Date: Thu, 23 Jul 2015 17:31:05 -0400
From: Steven Rostedt <rostedt@goodmis.org>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andy Lutomirski <luto@amacapital.net>, X86 ML <x86@kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Willy Tarreau <w@1wt.eu>, Borislav Petkov <bp@alien8.de>,
        Thomas Gleixner <tglx@linutronix.de>,
        Peter Zijlstra <peterz@infradead.org>, Brian Gerst <brgerst@gmail.com>
Subject: Re: Dealing with the NMI mess
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On Thu, 23 Jul 2015 14:08:59 -0700
Linus Torvalds <torvalds@linux-foundation.org> wrote:

> On Thu, Jul 23, 2015 at 1:49 PM, Andy Lutomirski <luto@amacapital.net> wrote:
> >
> > Issue A: to return with RF clear, we need to disarm the breakpoint.
> > If it's limited to the duration of the NMI, that's easy.  If not, when
> > do we re-arm?  New prepare_exit_to_usermode hook?  Hmm, setting ti
> > flags during context switch may target the wrong task.
> 
> We don't re-arm it.
> 

Let me get this straight. The idea is in the #DB handler to detect that
it was triggered in NMI context, and if so, simply disarm that
breakpoint permanently, right?

Nothing should be adding hw breakpoints to NMI code anyway. Sounds
perfectly reasonable to me. Of course, how we tell we are in NMI
brings back all the races as we had in the nesting code. We can check
the per-cpu variable that is set with nmi_enter() and cleared at
nmi_exit() but what happens if the breakpoint is outside those calls.
We can check the stack pointer, but then we are back to userspace
fooling us. Maybe add the DF trick again?

-- Steve
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