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

From: Eduardo Habkost
Subject: Re: [Qemu-devel] [qom-cpu PATCH 2/2] i386: disable PMU passthrough mode by default
Date: Wed, 24 Jul 2013 10:15:33 -0300
User-agent: Mutt/1.5.21 (2010-09-15)

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.

(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.)

> 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.


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