linux-kernel - Re: [patch V6 04/37] x86: Make hardware latency tracing explicit

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Date:   Mon, 18 May 2020 10:03:04 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Andy Lutomirski <luto@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Alexandre Chartre <alexandre.chartre@...cle.com>,
        Frederic Weisbecker <frederic@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Petr Mladek <pmladek@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>,
        Brian Gerst <brgerst@...il.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Will Deacon <will@...nel.org>,
        Tom Lendacky <thomas.lendacky@....com>,
        Wei Liu <wei.liu@...nel.org>,
        Michael Kelley <mikelley@...rosoft.com>,
        Jason Chen CJ <jason.cj.chen@...el.com>,
        Zhao Yakui <yakui.zhao@...el.com>,
        "Peter Zijlstra \(Intel\)" <peterz@...radead.org>
Subject: Re: [patch V6 04/37] x86: Make hardware latency tracing explicit

Andy Lutomirski <luto@...nel.org> writes:
> On Sun, May 17, 2020 at 1:48 AM Thomas Gleixner <tglx@...utronix.de> wrote:
>> Remember this is about ensuring that all the state is properly
>> established before any of this instrumentation muck can happen.
>>
>> DR7 handling is specific to #DB and done even before nmi_enter to
>> prevent recursion.
>
> So why is this change needed?

We really want nmi_enter() to be the carefully crafted mechanism which
establishes correct state in whatever strange context the exception
hits. Not more, not less.

Random instrumentation has absolutely no business there and I went a
long way to make sure that this is enforcible by objtool.

Aside of that the tracing which is contained in nmi_enter() is about
taking timestamps for hardware latency detection. If someone runs
hardware latency detection with active break/watchpoints then I really
can't help it.

Thanks,

        tglx