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

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Date:	Tue, 7 Jan 2014 18:11:10 +0200
From:	Gleb Natapov <gleb@...nel.org>
To:	Dirk Brandewie <dirk.brandewie@...il.com>
Cc:	Paolo Bonzini <pbonzini@...hat.com>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	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 Mon, Jan 06, 2014 at 09:18:44AM -0800, Dirk Brandewie wrote:
> On 01/03/2014 02:06 PM, Paolo Bonzini wrote:
> >Il 03/01/2014 21:00, Dirk Brandewie ha scritto:
> >>+    case MSR_IA32_MPERF:
> >>+    case MSR_IA32_APERF:
> >
> OK I will spin the patch to only add MSR_PLATFORM_INFO.
> 
> >These should never be accessed.  A KVM VM will always have
> >CPUID[06H].ECX = 0, and the Intel manual says that the MSRs are only
> >present if CPUID returns that value with bit 0 set.
> >
> >I think the actual bug is that intel_pstate_init does not check the
> >feature bits in CPUID (either manually or via x86_match_cpu).
> 
> I will add the feature check.
> 
> What are the differences between the first and the nested KVM's?
There shouldn't be any. There is a bug in nested emulation probably.

> At load time intel_pstate checks that APERF and MPERF are incrementing
> and that PLATFORM_INFO has some value.  Somehow these checks pass
> in the nested environment and we fall over when the CPU is being added
> by cpufreq.
> 
KVM does not emulate either of those and inject #GP if one is accessed. Linux
catches those #GPs and fixs up rdmsr to return zero. It would be interesting to
see ftrace for nested kvm run.

--
			Gleb.
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