Impact: cleanup

Get rid of unused debug code.
Signed-off-by: NIngo Molnar <mingo@elte.hu>

Impact: fix spurious missed counter wakeups

In the case of NMI events, close a race window that can occur if an NMI
hits counter code that temporarily disables+enables a counter, and the NMI
leaks into the disabled section.
Signed-off-by: NIngo Molnar <mingo@elte.hu>

Impact: make perfcounter NMI and IRQ sequence more robust

Make __smp_perf_counter_interrupt() a bit more conservative: first disable
all counters, then read out the status. Most invocations are because there
are real events, so there's no performance impact.

Code flow gets a bit simpler as well this way.
Signed-off-by: NIngo Molnar <mingo@elte.hu>

Implement performance counters for x86 Intel CPUs.

It's simplified right now: the PERFMON CPU feature is assumed,
which is available in Core2 and later Intel CPUs.

The design is flexible to be extended to more CPU types as well.
Signed-off-by: NIngo Molnar <mingo@elte.hu>