From mboxrd@z Thu Jan  1 00:00:00 1970
Received: from eggs.gnu.org ([140.186.70.92]:43586)
	by lists.gnu.org with esmtp (Exim 4.71)
	(envelope-from <anthony@codemonkey.ws>) id 1Rd08z-0007Zp-Lj
	for qemu-devel@nongnu.org; Tue, 20 Dec 2011 08:54:22 -0500
Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71)
	(envelope-from <anthony@codemonkey.ws>) id 1Rd08u-0000wR-Ef
	for qemu-devel@nongnu.org; Tue, 20 Dec 2011 08:54:13 -0500
Received: from mail-yx0-f173.google.com ([209.85.213.173]:38044)
	by eggs.gnu.org with esmtp (Exim 4.71)
	(envelope-from <anthony@codemonkey.ws>) id 1Rd08u-0000wI-Ao
	for qemu-devel@nongnu.org; Tue, 20 Dec 2011 08:54:08 -0500
Received: by yenm6 with SMTP id m6so4710259yen.4
	for <qemu-devel@nongnu.org>; Tue, 20 Dec 2011 05:54:07 -0800 (PST)
Message-ID: <4EF0937D.3090207@codemonkey.ws>
Date: Tue, 20 Dec 2011 07:54:05 -0600
From: Anthony Liguori <anthony@codemonkey.ws>
MIME-Version: 1.0
References: <cover.1323952402.git.jan.kiszka@siemens.com>	<c7844d420d29d281ec17416742cb18b464351d6a.1323952403.git.jan.kiszka@siemens.com>	<4EEFB72E.7030508@codemonkey.ws>	<4EEFC970.9030205@web.de>	<4EEFD69F.6080700@codemonkey.ws>	<4EEFD786.8030609@web.de>	<4EEFD90A.1000204@codemonkey.ws>	<4EF05BC4.8010905@redhat.com>
	<4EF09078.2030508@codemonkey.ws> <4EF092D2.6080009@redhat.com>
In-Reply-To: <4EF092D2.6080009@redhat.com>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Content-Transfer-Encoding: 7bit
Subject: Re: [Qemu-devel] [PATCH v5 06/16] apic: Introduce backend/frontend
 infrastructure for KVM reuse
List-Id: <qemu-devel.nongnu.org>
List-Unsubscribe: <https://lists.nongnu.org/mailman/options/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=unsubscribe>
List-Archive: <http://lists.nongnu.org/archive/html/qemu-devel>
List-Post: <mailto:qemu-devel@nongnu.org>
List-Help: <mailto:qemu-devel-request@nongnu.org?subject=help>
List-Subscribe: <https://lists.nongnu.org/mailman/listinfo/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=subscribe>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: 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 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
>
>