Date: Fri, 24 Jul 2015 12:02:49 -0400
From: Steven Rostedt <rostedt@goodmis.org>
To: Andy Lutomirski <luto@amacapital.net>
Cc: Peter Zijlstra <peterz@infradead.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        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>, Brian Gerst <brgerst@gmail.com>
Subject: Re: Dealing with the NMI mess
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On Fri, 24 Jul 2015 08:48:57 -0700
Andy Lutomirski <luto@amacapital.net> wrote:

> So by the time we detect that we've hit a watchpoint, the instruction
> that tripped it is done and we don't need RF.  Furthermore, after
> reading 17.3.1.1: I *think* that regs->flags withh have RF *clear* if
> we hit a watchpoint.  So this might be as simple as:
> 
> if ((dr6 && (0xf * DR_TRAP0) && (regs->flags & (X86_EFLAGS_RF |

Um, isn't 0xf * DR_TRAP0 same as a constant "true"?

-- Steve


> X86_EFLAGS_IF)) == X86_EFLAGS_RF && !user_mode(regs))
>   for (i = 0; i < 4; i++)
>     if (dr6 & (DR_TRAP0<<i)) {
>       /* hit a kernel breakpoint with IF clear */
>       dr7 &= ~(DR_GLOBAL_ENABLE << (i * DR_ENABLE_SHIFT));
>     }
> 
> I'm not saying that your code is wrong, but I think this is simpler
> and avoids poking at yet more per-cpu state from NMI context, which is
> kind of nice.
> 
> If you don't like the RF games above, it would also be straightforward
> to parse dr0..dr3 for each DR_TRAP bit that's set and see if it's a
> breakpoint.
> 
> --Andy

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