torvalds/linux

What:        /sys/devices/system/machinecheck/machinecheckX/tolerant
Contact:    Borislav Petkov <bp@suse.de>
Date:        Dec, 2021
Description:
        Unused and obsolete after the advent of recoverable machine
        checks (see last sentence below) and those are present since
        2010 (Nehalem).

        Original description:

        The entries appear for each CPU, but they are truly shared
        between all CPUs.

        Tolerance level. When a machine check exception occurs for a
        non corrected machine check the kernel can take different
        actions.

        Since machine check exceptions can happen any time it is
        sometimes risky for the kernel to kill a process because it
        defies normal kernel locking rules. The tolerance level
        configures how hard the kernel tries to recover even at some
        risk of    deadlock. Higher tolerant values trade potentially
        better uptime with the risk of a crash or even corruption
        (for tolerant >= 3).

        ==  ===========================================================
         0  always panic on uncorrected errors, log corrected errors
         1  panic or SIGBUS on uncorrected errors, log corrected errors
         2  SIGBUS or log uncorrected errors, log corrected errors
         3  never panic or SIGBUS, log all errors (for testing only)
        ==  ===========================================================

        Default: 1

        Note this only makes a difference if the CPU allows recovery
        from a machine check exception. Current x86 CPUs generally
        do not.