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From:   Thomas Gleixner <tglx@linutronix.de>
To:     Andy Lutomirski <luto@kernel.org>
Cc:     Andy Lutomirski <luto@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>, X86 ML <x86@kernel.org>,
        "Paul E. McKenney" <paulmck@kernel.org>,
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        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>,
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        Zhao Yakui <yakui.zhao@intel.com>,
        "Peter Zijlstra \(Intel\)" <peterz@infradead.org>
Subject: Re: [patch V6 04/37] x86: Make hardware latency tracing explicit
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Date:   Mon, 18 May 2020 10:03:04 +0200
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Andy Lutomirski <luto@kernel.org> writes:
> On Sun, May 17, 2020 at 1:48 AM Thomas Gleixner <tglx@linutronix.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