linux-kernel - Re: Dealing with the NMI mess

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Message-ID: <20150724103127.3c3f4693@gandalf.local.home>
Date:	Fri, 24 Jul 2015 10:31:27 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Willy Tarreau <w@....eu>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andy Lutomirski <luto@...capital.net>, 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, 24 Jul 2015 15:21:28 +0200
Willy Tarreau <w@....eu> wrote:

> My understanding is that by using RET we can't set the RF flag and #DB

But the RF flag is only set for instruction (executing) breakpoints. It
is not set for data (RW) ones.

-- Steve

> will immediately strike again when the operation is attempted again. Thus
> we have to completely disable the breakpoints on leaving after the first
> one strikes, resulting in some userland breakpoints being missed. Maybe
> it can be accepted as a limitation when perf is running. I don't know if
> the output of perf is that relevant when a debugger is present BTW.
> 
> Willy

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