linux-kernel - Re: [patch V4 part 4 15/24] x86/db: Split out dr6/7 handling

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Message-ID: <87wo5mk595.fsf@nanos.tec.linutronix.de>
Date:   Fri, 08 May 2020 13:58:30 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Peter Zijlstra <peterz@...radead.org>,
        Alexandre Chartre <alexandre.chartre@...cle.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Andy Lutomirski <luto@...nel.org>,
        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>
Subject: Re: [patch V4 part 4 15/24] x86/db: Split out dr6/7 handling

Peter Zijlstra <peterz@...radead.org> writes:
>> > +static __always_inline void debug_enter(unsigned long *dr6, unsigned long *dr7)
>> > +{
>> > +	/*
>> > +	 * Disable breakpoints during exception handling; recursive exceptions
>> > +	 * are exceedingly 'fun'.
>> > +	 *
>> > +	 * Since this function is NOKPROBE, and that also applies to
>> > +	 * HW_BREAKPOINT_X, we can't hit a breakpoint before this (XXX except a
>> > +	 * HW_BREAKPOINT_W on our stack)
>> > +	 *
>> > +	 * Entry text is excluded for HW_BP_X and cpu_entry_area, which
>> > +	 * includes the entry stack is excluded for everything.
>> > +	 */
>> > +	get_debugreg(*dr7, 6);
>> 
>> Do you mean  get_debugreg(*dr7, 7); ?
>
> Shees, I have to go buy a new stack of brown paper bags at this rate,
> don't I :/

Not only you, but it's also  amazing that the selftests didn't catch
that.