On Mon, Nov 09, 2020 at 06:33:04PM +0100, Paolo Bonzini wrote:
> On 09/11/20 18:16, Eduardo Habkost wrote:
> > On Mon, Nov 09, 2020 at 05:34:01PM +0100, Paolo Bonzini wrote:
> > > On 09/11/20 16:21, Eduardo Habkost wrote:
> > > > Nothing prevents us from describing those properties inside the
> > > > same property array.
> > >
> > > Do you mean adding PropertyInfos for them? Adding a once-only
> > > PropertyInfo
> > > is worse than writing a custom getter/setter pair, because:
> > >
> > > - without (DEFINE_)PROP_* you lose the type safety.
> > >
> > > - with (DEFINE_)PROP_* you have much more boilerplate to write
> >
> > I mean extending the API to let custom setters and getters appear
> > on the Property array, not using the existing API.
>
> That seems like conflicting goals. The field property API is based on
> getters and setters hidden in PropertyInfo. The "other" property API is
> based on getters and setters in plain sight in the declaration of the
> property.
There's nothing that prevents a
void object_class_add_properties(oc, Property *props);
function from supporting both.
>
> > > > > > I think having different ways for different things (class vs.
> > > > > > object) is
> > > > > > better than having different ways for the same things (class in
> > > > > > qdev vs.
> > > > > > class in non-qdev).
> > > > >
> > > > > Right, but qdev's DEFINE_PROP_STRING would be easy to change to
> > > > > something
> > > > > like
> > > > >
> > > > > - DEFINE_PROP_STRING("name", ...),
> > > > > + device_class_add_field_property(dc, "name", PROP_STRING(...));
> > > >
> > > > I'm not worried about this direction of conversion (which is
> > > > easy). I'm worried about the function call => QAPI schema
> > > > conversion. Function calls are too flexible and requires parsing
> > > > and executing C code.
> > >
> > > Converting DEFINE_PROP_STRING to a schema also requires parsing C code,
> > > since you can have handwritten Property literals (especially for custom
> > > PropertyInfo). Converting DEFINE_PROP_STRING it also requires matching
> > > the
> > > array against calls to object_class_add_field_properties (which could be
> > > hidden behind helpers such as device_class_set_props). (Plus matching
> > > class_init functions against TypeInfo).
> >
> > Parsing an array containing a handful of macros (a tiny subset of
> > C) isn't even comparable to parsing and executing C code where
> > object*_property_add*() calls can be buried deep in many levels
> > of C function calls (which may or may not be conditional).
>
> Finding the array would also require finding calls buried deep in C code,
> wouldn't they?
Yes, but I don't expect this to happen if the API doesn't
encourage that.
>
> > (Also, I don't think we should allow handwritten Property literals.)
>
> How would you do custom setters and getters then---without separate
> PropertyInfos, without Property literals, and without an exploding number of
> macros?
Property with struct field:
/* We call this DEFINE_PROP_UINT32 today. We can keep the
* existing name just to reduce churn.
*/
DEFINE_PROP_UINT32_FIELD("myproperty", MyState, my_field)
Prop with struct field but custom setter:
DEFINE_PROP_UINT32_FIELD("myproperty", MyState, my_field,
.custom_setter = my_custom_setter)
Prop with no struct field, and custom setter/getter:
DEFINE_PROP("myproperty", prop_type_uint32,
.custom_getter = my_getter,
.custom_setter = my_setter)
Definitions for above:
#define DEFINE_PROP(_name, _typeinfo, ...) \
{ .name = _name,
.info = &_typeinfo,
__VA_ARGS__
}
#define DEFINE_FIELD_PROP(name, typeinfo, type, state, field, ...) \
DEFINE_PROP(name, typeinfo,
.offset = offsetof(state, field) +
type_check(typeof_field(state, field), type),
__VA_ARGS__)
#define DEFINE_PROP_UINT32_FIELD(name, state, field, ...) \
DEFINE_FIELD_PROP(name, prop_type_uint32, uint32_t, state, field,
__VA_ARGS__)
Alternative DEFINE_FIELD_PROP definition if we implement some
macro magic to declare the expected type for each typeinfo
variable:
/* Will make ACTUAL_C_TYPE(prop_type_uint32) expand to uint32_t */
DECLARE_QOM_TYPE(prop_type_uint32, uint32_t)
/* Will make ACTUAL_C_TYPE(prop_type_uint64) expand to uint64_t)
DECLARE_QOM_TYPE(prop_type_uint64, uint64_t)
#define DEFINE_FIELD_PROP(name, typeinfo, state, field, ...) \
DEFINE_PROP(name, typeinfo,
.offset = offsetof(state, field) +
type_check(typeof_field(state, field),
ACTUAL_C_TYPE(typeinfo)),
__VA_ARGS__)
>
> > > So, you don't save any parsing by using arrays. (In fact I would probably
> > > skip the parsing, and use your suggestion of *executing* C code: write the
> > > QAPI schema generator in C, link into QEMU and run it just once to
> > > generate
> > > the QOM schema).
> >
> > If we do that with the existing code, we can't be sure the
> > generated schema doesn't depend on configure flags or run time
> > checks inside class_init.
>
> We can use grep or Coccinelle or manual code review to identify problematic
> cases.
We can, but I believe it is better and simpler to have an API
that enforces (or at least encourages) this.
>
> > Even locating the cases where this is
> > happening is being a challenge because the API is too flexible.
> >
> > However, if we require the property list to be always evaluated
> > at compile time, we can be sure that this method will be
> > reliable.
> >
> > > QOM has been using function calls for many years, are there any cases of
> > > misuse of that flexibility that you have in mind? I can only think of two
> > > *uses*, in fact. One is eepro100_register_types is the only case I can
> > > remember where types are registered dynamically. The other is S390 CPU
> > > features. [...]
> >
> > The list of tricky dynamic properties is large and I don't think
> > we even found all cases yet. John documented many of them here:
> >
> > https://gitlab.com/jsnow/qemu/-/blob/cli_audit/docs/cli_audit.md
> >
> > Look, for example, for the sections named "Features" for CPU
> > options.
>
> Yes, I'm only considering object_class_property calls. Those are the ones
> that I claim aren't being misused enough for this to be a problem.
>
instance-level properties are where most of the complexity was
introduced because the class API didn't exist yet. I don't think
we should ignore them, or we risk having the same issues when
converting them to class properties.
> Making instance-level properties appear in the schema is a completely
> different kind of conversion, because there is plenty of manual work (and
> unsolved problems for e.g. subobject property aliases).
I'd like us to convert instance-level properties to an API that
is easy to use and where the same problems won't happen again.
>
> > You are also ignoring the complexity of the code path that leads
> > to the object*_property_add*() calls, which is the main problem
> > on most cases.
>
> I would like an example of the complexity of those code paths. I don't see
> much complexity, as long as the object exists at all, and I don't see how it
> would be simpler to find the code paths that lead to
> object_class_add_field_properties.
Possibly the most complex case is x86_cpu_register_bit_prop().
The qdev_property_add_static() calls at arm_cpu_post_init() are
tricky too.
If object*_property_add*() is hidden behind a function call or a
`if` statement, it's already too much complexity to me. I don't
want us to need a second audit like the one John made when we
decide to represent QOM class properties in a QAPI schema.
--
Eduardo
