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

[David Gibson]

On Thu, Sep 29, 2016 at 09:14:16AM +0100, Daniel P. Berrange wrote:
> 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.

Ah, that makes sense.

> > 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.

Heh, ok.  Well, I'll keep that in mind when I'm adding properties in
future.  I wonder if there's a way we can better get the word out that
this is how properties should usually be done.

That said.. I'm still thinking we should remove
object_class_property_add_*_ptr().  Those take an actual pointer to
the value, meaning that it can't have different values per-instance.
These only create read-only properties, so they're not actually
dangerous, but they really don't seem very useful.

-- 
David Gibson                    | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au  | minimalist, thank you.  NOT _the_ _other_
                                | _way_ _around_!
http://www.ozlabs.org/~dgibson

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