linux-kernel - RE: [PATCH] x86/mce: drop cpu_missing since we have more capable mce_missing

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Date:   Tue, 9 Nov 2021 20:44:41 +0000
From:   "Luck, Tony" <tony.luck@...el.com>
To:     Borislav Petkov <bp@...en8.de>
CC:     Zhaolong Zhang <zhangzl2013@....com>,
        "x86@...nel.org" <x86@...nel.org>,
        "linux-edac@...r.kernel.org" <linux-edac@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Paul E . McKenney" <paulmck@...nel.org>
Subject: RE: [PATCH] x86/mce: drop cpu_missing since we have more capable
 mce_missing_cpus

> what do we do with the sysfs knob? It probably is an ABI:
>
> /sys/devices/system/machinecheck/machinecheck1/tolerant
> /sys/devices/system/machinecheck/machinecheck2/tolerant

$ git grep tolerant -- Documentation/ABI/
$

An undocumented ABI! Well, not documented with all the other sysfs bits.

It does appear in:
Documentation/x86/x86_64/machinecheck.rst

Of course, like a lot of documentation, it isn't accurate. It wasn't
updated to describe what happens with recoverable errors.
Final paragraph says:

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

Recovery was first introduced in the Nehalem generation which ark.intel.com
says was launched in Q1'2010. So over a decade.

Choices:
1) Leave the file there, but remove the code that uses the value
2) Delete the file too

Option 1 doesn't break any scripts that look for the file, but may make
people shout louder when they find it no longer does anything.

Option 2 is the more honest approach.

-Tony