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From:   Thomas Gleixner <tglx@linutronix.de>
To:     Andy Lutomirski <luto@amacapital.net>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        LKML <linux-kernel@vger.kernel.org>, x86@kernel.org,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Andy Lutomirski <luto@kernel.org>,
        Alexandre Chartre <alexandre.chartre@oracle.com>,
        Frederic Weisbecker <frederic@kernel.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Sean Christopherson <sean.j.christopherson@intel.com>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Petr Mladek <pmladek@suse.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Joel Fernandes <joel@joelfernandes.org>,
        Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        Juergen Gross <JGross@suse.com>,
        Brian Gerst <brgerst@gmail.com>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Will Deacon <will@kernel.org>,
        Tom Lendacky <Thomas.Lendacky@amd.com>,
        Wei Liu <wei.liu@kernel.org>,
        Michael Kelley <mikelley@microsoft.com>,
        Jason Chen CJ <jason.cj.chen@intel.com>,
        Zhao Yakui <yakui.zhao@intel.com>
Subject: Re: [patch V6 04/37] x86: Make hardware latency tracing explicit
In-Reply-To: <CF349A31-C401-4684-BA34-CD3359AF34E7@amacapital.net>
References: <87imgqv055.fsf@nanos.tec.linutronix.de> <CF349A31-C401-4684-BA34-CD3359AF34E7@amacapital.net>
Date:   Thu, 21 May 2020 00:20:07 +0200
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Andy Lutomirski <luto@amacapital.net> writes:
>> On May 20, 2020, at 1:10 PM, Thomas Gleixner <tglx@linutronix.de> wrote:
>> ﻿Peter Zijlstra <peterz@infradead.org> writes:
>>> We probably have to anyway. But I can do that later I suppose.
>> 
>> Second thoughts. For #DB and #INT3 we can just keep nmi_enter(), needs
>> just annotation in nmi_enter() around that trace muck.
>> 
>> For #NMI and #MCE I rather avoid the early trace call and do it once we
>> have reached "stable" state, i.e. avoid it in the whole nested NMI mess.
>
> What’s the issue?  The actual meat is mostly in the asm for NMI, and
> for MCE it’s just the sync-all-the-cores thing. The actual
> simultaneous NMI-and-MCE case is utterly busted regardless, and I’ve
> been thinking about how to fix it. It won’t be pretty, but nmi_enter()
> will have nothing to do with it.

The issue is that I want to avoid anything which is not essential just
for pure paranoia reasons.

I can drop that and just move the trace muck after RCU is safe and
annotate it properly.