Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
MIME-Version: 1.0
References: <20200515234547.710474468@linutronix.de> <20200515235124.783722942@linutronix.de>
 <CALCETrW2negGYRSQ8dqFVdiWGtYpYf6nKe5_hkotNv0e5mNBxA@mail.gmail.com>
 <87zha7c5h5.fsf@nanos.tec.linutronix.de> <CALCETrVuA25n_d-3KMvvDxuqZeBEEYb6n=QAXOhBFkgS1Dk+UA@mail.gmail.com>
 <877dx9zn47.fsf@nanos.tec.linutronix.de>
In-Reply-To: <877dx9zn47.fsf@nanos.tec.linutronix.de>
From:   Andy Lutomirski <luto@kernel.org>
Date:   Mon, 18 May 2020 13:42:52 -0700
Message-ID: <CALCETrW9sCjeRgNZ4h4f-zNzheerxWxEvUyCaPkO1nHqV99EpQ@mail.gmail.com>
Subject: Re: [patch V6 04/37] x86: Make hardware latency tracing explicit
To:     Thomas Gleixner <tglx@linutronix.de>
Cc:     Andy Lutomirski <luto@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>, X86 ML <x86@kernel.org>,
        "Paul E. McKenney" <paulmck@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>,
        "Peter Zijlstra (Intel)" <peterz@infradead.org>
Content-Type: text/plain; charset="UTF-8"
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Mon, May 18, 2020 at 1:03 AM Thomas Gleixner <tglx@linutronix.de> wrote:
>
> 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.
>

Okay.  I'll stop looking for the bug you're fixing, then.