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 14:33:15 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	"H. Peter Anvin" <hpa@...or.com>
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>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.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

On Wed, 2011-12-14 at 10:26 -0800, H. Peter Anvin wrote:
> On 12/13/2011 06:52 PM, Steven Rostedt wrote:
> > With i386, NMIs and breakpoints use the current stack and they
> > do not reset the stack pointer to a fix point that might corrupt
> > a previous NMI or breakpoint (as it does in x86_64). But NMIs are
> > still not made to be re-entrant, and need to prevent the case that
> > an NMI hitting a breakpoint (which does an iret), doesn't allow
> > another NMI to run.
> 
> Okay... what about the other way around: avoiding the IRET when invoked
> from an NMI context and therefore leaving NMI disabled until the
> appropriate time?

Linus was against this approach. He didn't want the ugliness of NMI to
spread to other code, by adding ugly handlers around the 'iret' of
breakpoints and exceptions.

-- Steve


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