Re: [Qemu-devel] [PATCH] target-ppc: Add POWER8E_v2.1 CPU model.

[Alexey Kardashevskiy <aik@ozlabs.ru>]

On 07/10/2015 07:19 PM, Andrea Bolognani wrote:
> On Fri, 2015-07-10 at 15:19 +1000, Alexey Kardashevskiy wrote:
>>
>>> If I'm reading the kernel source[1] correctly, there are actually
>>> subtle
>>> differences other than the number of cores:
>>>
>>>     #define CPU_FTRS_POWER8 (/* Bunch of features here */)
>>>     #define CPU_FTRS_POWER8E (CPU_FTRS_POWER8 | CPU_FTR_PMAO_BUG)
>>
>> POWER8E was the first family of POWER8. Some revisions of POWER8E
>> have this
>> bug. The bug is that if we migrate from a good POWER8 to POWER8 with
>> this
>> bug, perf counters will stop working as the source kernel did not
>> detect
>> broken CPU and did not enable a workaround. We do not really know how
>> many
>> of this chips are out there (not many I believe) and how many have
>> this
>> bug. And also I guess that more people will be annoyed by inability
>> to
>> migrate rather than by broken perf.
>>
>> I'd leave migration enabled but it is worth mentioning somewhere in
>> user's
>> guide that if the user migrates a guest to POWER8E with the specific
>> PVR,
>> perf might break.
>
> David said we might be able to just apply the kernel PMAO workaround
> unconditionally, which would of course be great. Even if that turns out
> not to be possible, I agree with you that allowing migration is more
> important than not breaking performance monitoring, and documenting
> this properly should be enough.
>
>>>     #define CPU_FTRS_POWER8_DD1 (CPU_FTRS_POWER8 & ~CPU_FTR_DBELL)
>>
>> This bug appears in POWER8E and POWER8 1.0 ("DD1"). The bug is that
>> when a
>> CPU thread wakes up after nap because of doorbell message, the
>> doorbell
>> interrupt is not pending. Guests cannot do nap themselves (they use
>> H_CEDE
>> hypercall for this), when a thread wakes up, it wakes in the host
>> context
>> so there is nothing to handle in the guest, it is purely for KVM to
>> workaround.
>
> Great news!
>
>>> So the PVR matching, as done currently in QEMU, will include
>>> POWER8DD1
>>> but exclude POWER8NVL, which according to to commit ddee09c0 and
>>> the
>>> code
>>> above is absolutely identical to POWER8.
>>
>> Yeah, we have to add 0x004c0000 into the POWER8 family as well, I'll
>> doublecheck.
>
> Okay. libvirt will soon advertise just POWER8 instead of specific
> models, and will consider the POWER8 guest CPU model to be compatible
> with any host having a POWER8* CPU, using the same PVR values and
> masks as the kernel. Once QEMU is updated to include POWER8NVL, the
> behavior will be consistent across the stack.


Correct.

> Just to be sure: the same applies to POWER7 as well, right? It
> certainly looks so from both the kernel and QEMU code.

Right.


> Thanks a lot for all the precious bits of information you've provided.

I've learnt a lot too :)


-- 
Alexey