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

[Paolo Bonzini]

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

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