linux-kernel - Re: Dealing with the NMI mess

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Message-ID: <CALCETrV2p9j5zQAmEL66YUQv33Z7Bk6mL4eeJUUpcXpvFFafYg@mail.gmail.com>
Date:	Fri, 24 Jul 2015 10:21:16 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Willy Tarreau <w@....eu>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Steven Rostedt <rostedt@...dmis.org>, X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Borislav Petkov <bp@...en8.de>,
	Thomas Gleixner <tglx@...utronix.de>,
	Brian Gerst <brgerst@...il.com>
Subject: Re: Dealing with the NMI mess

On Fri, Jul 24, 2015 at 9:25 AM, Willy Tarreau <w@....eu> wrote:
> On Fri, Jul 24, 2015 at 08:48:57AM -0700, Andy Lutomirski 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.
>
> Apparently after reading 17.3.1.1, it seems like RF can still be set
> if a data breakpoint triggers in a repeated string instruction before
> the last iteration. However I don't think we care because as long as
> we return to the string instruction, since the data location was already
> visited it won't trigger again so the loss of the flag should be safe.
>

Oh, right.  So my proposal is wrong: it'll clear a watchpoint
incorrectly if we hit it in the middle of a string operation.

So we should either parse dr0..dr3 (whichever one triggered) or do
Peter's think and clear dr7 entirely.  I still prefer just clearing
the breakpoint that triggered.

--Andy
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