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

From: Dirk Brandewie
Date: Mon Jan 06 2014 - 13:40:41 EST

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.


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.


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 }
+ (unsigned long)&policy }

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

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