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Date:   Wed, 26 Feb 2020 19:42:37 +0100
From:   Borislav Petkov <bp@alien8.de>
To:     Andy Lutomirski <luto@amacapital.net>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Andy Lutomirski <luto@kernel.org>,
        Frederic Weisbecker <frederic@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        LKML <linux-kernel@vger.kernel.org>, X86 ML <x86@kernel.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Brian Gerst <brgerst@gmail.com>,
        Juergen Gross <JGross@suse.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Arnd Bergmann <arnd@arndb.de>
Subject: Re: [patch 02/10] x86/mce: Disable tracing and kprobes on
 do_machine_check()
Message-ID: <20200226184237.GB16756@zn.tnic>
References: <20200226160818.GY18400@hirez.programming.kicks-ass.net>
 <C06C32B4-BD69-4287-BC67-C3E225061A46@amacapital.net>
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On Wed, Feb 26, 2020 at 09:28:51AM -0800, Andy Lutomirski wrote:
> > It entirely depends on what the goal is :-/ On the one hand I see why
> > people might want function tracing / kprobes enabled, OTOH it's all
> > mighty frigging scary. Any tracing/probing/whatever on an MCE has the
> > potential to make a bad situation worse -- not unlike the same on #DF.

FWIW, I had this at the beginning of the #MC handler in a feeble attempt
to poke at this:

+       hw_breakpoint_disable();
+       static_key_disable(&__tracepoint_read_msr.key);
+       tracing_off();

But then Tony noted that some recoverable errors do get reported with an
#MC exception so we would have to look at the error severity and then
decide whether to allow tracing or not.

But the error severity happens all the way down in __mc_scan_banks() -
i.e., we've executed the half handler already.

So, frankly, I wanna say, f*ck tracing etc - there are certain handlers
which simply don't allow it. And we'll only consider changing that when
a really good reason for it appears...

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette