Re: [PATCH] perf/x86/intel: ignore CondChgd bit to avoid false NMI handling

Peter Zijlstra
2014-06-11T08:55:00Z

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On Wed, Jun 11, 2014 at 04:30:28PM +0900, HATAYAMA Daisuke wrote:
> Currently, a NMI handler for NMI watchdog may falsely handle any NMI
> signaled for different purpose if CondChgd bit in
> MSR_CORE_PERF_GLOBAL_STATUS MSR is set.
>
> This commit deals with the issue simply by ignoring CondChgd bit.
>
> Here is explanation in detail.
>
> On x86 NMI watchdog uses performance monitoring feature to
> periodically signal NMI each time performance counter gets overflowed.
>
> intel_pmu_handle_irq() is called as a NMI_LOCAL handler from a NMI
> handler of NMI watchdog, perf_event_nmi_handler(). It identifies owner
> of a given NMI by looking at overflow status bits in
> MSR_CORE_PERF_GLOBAL_STATUS MSR. If some of the bits are set, then it
> handles the given NMI as its own NMI.
>
> The problem is that intel_pmu_handle_irq() doesn't distinguish
> CondChgd bit from other bits. Unlike the other status bits, CondChgd
> bit doesn't represent overflow status for performance counters. Thus,
> CondChgd bit cannot be thought of as a mark indicating a given NMI is
> NMI watchdog's.

So what was the problem? It ate another NMI?

> I noticed this behavior on systems with Ivy Bridge processors: Intel
> Xeon CPU E5-2630 v2 and Intel Xeon CPU E7-8890 v2. On both systems,
> CondChgd bit in MSR_CORE_PERF_GLOBAL_STATUS MSR has already been set
> in the beginning at boot. (then the CondChgd bit is cleared by next
> wrmsr to MSR_CORE_PERF_GLOBAL_CTRL MSR and appears to remain 0.)
>
> On the other hand, on older processors such as Nehalem, CondChgd bit
> is not set in the beginning at boot.
>
> 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?

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