Re: [PATCH v7 17/65] qom: add object_{class_}property_add_qapi_enum

[Marc-André Lureau <marcandre.lureau@xxxxxxxxxx>]

Hi

On Fri, Oct 30, 2026 at 2:02 PM Markus Armbruster <armbru@xxxxxxxxxx> wrote:
>
> Marc-André Lureau <marcandre.lureau@xxxxxxxxxx> writes:
>
> > Signed-off-by: Markus Armbruster <armbru@xxxxxxxxxx>
> > Signed-off-by: Marc-André Lureau <marcandre.lureau@xxxxxxxxxx>
> > ---
> >  include/qom/object.h | 74 ++++++++++++++++++++++++++++++++++++++++++
> >  qom/object.c         | 90 
> > +++++++++++++++++++++++++++++++++++++++++++++++++---
> >  2 files changed, 159 insertions(+), 5 deletions(-)
> >
> > diff --git a/include/qom/object.h b/include/qom/object.h
> > index eb61d8842b80..e04be4caed57 100644
> > --- a/include/qom/object.h
> > +++ b/include/qom/object.h
> > @@ -2027,6 +2027,80 @@ ObjectProperty 
> > *object_class_property_add_enum(ObjectClass *klass,
> >                                      int (*get)(Object *, Error **),
> >                                      void (*set)(Object *, int, Error **));
> >
> > +/**
> > + * struct QapiEnumProp: Descriptor for a property backed by a QAPI enum 
> > type
> > + *
> > + * Binds a QOM object property to a QAPI enum, providing automatic
> > + * string<->int conversion through QAPI visitors and optional
> > + * default-value initialization during object instance init.
> > + *
> > + * Use the QAPI_ENUM_PROP() macro to construct instances inline.
> > + *
> > + * @name: property name exposed on the QOM object
> > + * @description: human-readable description (shown in ``-device help``, 
> > etc.)
> > + * @default_value: initial enum value applied via @set during instance 
> > init,
> > + *                 or -1 (the QAPI_ENUM_PROP default) to skip 
> > initialization
> > + * @qapi_type: pointer to the generated QAPITypeInfo for the enum
> > + *             (provides the string<->int lookup table)
> > + * @get: getter that returns the current enum value as int, or -1 on error
> > + * @set: setter that receives the enum value as int; %NULL for read-only 
> > props
> > + */
> > +typedef struct QapiEnumProp {
> > +    const char *name;
> > +    const char *description;
> > +    const int default_value;
> > +    const QAPITypeInfo *qapi_type;
> > +    int (*get)(Object *, Error **);
> > +    void (*set)(Object *, int, Error **);
> > +} QapiEnumProp;
> > +
> > +#define QAPI_ENUM_PROP(...) ({                                           \
> > +    static const QapiEnumProp _prop = {                                  \
> > +        .default_value = -1, __VA_ARGS__                                 \
> > +    };                                                                   \
> > +    &_prop; })
>
> This is a cute and reasonably well-known macro trick for optional
> keyword arguments in C.
>
> The example [*] below specifies all arguments.  What happens when we
> omit any?  They default to zero via default initializaion, except for
> .default_value, which defaults to -1.
>
> Trap for developers: some arguments aren't actually optional!  If you
> forget to specify @name or @qapi_type, the code compiles, but crashes at
> run time.
>
> Not a fan.

Agree (not a fan of C either). We could probably add more
static/runtime checks. Or clearly state that there is room for
improvements, or just drop it altogether - either as follow up or with
the series. I leave that up to you since you are going to respin it.
thanks!



>
> > +
> > +/**
> > + * object_property_add_qapi_enum:
> > + * @obj: the object to add a property to
> > + * @prop: property descriptor
> > + *
> > + * Add an enum property with QAPI type association.
> > + *
> > + * Returns: The newly added property on success, or %NULL on failure.
> > + */
> > +ObjectProperty *
> > +object_property_add_qapi_enum(Object *obj, const QapiEnumProp *prop);
> > +
> > +/**
> > + * object_class_property_add_qapi_enum:
> > + * @klass: the class object to add a property to
> > + * @prop: property descriptor
> > + *
> > + * Add an enum property with QAPI type association.
> > + *
> > + * Use the QAPI_ENUM_PROP() macro to construct the property descriptor
> > + * inline. If .default_value is not set, the property is not initialized
> > + * (default_value is -1). Otherwise, the setter is called with
> > + * default_value during object instance init.
> > + *
> > + * Example::
>
> [*]
>
> > + *
> > + *   object_class_property_add_qapi_enum(oc, QAPI_ENUM_PROP(
> > + *       .name = "policy",
> > + *       .description = "Set the NUMA policy",
> > + *       .default_value = HOST_MEM_POLICY_DEFAULT,
> > + *       .qapi_type = &HostMemPolicy_type_info,
> > + *       .get = my_get_policy,
> > + *       .set = my_set_policy,
> > + *   ));
> > + *
> > + * Returns: The newly added property on success, or %NULL on failure.
> > + */
> > +ObjectProperty *
> > +object_class_property_add_qapi_enum(ObjectClass *klass,
> > +                                    const QapiEnumProp *prop);
> > +
> >  /**
> >   * object_property_add_qapi:
> >   * @obj: the object to add a property to
>
> [...]
>