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

[Alexander Graf]

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.


Alex