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

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Date:	Fri, 27 Dec 2013 14:47:49 +0200
From:	Gleb Natapov <gleb@...nel.org>
To:	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>
Cc:	Viresh Kumar <viresh.kumar@...aro.org>,
	Josh Boyer <jwboyer@...oraproject.org>,
	Dirk Brandewie <dirk.brandewie@...il.com>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	"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>
Subject: Re: intel_pstate divide error with v3.13-rc4-256-gb7000ad

On Fri, Dec 27, 2013 at 12:24:22PM +0000, One Thousand Gnomes wrote:
> On Tue, 24 Dec 2013 21:36:01 +0530
> Viresh Kumar <viresh.kumar@...aro.org> wrote:
> 
> > Adding Dirk..
> > 
> > On 24 December 2013 20:06, Josh Boyer <jwboyer@...oraproject.org> wrote:
> > > Hi All,
> > >
> > > We've had a report [1] that the pstate driver causes KVM guests to
> > > fail to boot because of a divide error.  See the backtrace below.
> > >
> > >     4.839784] Intel P-state driver initializing.
> > > [    4.859972] Intel pstate controlling: cpu 0
> > > [    4.867653] cpufreq: __cpufreq_add_dev: ->get() failed
> > 
> > After a call to ->init(), ->get() is supposed to work.
> > @Dirk: Any idea why it failed?
> > 
> > And then I don't know what made this divide by zero to happen :)
> 
> > > [    4.878127] Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011
> 
> Emulator bug perhaps ?
>
KVM does not emulate P-states at all. intel_pstate_init() calls
intel_pstate_msrs_not_valid() before printing "Intel P-state driver
initializing."  which suppose to fail since it checks that two reads of
MSR_IA32_APERF return different values, but KVM does not emulate this msr
at all, so both calls should return zero (KVM suppose to inject #GP, all rdmsrl
are patched to be rdmsrl_safe in a guest).

Anything interesting in host dmesg? Is it reproducible? 

--
			Gleb.
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