Prevent Kernel from resetting cycle counter

[Kim Phillips]

On Wed, 6 Sep 2017 17:36:36 -0700
Jason Teplitz <jason at tensyr.com> wrote:

[adding linux-arm-kernel]

> I'm reading the armv8 PMU cycle counter from userspace and noticing
> that it gets periodically reset by something. Does anything inside the
> kernel reset these counters? I am not using perf, so I'm not sure why
> anything else would be touching them.

PMCCNTR only shows up in kernel perf/PMU and KVM code, so yes, no
reason for it to be reset, esp. if no arm-pmu interrupts are triggered.

Is the period constant, as if to suggest a wraparound?  Can you use
CNTVCT instead?

Thanks,

Kim