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

[Alexey Kardashevskiy <aik@ozlabs.ru>]

On 30.09.2013 21:52, Paolo Bonzini wrote:
> Il 30/09/2013 13:25, Alexander Graf ha scritto:
>> On 27.09.2013, at 10:06, Alexey Kardashevskiy wrote:
>>
>>>> To be able to boot on newer hardware that the software support,
>>>> PowerISA defines a logical PVR, one per every PowerISA specification
>>>> version from 2.04.
>>>>
>>>> This adds the "compat" option which takes values 205 or 206 and forces
>>>> QEMU to boot the guest with a logical PVR (CPU_POWERPC_LOGICAL_2_05 or
>>>> CPU_POWERPC_LOGICAL_2_06).
>>>>
>>>> The guest reads the logical PVR value from "cpu-version" property of
>>>> a CPU device node.
>>>>
> 
> 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).

It is possible to run QEMU with -cpu POWER8 with some old distro which
does not know about power8, it will switch to power6-compat mode, the
the user does "yum update" (or analog) and then the new kernel (with
power8 support) will do H_CAS again and this time "raw" power8 should be
selected.


-- 
With best regards

Alexey Kardashevskiy -- icq: 52150396