hwpmc: let MD code see every context switch and exec

[afscoelho_gmail.com]

Summary

Branch records do not track address spaces, so the MD code must clear
them on every context switch and exec. Add optional pmd_lbr_csw()
and pmd_lbr_exec() hooks.

While a system-mode PMC that needs them runs, call the switch hooks for
all threads. Otherwise the scheduler fast path is unchanged.

Depends on D60566

Test Plan

Built amd64 GENERIC + hwpmc.ko, GENERIC without HWPMC_HOOKS, and arm64.
Ran tests/sys/pmc on AMD Zen 6 (LBR v2).

[kib]

sys/dev/hwpmc/hwpmc_mod.c

1781

Why do you need the critical section there?
PMC_FN_CSW_OUT is always called under some spinlock: either the thread lock when done during normal context switch, or the process spinlock if called during thread exit. As is, the context switch is not possible already there.

You might add CRITICAL_ASSERT() there instead, if needed, I believe.

[afscoelho_gmail.com]

sys/dev/hwpmc/hwpmc_mod.c

1781

Good catch, thanks! You're right: we're already under a spinlock there, so the critical section isn't needed. I'll replace it with CRITICAL_ASSERT() and do the same in the CSW_IN early path, which has the same problem. Thanks for taking the time to review this!

[afscoelho_gmail.com]

Replace critical_enter()/critical_exit() with CRITICAL_ASSERT() in the CSW early-return paths; the hooks already run under a spinlock.

[afscoelho_gmail.com]

Rebase only. No code change.