Re: [Qemu-devel] [RFC PATCH] powerpc: add PVR mask support

[Andreas Färber]

Am 15.08.2013 17:58, schrieb Alexander Graf:
> 
> On 15.08.2013, at 17:48, Andreas Färber wrote:
> 
>> Am 15.08.2013 17:30, schrieb Alexander Graf:
>>>
>>> On 15.08.2013, at 17:11, Andreas Färber wrote:
>>>
>>>> Am 15.08.2013 15:12, schrieb Anthony Liguori:
>>>>> Everyone is talking past each other and no one is addressing the real
>>>>> problem.  There are two distinct issues here:
>>>>>
>>>>> 1) We have two ABIs that cannot be changed unless there's a very good
>>>>>  reason to.  Alexey's original patch breaks both.  The guest ABI
>>>>>  cannot change given a fixed command line.
>>>>>
>>>>>  IOW, the exposed PVR value for -cpu POWER7 cannot change across
>>>>>  versions of QEMU or when running on different hardware.  This breaks
>>>>>  live migration and save/resume.
>>>>>
>>>>>  We also cannot break the command line interface.  If the last version
>>>>>  of QEMU supported -cpu POWER7_v2.1, then we must continue to support
>>>>>  that.
>>>>
>>>> 1a) How should -cpu 0xDEADBEEF or -cpu DEADBEEF behave.
>>>>
>>>>   I expect it to error out as before
>>>>   rather than applying the same fuzz/mask that -cpu host might.
>>>
>>> I actually think it'd make sense to apply the same fuzz/mask, don't you 
>>> think?
>>
>> I think "-cpu host" has the semantics of give-me-what-the-host-has. But
>> -cpu 0xDEADBEEF is asking for PVR DEADBEEF and having it silently return
>> a guest-visible DEADBEBE is going to be undesired.
> 
> -cpu host on 0xDEADBEEF should give us a 0xDEADBEEF cpu. -cpu 0xDEADBEEF 
> should give us a 0xDEADBEEF cpu :).

Then we mustn't tweak translate_init.c:cpu_class_by_pvr() to return
deviating results! Which is what the change to
ppc_cpu_compare_class_pvr() is essentially resulting in if I am not
completely off track. And therefore my calling to handle this at a
higher level (KVM init), where the user's intentions are clear, rather
than to blur our internal API. Otherwise the _by_pvr() function would
need to create a new class or modify an existing one when the function
can't know what the function call was actually intended for.

Andreas

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