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

[Paolo Bonzini <pbonzini@redhat.com>]

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.

Paolo

> and I
> named that option "pmu-passthrough". How do you think we should name it?