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

[Alexander Graf]

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.


Alex