Re: [Qemu-devel] [PATCH v5 06/16] apic: Introduce backend/frontend infrastructure for KVM reuse

[Anthony Liguori]

On 12/20/2011 07:51 AM, Paolo Bonzini wrote:
> On 12/20/2011 02:41 PM, Anthony Liguori wrote:
> > On 12/20/2011 03:56 AM, Avi Kivity wrote:
> > > On 12/20/2011 02:38 AM, Anthony Liguori wrote:
> > > > > That was v1 of my patches. Avi didn't like it, I tried it like this,
> > > > > and
> > > > > in the end I had to agree. So, no, I don't think we want such a model.
> > > >
> > > >
> > > > Yes, we do :-)
> > > >
> > > > The in-kernel APIC is a different implementation of the APIC device.
> > > > It's not an "accelerator" for the userspace APIC.
> > >
> > > A different implementation but not a different device. Device == spec.
> >
> > If it was hardware, it'd be a fully compatible clone. The way we would
> > model this is via inheritance.
>
> I see your fully compatible clone, and I raise my bridge with a different
> implementation underneath. It's the same old debate on is-a vs has-a.
>
> In QOM parlance Jan implemented this:
>
> abstract class Object
> abstract class Device
> class APIC: { backend: link<APICBackend> }
> abstract class APICBackend
> class QEMU_APICBackend
> class KVM_APICBackend

I don't fundamentally object to modeling it like this provided that it's modeled 
(and visible) through qdev and not done through a one-off infrastructure.

But yes, you are exactly correct in your observation (and that both can be 
right).

Regards,

Anthony Liguori

> and you're proposing this:
>
> abstract class Object
> abstract class Device
> abstract class APIC
> class QEMU_APIC
> class KVM_APIC
>
> Both can be right, both can be wrong.
>
> Paolo