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

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Date:	Mon, 06 Jan 2014 10:40:39 -0800
From:	Dirk Brandewie <dirk.brandewie@...il.com>
To:	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Paolo Bonzini <pbonzini@...hat.com>
CC:	Gleb Natapov <gleb@...antech.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 01/06/2014 03:37 AM, Rafael J. Wysocki wrote:
> 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. :-)
>

The patch below adds a feature check for APERF/MPERF.  With this patch
you should NOT see "Intel P-state driver initializing." in dmesg for KVM.

commit 4279f36818bd3ac42f077de114b17eb27d81d482
Author: Dirk Brandewie <dirk.j.brandewie@...el.com>
Date:   Mon Jan 6 10:19:38 2014 -0800

     intel_pstate: Add X86_FEATURE_APERFMPERF to cpu match parameters.


     KVM environments do not support APERF/MPERF MSRs. intel_pstate cannot
     operate without these registers.

     The previous validity checks in intel_pstate_msrs_not_valid() are
     insufficent in nested KVMs.

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

     Signed-off-by: Dirk Brandewie <dirk.j.brandewie@...el.com>
---
  drivers/cpufreq/intel_pstate.c | 3 ++-
  1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/cpufreq/intel_pstate.c b/drivers/cpufreq/intel_pstate.c
index 0f63f5d..fe91dad 100644
--- a/drivers/cpufreq/intel_pstate.c
+++ b/drivers/cpufreq/intel_pstate.c
@@ -619,7 +619,8 @@ static void intel_pstate_timer_func(unsigned long __data)
  }

  #define ICPU(model, policy) \
-	{ X86_VENDOR_INTEL, 6, model, X86_FEATURE_ANY, (unsigned long)&policy }
+	{ X86_VENDOR_INTEL, 6, model, X86_FEATURE_APERFMPERF,\
+			(unsigned long)&policy }

  static const struct x86_cpu_id intel_pstate_cpu_ids[] = {
  	ICPU(0x2a, core_params),

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