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

["Andreas Färber" <afaerber@suse.de>]

Am 05.11.2013 10:52, schrieb Paolo Bonzini:
> 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.

PowerPC currently does not have -cpu option parsing. If you need to
implement it, I would ask for a generic hook in CPUClass set by
TYPE_POWERPC_CPU, so that the logic does not get hardcoded in cpu_init,
and for the p=v parsing logic to be so generic as to just set property p
to value v on the CPU instance. I.e. please make the compatibility
settings a static property or dynamic property of the CPU.

Maybe the parsing code could even live in generic qom/cpu.c, overridden
by x86/sparc and reused for arm? Somewhere down my to-do list but
patches appreciated...

Andreas

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