable/gobject-Type-Information.html#G-DECLARE-FINAL-TYPE:CAPS
>
> Or 
>
>   #define VIR_TYPE_IDENTITY vir_identity_get_type()
>   G_DECLARE_DERIVABLE_TYPE(virIdentity, vir_identity, VIR, IDENTITY, GObject);
>
>   
> https://developer.gnome.org/gobject/stable/gobject-Type-Information.html#G-DECLARE-DERIVABLE-TYPE:CAPS
>
>
> It would be nice to have a QOM code generator so that we can statically
> declare properties & parent/child/interface relationships, but for an
> immediate low cost win, better macros would be very useful IMHO.

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