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.