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Re: [Qemu-ppc] [PATCH 02/17] pseries: rework XICS

From: Alexander Graf
Subject: Re: [Qemu-ppc] [PATCH 02/17] pseries: rework XICS
Date: Tue, 2 Jul 2013 02:21:59 +0200

On 02.07.2013, at 02:06, David Gibson wrote:

> On Thu, Jun 27, 2013 at 10:17:19PM +1000, Alexey Kardashevskiy wrote:
>> On 06/27/2013 09:47 PM, David Gibson wrote:
>>> On Thu, Jun 27, 2013 at 04:45:45PM +1000, Alexey Kardashevskiy wrote:
>>>> Currently XICS interrupt controller is not a QEMU device. As we are going
>>>> to support in-kernel emulated XICS which is a part of KVM, it make
>>>> sense not to extend the existing XICS and have multiple KVM stub functions
>>>> but to create yet another device and share pieces between fully emulated
>>>> XICS and in-kernel XICS.
>>> Hmm.  So, I think changing the xics to the qdev/qom framework is a
>>> generally good idea.  But I'm not convinced its a good idea to have
>>> different devices for the kernel and non-kernel xics.
>> The idea came from Alex Graf, this is already done for openpic/openpic-kvm.
>> The normal practice is to move ioctls to KVM to KVM code and provide empty
>> stubs for non-KVM case. There were too many so having a separate xics-kvm
>> is kind of help.
>>> Won't that
>>> prevent migrating from a system with a kernel xics to one without, or
>>> vice versa?
>> Mmm. Do we care much about that?...
> Enough to avoid making it impossible by design.

We went that route with x86 too after lots of hassle trying to shoehorn the 
in-kernel APIC into the emulation device. It's more hassle than gain.

>> At the moment it is not supported that as VMStateDescription have different
>> .name for xics and xics-kvm but easy to fix. And we do not pass a device to
>> vmstate_register so that must be it.
> Ok, if you can make the ids in the vmsd match, then that should be ok.

I really just wouldn't bother too much about it. Sooner or later QEMU-XICS is 
going to be a legacy and debug only option.


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