linux-kernel - NMI vs #PF clash

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Date:	Tue, 22 May 2012 15:53:20 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Steven Rostedt <rostedt@...dmis.org>
CC:	linux-kernel <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Paul Turner <pjt@...gle.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Subject: NMI vs #PF clash

The recent changes to NMI allow exceptions to take place in NMI
handlers, but I think that a #PF (say, due to access to vmalloc space)
is still problematic.  Consider the sequence

  #PF  (cr2 set by processor)
    NMI
      ...
      #PF (cr2 clobbered)
        do_page_fault()
        IRET
      ...
      IRET
    do_page_fault()
      address = read_cr2()

The last line reads the overwritten cr2 value.

I vaguely remember some discussion about this back in the day, but I
can't find anything in the code to save/restore cr2 in the NMI handler. 
Did I miss it?  Or perhaps the page fault handler ignores the incorrect
cr2 and IRETs, to fault back immediately?

-- 
error compiling committee.c: too many arguments to function

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