Several people have reported spurious unknown NMI
messages on some P4 CPUs.

This patch fixes it by checking for an overflow (negative
counter values) directly, instead of relying on the
P4_CCCR_OVF bit.
Reported-by: NGeorge Spelvin <linux@horizon.com>
Reported-by: NMeelis Roos <mroos@linux.ee>
Reported-by: NDon Zickus <dzickus@redhat.com>
Reported-by: NDave Airlie <airlied@gmail.com>
Signed-off-by: NCyrill Gorcunov <gorcunov@openvz.org>
Cc: Lin Ming <ming.m.lin@intel.com>
Cc: Don Zickus <dzickus@redhat.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <AANLkTinfuTfCck_FfaOHrDqQZZehtRzkBum4SpFoO=KJ@mail.gmail.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>