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Re: [Qemu-devel] [PATCH v5 06/16] apic: Introduce backend/frontend infra

From: Anthony Liguori
Subject: Re: [Qemu-devel] [PATCH v5 06/16] apic: Introduce backend/frontend infrastructure for KVM reuse
Date: Tue, 20 Dec 2011 07:54:05 -0600
User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv: Gecko/20110922 Lightning/1.0b2 Thunderbird/3.1.15

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,
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 


Anthony Liguori

and you're proposing this:

abstract class Object
abstract class Device
abstract class APIC
class KVM_APIC

Both can be right, both can be wrong.


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