> On 21 Jan 2020, at 16:11, Marc-André Lureau <marcandre.lur...@gmail.com>
> wrote:
>
> Hi
>
> On Tue, Jan 21, 2020 at 7:01 PM Markus Armbruster <arm...@redhat.com> wrote:
>>
>> Daniel P. Berrangé <berra...@redhat.com> writes:
>>
>>> On Tue, Jan 21, 2020 at 02:36:17PM +0100, Markus Armbruster wrote:
>>>> Marc-André Lureau <marcandre.lur...@gmail.com> writes:
>>>>
>>>>> Hi
>>>>>
>>>>> On Tue, Jan 21, 2020 at 3:32 PM Stefan Hajnoczi <stefa...@gmail.com>
>>>>> wrote:
>>>>>>
>>>>>> On Tue, Jan 21, 2020 at 06:42:47AM +0100, Markus Armbruster wrote:
>>>>>>> Stefan Hajnoczi <stefa...@gmail.com> writes:
>>>>>>>
>>>>>>>> On Wed, Jan 15, 2020 at 01:15:17PM +0100, Markus Armbruster wrote:
>>>>>>>>> Christophe de Dinechin <dinec...@redhat.com> writes:
>>>>>>>>>>> On 15 Jan 2020, at 10:20, Markus Armbruster <arm...@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++.
I’m serious.
>
> There are plenty of refactoring to do. The problem when touching the
> whole code-base, imho, is review time. It may take a couple of
> hours/days to come up with a cocci/spatch, and make various patches
> here and there. But it takes often weeks and a lot of constant push to
> various folks to get all the reviews (as seens by the qdev prop-ptr
> series earlier for example). How can we better address whole code-base
> changes?
>
>
>
> --
> Marc-André Lureau
>