Re: [Qemu-devel] [qom-cpu PATCH 2/2] i386: disable PMU passthrough mode by default

[Paolo Bonzini]

Il 24/07/2013 15:15, Eduardo Habkost ha scritto:
> On Tue, Jul 23, 2013 at 09:43:06PM +0200, Paolo Bonzini wrote:
>> Il 23/07/2013 19:41, Eduardo Habkost ha scritto:
>>> On Tue, Jul 23, 2013 at 06:23:08PM +0200, Paolo Bonzini wrote:
>>>> Il 23/07/2013 17:40, Eduardo Habkost ha scritto:
>>>>> On Tue, Jul 23, 2013 at 05:09:02PM +0200, Paolo Bonzini wrote:
>>>>>> Il 23/07/2013 16:13, Eduardo Habkost ha scritto:
>>>>>>> On Tue, Jul 23, 2013 at 11:18:03AM +0200, Paolo Bonzini wrote:
>>>>>>>> Il 22/07/2013 21:25, Eduardo Habkost ha scritto:
>>>>>>>>> Bug description: QEMU currently gets all bits from GET_SUPPORTED_CPUID
>>>>>>>>> for CPUID leaf 0xA and passes them directly to the guest. This makes
>>>>>>>>> the guest ABI depend on host kernel and host CPU capabilities, and
>>>>>>>>> breaks live migration if we migrate between host with different
>>>>>>>>> capabilities (e.g. different number of PMU counters).
>>>>>>>>>
>>>>>>>>> This patch adds a "pmu-passthrough" property to X86CPU, and set it to
>>>>>>>>> true only on "-cpu host", or on pc-*-1.5 and older machine-types.
>>>>>>>>
>>>>>>>> Can we just call the property "pmu"?  It doesn't have to be passthough.
>>>>>>>
>>>>>>> Yes, but the only options we have today are "no PMU" and "passthrough
>>>>>>> PMU". I wouldn't like to make "pmu=on" enable the passthrough behavior
>>>>>>> implicitly (I don't want things that break live-migration to be enabled
>>>>>>> without making it explicit that it is a host-dependent/passthrough
>>>>>>> mode).
>>>>>>
>>>>>> I think "passthrough PMU" should be considered a bug except of course
>>>>>> with "-cpu host".
>>>>>>
>>>>>> If "-cpu Nehalem,pmu=on" goes from passthrough to Nehalem-compatible in
>>>>>> a future QEMU release, that'll be a bugfix.
>>>>>
>>>>> Exactly. But then I don't understand your suggestion. We still need a
>>>>> property to enable pasthrough behavior on old machine-types (not
>>>>> perfect, but a best-effort way to try to keep compatibility),
>>>>
>>>> Do we?
>>>>
>>>> We only need "pmu=on"---which right now is buggy on old machine types
>>>> because it will always passthrough.
>>>
>>> I am not sure I understand what you are arguing for.
>>>
>>> You agree that pmu=on needs to keep the buggy passthrough behavior on
>>> pc-1.5 and older, right?
>>
>> I agree it needs to remain enabled on 1.5.  But if, for example, 1.8
>> makes pmu=on emulate a Nehalem-compatible PMU, I think it is fine if
>> pc-1.5 moves from a host-compatible PMU to a Nehalem-compatible PMU.
> 
> That's where I disagree. Today users are (luckily) able to migrate
> safely between hosts with the same number of PMU counters. But if we
> make, e.g., "qemu-1.6 -machine pc-1.5 -cpu Westmere" present a smaller
> number of PMU counters than "qemu-1.5 -machine pc-1.5 -cpu Westmere" on
> the same host, we will break an existing setup where everything was
> working before, which is something we could have easily avoided.

But at the same time we will fix live migration from a Sandy Bridge host
to a Westmere.  So it's a choice we have to make anyway.

> (Just to clarify what breaking this means in practice: changing the
> number of PMU counters under the guest on live-migration means the guest
> will crash when trying to use counters that suddenly went away, and it
> may crash a very long time after it was migrated.)

And at the same time we fix live migration of a Sandy Bridge to a Westmere.

>> The reason is that pc-1.5 has never guaranteed any feature of the
>> emulated PMU.
> 
> Right, current behavior is buggy and we never guaranteed anything, but
> IMO we shouldn't break on purpose something that is working today.

Even if it is to fix something else?

Paolo