kmsan: avoid false reports while processing NMIs

[chs]

Summary

The NMI asm path needs two KMSAN hooks to avoid false reports:

• KMSAN_INIT_ARG clears the parameter shadow for calling an instrumented C function from asm code
• KMSAN_MARK_TRAPFRAME marks a trapframe that was initialized in asm code as initialized.

Sponsored by: Netflix
Assisted-By: Claude Code - Opus 5 (1M context)

[markj]

sys/amd64/amd64/exception.S

879

Why are we doing this here? We're either about to return back to the interrupted context via iret, or we're going to call pmc_hook. In the latter case, the trapframe has already been marked by the trap() call above.