s390/cpumf: cpum_cf PMU displays invalid value after hotplug remove

When a CPU is hotplugged while the perf stat -e cycles command is
running, a wrong (very large) value is displayed immediately after the
CPU removal:

  Check the values, shouldn't be too high as in
            time             counts unit events
     1.001101919           29261846      cycles
     2.002454499           17523405      cycles
     3.003659292           24361161      cycles
     4.004816983 18446744073638406144      cycles
     5.005671647      <not counted>      cycles
     ...

The CPU hotplug off took place after 3 seconds.
The issue is the read of the event count value after 4 seconds when
the CPU is not available and the read of the counter returns an
error. This is treated as a counter value of zero. This results
in a very large value (0 - previous_value).

Fix this by detecting the hotplugged off CPU and report 0 instead
of a very large number.

Cc: stable@vger.kernel.org
Fixes: a029a4e ("s390/cpumf: Allow concurrent access for CPU Measurement Counter Facility")
Reported-by: Sumanth Korikkar <sumanthk@linux.ibm.com>
Signed-off-by: Thomas Richter <tmricht@linux.ibm.com>
Reviewed-by: Sumanth Korikkar <sumanthk@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

Author: Thomas Richter

arch/s390/kernel/perf_cpum_cf.c

@@ -687,8 +687,10 @@ static void cpumf_pmu_stop(struct perf_event *event, int flags)
 						      false);
 			if (cfdiag_diffctr(cpuhw, event->hw.config_base))
 				cfdiag_push_sample(event, cpuhw);
-		} else
+		} else if (cpuhw->flags & PMU_F_RESERVED) {
+			/* Only update when PMU not hotplugged off */
 			hw_perf_event_update(event);
+		}
 		hwc->state |= PERF_HES_UPTODATE;
 	}
 }