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

From: Paolo Bonzini
Subject: Re: [Qemu-devel] [qom-cpu PATCH 2/2] i386: disable PMU passthrough mode by default
Date: Tue, 23 Jul 2013 21:43:06 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130625 Thunderbird/17.0.7

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.

The reason is that pc-1.5 has never guaranteed any feature of the
emulated PMU.


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