Re: x86 PMU broken in current Linus' tree

[Peter Zijlstra]

On Tue, Aug 02, 2016 at 02:04:58PM +0200, Jiri Kosina wrote:
> With current Linus' tree (HEAD == 731c7d3a20), I am getting bogus MSR 
> write warning during bootup, and kernel panic when shutting PMUs down 
> during poweroff.
> 
> The MSR warning is below, the camera capture of the poweroff panic can be 
> found at
> 
> 	http://www.jikos.cz/jikos/junk/pmu-panic.jpg
> 
> The last previous kernel version that I've booted on this particular 
> machine was 4.7.0-rc4, and it had neither of those symptoms, so I can 
> eventually bisect if needed.
> 
> === [ snip ] ==
> [    0.136000] smpboot: CPU0: Intel(R) Core(TM)2 Duo CPU     L9400  @ 1.86GHz (family: 0x6, model: 0x17, stepping: 0x6)
> [    0.136000] Performance Events: PEBS fmt0+, Core2 events, Intel PMU driver.
> [    0.136000] ... version:                2
> [    0.136000] ... bit width:              40
> [    0.136000] ... generic registers:      2
> [    0.136000] ... value mask:             000000ffffffffff
> [    0.136000] ... max period:             000000007fffffff
> [    0.136000] ... fixed-purpose events:   3
> [    0.136000] ... event mask:             0000000700000003
> [    0.136000] NMI watchdog: enabled on all CPUs, permanently consumes one hw-PMU counter.
> [    0.136000] unchecked MSR access error: WRMSR to 0xdf (tried to write 0x000000ff80000001) at rIP: 0xffffffff90004acc (x86_perf_event_set_period+0xdc/0x190)

'Curious'.. :/

x86_perf_event_set_period() only does:

  wrmsrl(hwc->event_base, (u64)(-left) & x86_pmu.cntval_mask);

and hwc->event ends up being:

  MSR_ARCH_PERFMON_PERFCTR0 + index

>From which we can deduce that index = 0xdf - 0xc1 = 30, which is
somewhat larger than the max reported number of counters (2).

Lemme go see how that can happen.