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Date: Tue, 05 Nov 2013 10:52:48 +0100
From: Paolo Bonzini <pbonzini@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH] spapr: add "compat" machine option
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To: Alexander Graf <agraf@suse.de>
Cc: Nikunj A Dadhania <nikunj@linux.vnet.ibm.com>, Alexey Kardashevskiy <aik@ozlabs.ru>, QEMU Developers <qemu-devel@nongnu.org>, "list@suse.de:PReP" <qemu-ppc@nongnu.org>, Paul Mackerras <paulus@samba.org>, =?ISO-8859-1?Q?Andreas_?= =?ISO-8859-1?Q?F=E4rber?= <afaerber@suse.de>

Il 05/11/2013 10:16, Alexander Graf ha scritto:
> 
> On 05.11.2013, at 10:06, Paolo Bonzini <pbonzini@redhat.com> wrote:
> 
>> Il 30/09/2013 14:57, Alexey Kardashevskiy ha scritto:
>>>>> Why is the option under -machine instead of -cpu?
>>> Because it is still the same CPU and the guest will still read the real
>>> PVR from the hardware (which it may not support but this is why we need
>>> compatibility mode).
>>
>> How do you support migration from a newer to an older CPU then?  I think
>> the guest should never see anything about the hardware CPU model.
> 
> POWER can't model that. It always leaks the host CPU information into the guest. It's the guest kernel's responsibility to not expose that change to user space.
> 
> Yes, it's broken :). I'm not even sure there is any sensible way to do live migration between different CPU types.

Still in my opinion it should be "-cpu", not "-machine".  Even if it's
just a "virtual" CPU model.

Paolo