Re: [Qemu-devel] [PATCH] spapr: add "compat" machine option

[Paolo Bonzini]

Il 05/11/2013 11:27, Alexander Graf ha scritto:
> 
> On 05.11.2013, at 10:52, Paolo Bonzini <address@hidden> wrote:
> 
>> Il 05/11/2013 10:16, Alexander Graf ha scritto:
>>>
>>> On 05.11.2013, at 10:06, Paolo Bonzini <address@hidden> 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.
> 
> The only thing that this really changes is an SPR (MSR in x86 speech)
> on an existing cpu model. It's definitely not a new CPU type. If
> anything it'd be an option to an existing type.

Agreed.

Paolo