Improvements to HWPMC support on FreeBSD/arm64 to handle counter wraparound

[rwatson]

Summary

Starting with a patch from Andrew Turner and Ruslan Bukin to improve HWPMC support for ARMv8-A counter wraparound:

• Move to per-CPU overflow entries, since we model these as 32-bit extensions to 32-bit per-CPU registers.

• Attempt to address a race between overflow counting and overflows, which may occur (for example) if the overflow happens while interrupts are off during context switching.

With these changes, I no longer experience panics when using per-process
counters on a multithreaded process.

[andrew]

sys/dev/hwpmc/hwpmc_arm64.c

235

Why is this per-CPU? pm should already be per-CPU from arm64_pcpu[cpu]

[andrew]

I think we now handle this with my recent commits.

[rwatson]

A modified version of this merged in recent changes by @andrew.