Bypassing HIS to exploit the CPU Measurement Facility

HIS is designed to have exclusive access to the CPU Measurement Facility (CPU/MF). Start of change The HISSERV service interface is provided to allow authorized programs to collect instrumentation data using HIS functions and should be used by exploiters. There is no need to bypass HIS. See HISSERV service overview. End of change Other programs which use instructions provided by the CPU/MF must be careful not to interfere with HIS processing. For example, if a system is running HIS and another program is executing CPU/MF instructions, HIS may believe the counters are enabled when the other program has disabled them. Also turning on the sampling facility will cause external interrupts as sampling buffers are filled, which might not be properly handled due to the illicit enablement. Start of change For these reasons, the HISSERV service interface should be used. End of change If execution of the instructions by the CPU/MF is still desired, the following guidelines should be used.

Instructions that may be used:

These instructions are typically query instructions which do not change the state of the system. There will be no side effects if multiple programs issue these instructions.

ECA (Extract Coprocessor Group Address)
ECCTR (Extract CPU Counter)
EPCTR (Extract Peripheral Counter)
QCTRI (Query Counter Information)
QSI (Query Sampling Information)

Instructions that should not be used:

These instructions change the state of the system, executing them will cause side effects to any other program using the CPU/MF. It is recommended these instructions not be executed.

SCCTR (Set CPU Counter)
SCCTL (Set CPU Counter Set Controls)
SPCTR (Set Peripheral Counter)
SPCTL (Set Peripheral Counter Controls)
SSCTL (Set Sampling Controls)