powerpc/perf: power_pmu_start restores incorrect values, breaking frequency events

powerpc/perf: power_pmu_start restores incorrect values, breaking frequency events

commit 9a45a9407c69d068500923480884661e2b9cc421 upstream.

perf on POWER stopped working after commit e050e3f0a71b (perf: Fix
broken interrupt rate throttling). That patch exposed a bug in
the POWER perf_events code.

Since the PMCs count upwards and take an exception when the top bit
is set, we want to write 0x80000000 - left in power_pmu_start. We were
instead programming in left which effectively disables the counter
until we eventually hit 0x80000000. This could take seconds or longer.

With the patch applied I get the expected number of samples:

          SAMPLE events:       9948

Signed-off-by: Anton Blanchard <anton@samba.org>
Acked-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>