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

[Dirk Brandewie]

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@xxxxxxxxx>
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@xxxxxxxxx>
---
 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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