linux-kernel - Re: [patch 2/2] x86 NMI-safe INT3 and Page Fault

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Message-ID: <alpine.LFD.2.00.1007141732200.11778@eddie.linux-mips.org>
Date:	Wed, 14 Jul 2010 17:42:21 +0100 (BST)
From:	"Maciej W. Rozycki" <macro@...ux-mips.org>
To:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
cc:	LKML <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Steven Rostedt <rostedt@...tedt.homelinux.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Christoph Hellwig <hch@....de>, Li Zefan <lizf@...fujitsu.com>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Johannes Berg <johannes.berg@...el.com>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Tom Zanussi <tzanussi@...il.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Andi Kleen <andi@...stfloor.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	akpm@...l.org, "H. Peter Anvin" <hpa@...or.com>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	"Frank Ch. Eigler" <fche@...hat.com>
Subject: Re: [patch 2/2] x86 NMI-safe INT3 and Page Fault

On Wed, 14 Jul 2010, Mathieu Desnoyers wrote:

> This patch makes all faults, traps and exception safe to be called from NMI
> context *except* single-stepping, which requires iret to restore the TF (trap
> flag) and jump to the return address in a single instruction. Sorry, no kprobes

 Watch out for the RF flag too, that is not set correctly by POPFD -- that 
may be important for faulting instructions that also have a hardware 
breakpoint set at their address.

> support in NMI handlers because of this limitation. This cannot be emulated
> with popf/lret, because lret would be single-stepped. It does not apply to
> "immediate values" because they do not use single-stepping. This code detects if
> the TF flag is set and uses the iret path for single-stepping, even if it
> reactivates NMIs prematurely.

 What about the VM flag for VM86 tasks?  It cannot be changed by POPFD 
either.

 How about only using the special return path when a nested exception is 
about to return to the NMI handler?  You'd avoid all the odd cases then 
that do not happen in the NMI context.

  Maciej
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