Re: [Qemu-devel] [RFC 0/4] QOM class properties - do we need them?

[Daniel P. Berrange]

On Thu, Sep 29, 2016 at 12:12:32PM +0200, Andreas Färber wrote:
> Am 29.09.2016 um 10:14 schrieb Daniel P. Berrange:
> > On Thu, Sep 29, 2016 at 10:16:41AM +1000, David Gibson wrote:
> >> QOM has the concept of both "object class" properties and "object
> >> instance" properties.
> >>
> >> The accessor functions installed for the rarely-used class properties
> >> still take an Object *, so the *value* of such properties is still
> >> per-instance; it's just the *existence* (and type) of the property
> >> that is per-class.
> > 
> > Yes, of course. This is the whole point of class properties. It avoids
> > allocating the same ObjectProperty struct against every object instance
> > which wastes massive amounts of memory in scenarios where there are lots
> > of instances created.
> 
> +1
> 
> >> Of course, that's also true in practice for the great majority of
> >> "instance" properties, because they're created identically and
> >> unconditionally for every instance from the per-class instance_init
> >> hook.
> >>
> >> This also means that the (unused) object_class_property_add_*_ptr()
> >> functions don't make a lot of sense, since they require a fixed
> >> pointer which means the value of such a property would only be
> >> per-class.
> >>
> >> Given that, is there really any value to supporting the "class"
> >> properties in addition to the "instance" properties?  This series is
> >> an RFC which removes all support for class properties, changing the
> >> few existing users to instance properties instead.
> >>
> >> Alternatively, if we *don't* want to remove class properties, should
> >> we instead be trying to convert the many, many "instance" properties
> >> whose existence is actually per-class to be class properties?
> > 
> > Practically all instances properties should become class properties
> > as its going to save wasting memory once most are converted.
> 
> Not all, but most. child<> properties were the reason to have properties
> on the instance.

That's why I said "Practically all", instead of just "all" :-)

Regards,
Daniel
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