Changes after perfctr-2.6.0:
- See if we can work around the PowerPC PMI/DECR/TAU erratum.
- Automate setting of usr.lib/Makefile:SO_LIBVER !
- Install perfex with dynamically linked libperfctr.so.
- Fix vperfctr_reset_sampling_timer() to be cpu_khz-dependent.
  Assuming 32-bit sampling precision and max sustained rate of
  one event per core clock, then set the sampling timer to a
  value N such that N < (2^32 * HZ) / (cpu_khz * 1000).
- Convert NR_CPUS arrays to per_cpu().

Changes after perfctr-2.7:
- Implement a patch-less version of the driver. Insert a glue module
  that hooks into the kernel via code backpatching and symbol table
  information. Afterwards, the driver module proper can interface with
  the glue module for the kernel callbacks, IDT, and irq return path.
  This requires the /proc/<pid>/perfctr removal in perfctr-2.7.

Possible Changes:
- Change NR_CPUS loops to use the 2.6.2-rc1+ for_each_cpu() macro?
- Increase [18] arrays in i386 to [24] or [32]? Just in case...
- SuSE kernels don't use EXTRAVERSION. Fix update-kernel to use
  rpm to identify the exact version instead.
- Pull out common parts of *_check_control() and do those once.
- Consider specialised read_counter() procedures for different uses.
- Investigate the feasibility of adding back inheritance support.
- When an overflown perfctr is reset, we should take into account
  how many events past 0 or 1 it is at.
- Implement gethrvtime(). Don't ever STOP the counters. To stop PMC
  updates, call CONTROL with tsc_on == 1 and nractrs == nrictrs == 0.
  The driver will continue sampling the TSC. Then gethrvtime() reduces
  to scaling the virtualised TSC with cpu_khz.
- Describe derived events in event_set.c.
- Write some documentation :-(
- The P6 and P4 sub-models don't matter for the driver. Should the driver
  just export the major model and the cpuid, and let user-space figure
  out sub-model details?
- Access control mechanism for global-mode perfctrs?
- Interrupt support for global-mode perfctrs?
- Multiplexing support? PAPI seems to do fine w/o it.
- A "kernel profiling" mode which uses global-mode perfctrs in
  interrupt mode to profile the kernel?
- Buffer interrupts and signal user-space when buffer is nearly full?