linux-kernel - Re: intel_pstate divide error with v3.13-rc4-256-gb7000ad

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Date:	Mon, 06 Jan 2014 12:37:19 +0100
From:	"Rafael J. Wysocki" <rjw@...ysocki.net>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	Gleb Natapov <gleb@...antech.com>,
	Dirk Brandewie <dirk.brandewie@...il.com>,
	Kashyap Chamarthy <kchamart@...hat.com>,
	Josh Boyer <jwboyer@...oraproject.org>,
	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	"cpufreq@...r.kernel.org" <cpufreq@...r.kernel.org>,
	Linux PM list <linux-pm@...r.kernel.org>,
	"Linux-Kernel@...r. Kernel. Org" <linux-kernel@...r.kernel.org>,
	"Richard W.M. Jones" <rjones@...hat.com>
Subject: Re: intel_pstate divide error with v3.13-rc4-256-gb7000ad

On Monday, January 06, 2014 12:20:32 PM Paolo Bonzini wrote:
> Il 04/01/2014 22:38, Rafael J. Wysocki ha scritto:
> > On Saturday, January 04, 2014 07:48:13 PM Gleb Natapov wrote:
> >> On Sat, Jan 04, 2014 at 06:38:59PM +0100, Paolo Bonzini wrote:
> >>> Il 04/01/2014 15:38, Rafael J. Wysocki ha scritto:
> >>>> Well, it's just a sanity check and it makes the problem go away for the reporter.
> >>>>
> >>>>>> Your patch is welcome but perhaps it should have a WARN_ON too.
> >>>> It has been pulled in already, so the WARN_ON() can only be added via a separate
> >>>> patch now.  Would you like to prepare that patch?
> >>>
> >>> Yes, I'll add it together with the CPUID check.  I'll send the patch so
> >>> that it can get into 3.14.
> >>>
> >> CPUID check, while correct, will sweep the problem under the rug. Current
> >> Linux logic should detect non working pstate in KVM. We should look into
> >> why this is not happening for nested.
> > 
> > I agree.  It's better not to use CPUID for that in my opinion.
> 
> Among hypervisors, RHEL5's Xen is probably one of the oldest in actual
> use with new hardware and new kernels, and the CPUID bit has been fixed
> in 2011.  Older versions wouldn't run new kernels due to other CPUID
> bits not being cleared properly in VMs.
> 
> Is there real hardware that has the CPUID bit set and non-working
> pstate?  If there's no such real hardware, CPUID is what the SDM says
> you should use to detect presence of the APERF/MPERF msrs.

OK

> Having extra safety checks is fine on top of what the SDM says, but IMO
> they should be WARN_ONs.  Otherwise you are sweeping bugs under the rug
> just as much.

As I said I'm not against adding WARN_ON()s there. :-)

Thanks!

-- 
I speak only for myself.
Rafael J. Wysocki, Intel Open Source Technology Center.
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