Date: Fri, 24 Jul 2015 21:55:09 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Willy Tarreau <w@1wt.eu>, Steven Rostedt <rostedt@goodmis.org>,
        Andy Lutomirski <luto@amacapital.net>, X86 ML <x86@kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Borislav Petkov <bp@alien8.de>, Thomas Gleixner <tglx@linutronix.de>,
        Brian Gerst <brgerst@gmail.com>
Subject: Re: Dealing with the NMI mess
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On Fri, Jul 24, 2015 at 11:29:29AM -0700, Linus Torvalds wrote:
> On Fri, Jul 24, 2015 at 8:30 AM, Peter Zijlstra <peterz@infradead.org> wrote:
> > On Fri, Jul 24, 2015 at 05:26:37PM +0200, Willy Tarreau wrote:
> >> >
> >> > The point is, if we trigger a #DB on an instruction breakpoint
> >> > while !IF, then we simply disable that breakpoint and do the RET.
> >>
> >> Yes but the breakpoint remains disabled then. Or I'm missing
> >> something.
> >
> > http://marc.info/?l=linux-kernel&m=143773601130974
> >
> > We re-enable before going back to userspace.
> 
> Actually, Andy had a good argument that we don't even need this.
> 
> We just don't ever need to disable data breakpoints. Even if we end up doing
> 
>         cli();
>         copy_from_user_inatomic();
> 
> that actually works fine. If there are data breakpoints, we will have

I worry that we'll end up running the do_debug() handlers from effective
NMI context.

The NMI might have preempted locks which these handlers require etc..
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