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Date: Tue, 20 Dec 2011 14:51:14 +0100
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Subject: Re: [Qemu-devel] [PATCH v5 06/16] apic: Introduce backend/frontend
 infrastructure for KVM reuse
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To: Anthony Liguori <anthony@codemonkey.ws>
Cc: Anthony Liguori <aliguori@us.ibm.com>, kvm@vger.kernel.org, "Michael S. Tsirkin" <mst@redhat.com>, Marcelo Tosatti <mtosatti@redhat.com>, qemu-devel <qemu-devel@nongnu.org>, Blue Swirl <blauwirbel@gmail.com>, Jan Kiszka <jan.kiszka@web.de>, Avi Kivity <avi@redhat.com>

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