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

From: Alexey Kardashevskiy <aik@ozlabs.ru>
To: Alexander Graf <agraf@suse.de>
Cc: Anthony Liguori <aliguori@us.ibm.com>,
	qemu-devel@nongnu.org, qemu-ppc@nongnu.org,
	Paolo Bonzini <pbonzini@redhat.com>,
	Paul Mackerras <paulus@samba.org>,
	David Gibson <david@gibson.dropbear.id.au>
Subject: Re: [Qemu-devel] [PATCH 02/17] pseries: rework XICS
Date: Tue, 02 Jul 2013 12:08:39 +1000	[thread overview]
Message-ID: <51D23627.5090308@ozlabs.ru> (raw)
In-Reply-To: <6A17A370-6FC7-4912-8B9E-9E51280F3BAB@suse.de>

On 07/02/2013 10:21 AM, Alexander Graf wrote:
> 
> 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 can be supported at no cost so next time I'll post it with
matched vmsd.

>>> 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.

-- 
Alexey