linux-kernel - Re: Dealing with the NMI mess

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Message-ID: <20150724152637.GC3612@1wt.eu>
Date:	Fri, 24 Jul 2015 17:26:37 +0200
From:	Willy Tarreau <w@....eu>
To:	Steven Rostedt <rostedt@...dmis.org>
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, Jul 24, 2015 at 11:16:21AM -0400, Steven Rostedt wrote:
> On Fri, 24 Jul 2015 16:59:01 +0200
> Willy Tarreau <w@....eu> wrote:
> 
> > On Fri, Jul 24, 2015 at 10:31:27AM -0400, Steven Rostedt wrote:
> > > 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.
> > 
> > True but these also are the most complicated to deal with. The data
> > accesses can always be emulated (not what I'm suggesting here) while
> > instructions are much harder to emulate.
> 
> 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.

> What emulation is needed?

I was speaking about redoing the operation with BP disabled before
re-enabling it. But that's not the point here anyway.

Willy

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