s390/cpum_cf: Add minimal in-kernel interface for counter measurements

s390/cpum_cf: Add minimal in-kernel interface for counter measurements

Introduce a minimal interface for doing counter measurements of small
units of work within the kernel.  Use the kernel_cpumcf_begin() function
start a measurement session and, later, stop it with kernel_cpumcf_end().

During the measreument session, you can enable and start/stop counter sets
by using ctr_set_* functions.  To make these changes effective use the
lcctl() function.  You can then use the ecctr() function to extract counters
from the different counter sets.
Please note that you have to check whether the counter sets to be enabled
are authorized.

Note that when a measurement session is active, other users cannot perform
counter measurements.  In such cases, kernel_cpumcf_begin() indicates this
with returning -EBUSY.  If the counter facility is not available,
kernel_cpumcf_begin() returns -ENODEV.

Note that this interface is restricted to the current CPU and, thus,
preemption must be turned off.

Example:

	u32 state, err;
	u64 cycles, insn;

	err = kernel_cpumcf_begin();
	if (err)
		goto out_busy;

	state = 0;
	ctr_set_enable(&state, CPUMF_CTR_SET_BASIC);
	ctr_set_start(&state, CPUMF_CTR_SET_BASIC);

	err = lcctl(state);
	if (err)
		goto ;

	/* ... do your work ... */

	ctr_set_stop(&state, CPUMF_CTR_SET_BASIC);
	err = lcctl(state);
	if (err)
		goto out;

	cycles = insn = 0;
	ecctr(0, &cycles);
	ecctr(1, &insn);

	/* ... */

	kernel_cpumcf_end();
out_busy:

Signed-off-by: Hendrik Brueckner <brueckner@linux.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>