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

[Anthony Liguori <anthony@codemonkey.ws>]

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
>
>