x86_energy_perf_policy(8) was created as an example
of how the user, or upper-level OS, can manage
MSR_IA32_ENERGY_PERF_BIAS (EPB).

Hardware consults EPB when it makes internal decisions
balancing energy-saving vs performance.
For example, should HW quickly or slowly
transition into and out of power-saving idles states?
Should HW quickly or slowly ramp frequency up or down
in response to demand in the turbo-frequency range?

Depending on the processor, EPB may have package, core,
or CPU thread scope.  As such, the only general policy
is to write the same value to EPB on every CPU in the system.

Recent platforms add support for Hardware Performance States (HWP).
HWP effectively extends hardware frequency control from
the opportunistic turbo-frequency range to control the entire
range of available processor frequencies.

Just as turbo-mode used EPB, HWP can use EPB to help decicde
how quickly to ramp frequency and voltage up and down
in response to changing demand.  Indeed, BDX and BDX-DE,
the first processors to support HWP, use EPB for this purpose.

Starting in SKL, HWP no longer looks to EPB for influence.
Instead, it looks in a new MSR specifically for this purpose:
IA32_HWP_REQUEST.Energy_Performance_Preference (HWP.EPP).
HWP.EPP is like EPB, except that it is specific to HWP-mode
frequency selection.  Also, HWP.EPP is defined to have
per CPU-thread scope.

Starting in SKX, IA32_HWP_REQUEST is augmented by
IA32_HWP_REQUEST_PKG -- which has the same function, but is
defined to have package-wide scope.  A new bit in IA32_HWP_REQUEST
determines if it over-rides the IA32_HWP_REQUEST_PKG or not.

Note that HWP-mode can be enabled in several ways.
The "in-band" method is for HWP to be exposed in CPUID,
and for the Linux intel_pstate driver to recognized that,
and thus enable HWP.  In this case, starting in Linux 4.10, intel_pstate
exports cpufreq sysfs attribute "energy_performance_preference"
which can be used to manage HWP.EPP.  This interface can be
used to set HWP.EPP to these values:

0 performance
128 balance_performance (default)
192 balance_power
255 power

Here, x86_energy_performance_policy is updated to use
idential strings and values as intel_pstate.

But HWP-mode may also be enabled by firmware before the OS boots,
and the OS may not be aware of HWP.  In this case, intel_pstate
is not available to provide sysfs attributes, and x86_energy_perf_policy
or a similar utility is invaluable for managing HWP.EPP, for
this utility works the same, no matter if cpufreq is enabled or not.
Signed-off-by: NLen Brown <len.brown@intel.com>