x86, nmi: Add in logic to handle multiple events and unknown NMIs

Previous patches allow the NMI subsystem to process multipe NMI events
in one NMI.  As previously discussed this can cause issues when an event
triggered another NMI but is processed in the current NMI.  This causes the
next NMI to go unprocessed and become an 'unknown' NMI.

To handle this, we first have to flag whether or not the NMI handler handled
more than one event or not.  If it did, then there exists a chance that
the next NMI might be already processed.  Once the NMI is flagged as a
candidate to be swallowed, we next look for a back-to-back NMI condition.

This is determined by looking at the %rip from pt_regs.  If it is the same
as the previous NMI, it is assumed the cpu did not have a chance to jump
back into a non-NMI context and execute code and instead handled another NMI.

If both of those conditions are true then we will swallow any unknown NMI.

There still exists a chance that we accidentally swallow a real unknown NMI,
but for now things seem better.

An optimization has also been added to the nmi notifier rountine.  Because x86
can latch up to one NMI while currently processing an NMI, we don't have to
worry about executing _all_ the handlers in a standalone NMI.  The idea is
if multiple NMIs come in, the second NMI will represent them.  For those
back-to-back NMI cases, we have the potentail to drop NMIs.  Therefore only
execute all the handlers in the second half of a detected back-to-back NMI.

Signed-off-by: Don Zickus <dzickus@redhat.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1317409584-23662-5-git-send-email-dzickus@redhat.com
Signed-off-by: Ingo Molnar <mingo@elte.hu>

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