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Date: Wed, 04 Sep 2013 22:59:38 +1000
From: Alexey Kardashevskiy <aik@ozlabs.ru>
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Subject: Re: [Qemu-devel] [RFC PATCH] spapr: add initial ibm,
 client-architecture-support rtas call support
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To: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Nikunj A Dadhania <nikunj@linux.vnet.ibm.com>, Alexander Graf <agraf@suse.de>, qemu-devel@nongnu.org, qemu-ppc@nongnu.org, Paul Mackerras <paulus@samba.org>, =?UTF-8?B?QW5kcmVhcyBGw6RyYmVy?= <afaerber@suse.de>

On 09/04/2013 09:54 PM, Benjamin Herrenschmidt wrote:
> On Wed, 2013-09-04 at 21:40 +1000, Alexey Kardashevskiy wrote:
>> One of the examples (came from Paul) is:
>> the host runs on POWER8, the guest boots a kernel which is capable of
>> POWER7 only + POWER7-compat. We do this reboot tweak and boot in
>> POWER7-compat mode. Then the guest does "yum update" and gets POWER8
>> kernel
>> installed so when it reboots, it will boot in normal POWER8 mode.
>> Everybody
>> is happy.
>>
>> Having POWER7-compat mode set from the very beginning will break this
>> behaviour.
> 
> But it will allow migrating that partition to a P7 machine. It's
> important to be able to specify the compat mode for that reason, to
> create guests that can be migrated to older machines.

I am not saying we do not need this feature, I am saying we do not always
want it.


-- 
Alexey