Re: [Qemu-devel] [PATCH 27/28] sysbus: apic: ioapic: convert to QEMU Object Model

[Anthony Liguori]

On 01/24/2012 03:31 PM, Jan Kiszka wrote:
> On 2012-01-24 22:11, Anthony Liguori wrote:
> > On 01/24/2012 03:01 PM, Jan Kiszka wrote:
> > > On 2012-01-24 21:21, Anthony Liguori wrote:
> > > > > Also, I see a lot of programmatic initialization and a lot of repeating
> > > > > patterns (specifically regarding trivial class initialization) - there
> > > > > is no better alternative?
> > > >
> > > > Not really, no.  It looks bad now because you have DeviceInfo still.
> > > > Once DeviceInfo goes away, all of the initialization will happen in the
> > > > class_init function.
> > > >
> > > > The design of QOM is such that a lot of what was previously done via
> > > > declarative structures is now done imperatively.  But the code bloat
> > > > that came in this patch series will decrease significantly with the next
> > > > series as we eliminate DeviceInfo.
> > >
> > > Are there examples of fully converted devices to get an impression?
> >
> > https://github.com/aliguori/qemu/tree/qom-rebase.8
> >
> > Has everything fully converted (including BusState).
> >
> > If you look at qdev.[ch], you'll notice the remaining qdev
> > infrastructure becomes greatly simplified.
>
> But I don't get yet why all these repeating initialization tasks need to
> be open-coded instead of remaining declarative.

It would look like:

static void device_generic_class_init(ObjectClass *klass, void *data)
{
    DeviceClass *dc = DEVICE_CLASS(klass);
    DeviceTypeInfo *ti = data;

    if (ti->reset) {
       dc->reset = ti->reset;
    }
    if (ti->vmsd) {
       dc->vmsd = ti->vmsd;
    }
}

void device_type_register_static(DeviceTypeInfo *ti)
{
    TypeInfo ti = {
      .class_init = device_generic_class_init,
      .class_data = ti,
      // ...
    }
    type_register_static(&ti);
}

But I don't like this.  The problem is that the declarative syntax we have 
doesn't distinguish between "not-specified" and "zero-initialized".  It's also 
tempting to just drop the if (ti->reset) check but that means that you 
unconditionally override the base class implementation even if it's not specified.

I don't see any tangible benefits to a declarative syntax except that it makes 
it harder to get right because you have to write per-base class initialization 
functions and it's very easy to get polymorphism wrong there.

We're not talking about a code size difference.  It's a wash in terms of number 
of lines of code.

Imperative allows you to explicit zero-initialize, accept the previous version 
(from the base class), or override with a new value.

Regards,

Anthony Liguori

> Jan