Re: Integrating QOM into QAPI

["Marc-André Lureau" <marcandre.lureau@gmail.com>]

Hi

On Sun, Jan 26, 2020 at 9:10 AM Christophe de Dinechin
<dinechin@redhat.com> wrote:
>
>
>
> > On 21 Jan 2020, at 16:11, Marc-André Lureau <marcandre.lureau@gmail.com> wrote:
> >
> > Hi
> >
> > On Tue, Jan 21, 2020 at 7:01 PM Markus Armbruster <armbru@redhat.com> wrote:
> >>
> >> Daniel P. Berrangé <berrange@redhat.com> writes:
> >>
> >>> On Tue, Jan 21, 2020 at 02:36:17PM +0100, Markus Armbruster wrote:
> >>>> Marc-André Lureau <marcandre.lureau@gmail.com> writes:
> >>>>
> >>>>> Hi
> >>>>>
> >>>>> On Tue, Jan 21, 2020 at 3:32 PM Stefan Hajnoczi <stefanha@gmail.com> wrote:
> >>>>>>
> >>>>>> On Tue, Jan 21, 2020 at 06:42:47AM +0100, Markus Armbruster wrote:
> >>>>>>> Stefan Hajnoczi <stefanha@gmail.com> writes:
> >>>>>>>
> >>>>>>>> On Wed, Jan 15, 2020 at 01:15:17PM +0100, Markus Armbruster wrote:
> >>>>>>>>> Christophe de Dinechin <dinechin@redhat.com> writes:
> >>>>>>>>>>> On 15 Jan 2020, at 10:20, Markus Armbruster <armbru@redhat.com> 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.
> >>
> >> Volunteers?
> >>
> >
> > Actually, we are not that far off from being able to use GObject
> > altogether (I hacked something like that to play with), but I
> > disgress...
> >
> > So introducing GObject-like macros? sure!
>
> I’m still puzzled as to why anybody would switch to something like
> GObject when there is C++.

C++ is another level of complexity.

Replacing QOM with GObject would mainly bring us a more solid type
system with better tooling/features, gobject-introspection support,
and remove the burden of having our own OO from QEMU code base.

It sufficiently hard for GObject developers to allow writing GObjects
from Rust, I don't think anyone want to repeat that work for QOM/QDev.
I don't know how c++ and rust would interoperate, but that seems even
more complicated to me.


-- 
Marc-André Lureau