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

[Eduardo Habkost]

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?

In that case, how do you suggest I let QEMU know that only pc-1.5 and
older should have the buggy behavior enabled when pmu=on? I understand
that compat_props is the appropriate place for that, and that's why I
need a "please-enable-the-old-buggy-pmu-passthrough-behavior" property
that I can add to PC_COMPAT_1_5.

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

-- 
Eduardo