x86: Save cr2 in NMI in case NMIs take a page fault (for i386)

x86: Save cr2 in NMI in case NMIs take a page fault (for i386)

Avi Kivity reported that page faults in NMIs could cause havic if
the NMI preempted another page fault handler:

   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.

This is the i386 version, which has the luxury of doing the work
in C code.

Link: http://lkml.kernel.org/r/4FBB8C40.6080304@redhat.com
Reported-by: Avi Kivity <avi@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>