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Date:   Fri, 8 May 2020 14:45:56 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Thomas Gleixner <tglx@linutronix.de>
Cc:     Alexandre Chartre <alexandre.chartre@oracle.com>,
        LKML <linux-kernel@vger.kernel.org>, x86@kernel.org,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Andy Lutomirski <luto@kernel.org>,
        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>
Subject: Re: [patch V4 part 4 15/24] x86/db: Split out dr6/7 handling
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On Fri, May 08, 2020 at 01:58:30PM +0200, Thomas Gleixner wrote:
> Peter Zijlstra <peterz@infradead.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.

I don't think the selftests try and set hardware breakpoints in the
kernel.