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

From: Paolo Bonzini
Subject: Re: [Qemu-devel] [PATCH v5 06/16] apic: Introduce backend/frontend infrastructure for KVM reuse
Date: Tue, 20 Dec 2011 14:51:14 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:7.0.1) Gecko/20110930 Thunderbird/7.0.1

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

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.


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