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

[Paolo Bonzini]

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.

Paolo