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SubjectRe: [PATCH] perf/x86/intel: ignore CondChgd bit to avoid false NMI handling
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From: Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH] perf/x86/intel: ignore CondChgd bit to avoid false NMI handling
Date: Wed, 11 Jun 2014 13:54:13 +0200

> On Wed, Jun 11, 2014 at 10:54:48AM +0200, Peter Zijlstra wrote:
>> > I'm not sure about exact behavior of CondChgd bit, in particular when
>> > this bit is set. Although I read Intel System Programmer's Manual to
>> > figure out but I have yet completed that. At least, I think ignoring
>> > CondChgd bit should be enough for NMI watchdog perspective.
>>
>> So yes, the SDM lists the bit as existing but never once mentions it
>> outside of that, and its been doing that at least back to 2008.
>>
>> Ooh, I found it:
>>
>> "The IA32_PERF_GLOBAL_STATUS MSR also provides a ‘sticky bit’ to
>> indicate changes to the state of performance monitoring hardware (see
>> Figure 18-29)."
>>
>> Which is of course completely useless, not to mention inconsistent with
>> the later CondChgd name.
>>
>> HPA, can you explain wtf that bit does and why hatayama-san's ivb feels
>> like having that set on boot?
>
> Matt found in the MSR listing for GLOBAL_STATUS:
>
> 63 CondChg: status bits of this register has changed. If CPUID.0AH: EAX[7:0] > 0
>
> Which brings us to a grand total of 3 different names for this bit.
>
> If it indeed does what it says on the tin, set every time the status
> changes its like the most useless bit ever and I wonder why people
> bothered to spend silicon on it.
>

Yes, I didn't mention in patch description this, but I reached the
same conclusion. The description confuses me because the desciption
and the behaviour of CondChg bit I see on the actual system is not
coincide.

Also, I checked cpuid on the system with Neharlem processor where I
have never seen CondChg bit is set.

[root@localhost ~]# ./cpuid -r
CPU 0:
0x00000000 0x00: eax=0x0000000b ebx=0x756e6547 ecx=0x6c65746e edx=0x49656e69
0x00000001 0x00: eax=0x000206e6 ebx=0x40200800 ecx=0x00bce3bd edx=0xbfebfbff
<snip>
0x0000000a 0x00: eax=0x07300403 ebx=0x00000044 ecx=0x00000000 edx=0x00000603
^^^^^^^^^^^^^^
So, cpuid tells that CondChg bit is supported on this processor, too.

> In any case, the proposed patch seems fine, just needs a better
> changelog.
>

I see.

I'll write that the problem is that any NMI could be robbed by NMI
watchdog explicitly. Now only patch title says this explicitly. This
is your first comment.

About CondChgd bit, I cannot write more than I see on actual
system. If it's necessary to describe more about CondChgd bit, it
would be appreciated if someone tell me more information about it.

Thanks.
HATAYAMA, Daisuke
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