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

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Date:	Fri, 27 Dec 2013 08:46:25 -0500
From:	Josh Boyer <jwboyer@...oraproject.org>
To:	Gleb Natapov <gleb@...nel.org>
Cc:	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>,
	Viresh Kumar <viresh.kumar@...aro.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>,
	Kashyap Chamarthy <kchamart@...hat.com>,
	"Richard W.M. Jones" <rjones@...hat.com>
Subject: Re: intel_pstate divide error with v3.13-rc4-256-gb7000ad

On Fri, Dec 27, 2013 at 7:47 AM, Gleb Natapov <gleb@...nel.org> wrote:
> 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?

Seems reproducible.  I forgot to link to the actual bug in my original
report, but you can find it here:

https://bugzilla.redhat.com/show_bug.cgi?id=1046317

Adding Richard and Kashyap on CC.

josh
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