linux-kernel - Re: [RFC][PATCH 5/5 v2] x86: Allow NMIs to hit breakpoints in i386

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Date:	Wed, 14 Dec 2011 08:44:13 -0500
From:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"H. Peter Anvin" <hpa@...or.com>, Andi Kleen <andi@...stfloor.org>,
	"H. Peter Anvin" <hpa@...ux.intel.com>,
	Paul Turner <pjt@...gle.com>
Subject: Re: [RFC][PATCH 5/5 v2] x86: Allow NMIs to hit breakpoints in i386

* Steven Rostedt (rostedt@...dmis.org) wrote:
> On Wed, 2011-12-14 at 08:30 -0500, Mathieu Desnoyers wrote:
> 
> > Just to make sure I understand: if an NMI nests over do_nmi between
> > nmi_postprocess() and the following iret (in which case the CPU is in
> > state NMI_NOT_RUNNING), we will end up with two NMI handlers nested on
> > the stack, right ? Given that there is no upper-bound on the nesting
> > level of this situation (although nesting like this more than once is
> > extremely unlikely), is this side-effect something we should care about
> > in terms of stack space usage ?
> 
> At that point, there's very little on the stack to begin with. Just the
> one irq frame, and saved regs, plus the stack frame of this function. If
> we are hitting that many NMIs to cause a stack overflow, then I believe
> there's more issues than the overflow itself. Say, a livelock of NMIs?

Yep, if it's small then it's fine I guess.

> 
> >  Also, is the stack dump OOPS handler
> > aware of this stack layout that was until now impossible ?
> 
> Since NMIs on i386 doesn't change the stack when interrupting the
> kernel, the OOPs handler never was aware of the NMI stack layout.

Makes sense, sounds good,

Thanks!

Mathieu

> 
> -- Steve
> 
> 
> 

-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com
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