==================

  Guest halt polling
  ==================
  
  The cpuidle_haltpoll driver, with the haltpoll governor, allows
  the guest vcpus to poll for a specified amount of time before
  halting.

  This provides the following benefits to host side polling:
  
  	1) The POLL flag is set while polling is performed, which allows
  	   a remote vCPU to avoid sending an IPI (and the associated
  	   cost of handling the IPI) when performing a wakeup.
  
  	2) The VM-exit cost can be avoided.
  
  The downside of guest side polling is that polling is performed
  even with other runnable tasks in the host.
  
  The basic logic as follows: A global value, guest_halt_poll_ns,
  is configured by the user, indicating the maximum amount of
  time polling is allowed. This value is fixed.
  
  Each vcpu has an adjustable guest_halt_poll_ns
  ("per-cpu guest_halt_poll_ns"), which is adjusted by the algorithm
  in response to events (explained below).
  
  Module Parameters
  =================
  
  The haltpoll governor has 5 tunable module parameters:
  
  1) guest_halt_poll_ns:

  Maximum amount of time, in nanoseconds, that polling is
  performed before halting.
  
  Default: 200000
  
  2) guest_halt_poll_shrink:

  Division factor used to shrink per-cpu guest_halt_poll_ns when
  wakeup event occurs after the global guest_halt_poll_ns.
  
  Default: 2
  
  3) guest_halt_poll_grow:

  Multiplication factor used to grow per-cpu guest_halt_poll_ns
  when event occurs after per-cpu guest_halt_poll_ns
  but before global guest_halt_poll_ns.
  
  Default: 2
  
  4) guest_halt_poll_grow_start:

  The per-cpu guest_halt_poll_ns eventually reaches zero
  in case of an idle system. This value sets the initial
  per-cpu guest_halt_poll_ns when growing. This can
  be increased from 10000, to avoid misses during the initial
  growth stage:
  
  10k, 20k, 40k, ... (example assumes guest_halt_poll_grow=2).
  
  Default: 50000
  
  5) guest_halt_poll_allow_shrink:
  
  Bool parameter which allows shrinking. Set to N
  to avoid it (per-cpu guest_halt_poll_ns will remain
  high once achieves global guest_halt_poll_ns value).
  
  Default: Y

  The module parameters can be set from the debugfs files in::

  
  	/sys/module/haltpoll/parameters/
  
  Further Notes
  =============
  
  - Care should be taken when setting the guest_halt_poll_ns parameter as a

    large value has the potential to drive the cpu usage to 100% on a machine
    which would be almost entirely idle otherwise.