linux-kernel - Re: [RFC] #MC mess

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Message-Id: <A3AAF46E-437D-420C-BF0C-C2394B48C9F4@amacapital.net>
Date:   Tue, 18 Feb 2020 15:17:18 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     "Luck, Tony" <tony.luck@...el.com>, Borislav Petkov <bp@...en8.de>,
        Steven Rostedt <rostedt@...dmis.org>,
        Andy Lutomirski <luto@...nel.org>, x86-ml <x86@...nel.org>,
        lkml <linux-kernel@...r.kernel.org>
Subject: Re: [RFC] #MC mess



> On Feb 18, 2020, at 12:02 PM, Peter Zijlstra <peterz@...radead.org> wrote:
> 
> On Tue, Feb 18, 2020 at 06:20:38PM +0000, Luck, Tony wrote:
>>> Anything else I'm missing? It is likely...
>> 
>> +    hw_breakpoint_disable();
>> +    static_key_disable(&__tracepoint_read_msr.key);
>> +    tracing_off();
>> +
>>    ist_enter(regs);
>> 
>> How about some code to turn all those back on for a recoverable (where we actually recovered) #MC?
> 
> Then please rewrite the #MC entry code to deal with nested exceptions
> unmasking the MCE, very similr to NMI.
> 
> The moment you allow tracing, jump_labels or anything else you can
> expect #PF, #BP and probably #DB while inside #MC, those will then IRET
> and re-enable the #MC.

Huh?  As I understand it, there is no such thing as MCE masking.  There are two states:

CR4.MCE=1: MCE is delivered when it occurs.

CR4.MCE=0: MCE causes shutdown

MC delivery sets MCE=0.

So, basically, without LMCE, we are irredeemably screwed.  With LMCE, we are still hosed if we nest an MCE inside a recoverable MCE.  We can play some games to make the OOPS more reliable, but we are still mostly screwed.

The x86 MCE architecture sucks.

> 
> The current situation is completely and utterly buggered.