s390/pai: Fix missing PAI counter increments under heavy load

s390/pai: Fix missing PAI counter increments under heavy load

Machines with a larger number of CPUs and under heavy load sometimes
loose PAI counter increments during recording using events
-e CRYPTO_ÂLL or -e NNPA_ALL. Counting is not affected.
This happens when several PAI crypto counters are incremented during
the same cryptographic operation.

During schedule out the functions

paiXXX_sched_task() (with XXX either crypt or ext)
+--> pai_have_samples()
   +--> pai_have_sample()
	+--> pai_copy()
	+--> pai_push_sample()

are called to read out PAI counter values.
In pai_copy() the current values of PAI counters are read from the
PMU memory mapped page and compared to the values read during last
schedule out operation, which have been saved in a backup page
named PAI_SAVE_AREA(event). For each PAI counter a delta is calculated
and when the delta is positive, that PAI counter was incremented by
hardware. This positve delta is reported as raw data record attached
to a sample.
After all deltas have been calculated, the new PAI counter values
are saved in the backup page PAI_SAVE_AREA(event). However this is
done in pai_push_sample(), leaving a small window for missing hardware
triggered updates. Here is one scenario:

  PAI counter idx:   0   1   2   3   4   5   6   7  ....  N
                   +---+---+---+---+---+---+---+---+    +---+
  PAI counter page:|   |   | X |   |   |   |   |   |....| Y |
                   +---+---+---+---+---+---+---+---+    +---+

In pai_copy() each PAI counter value is read and compared
to its old value. This is done in a loop. When PAI counter indexed
N is read, the hardware might increment PAI counter indexed 2 again,
updating its value from X to X+1.
Later pai_push_sample() simply mem-copies the complete PAI counter
page to a backup page and the increment of X+1 is lost, because the
backup page now contains the new value.

Read each PAI counter and save this value in the backup page when
there is a positive delta. This omits any time window between read
and store. This also reduced the work load as only modified PAI
counters are saved.

Cc: stable@vger.kernel.org
Fixes: fe861b0c8d06 ("s390/pai: save PAI counter value page in event structure")
Signed-off-by: Thomas Richter <tmricht@linux.ibm.com>
Reviewed-by: Sumanth Korikkar <sumanthk@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>