kernel/Documentation/admin-guide/pm/intel_idle.rst

.. SPDX-License-Identifier: GPL-2.0
.. include:: <isonum.txt>

==============================================
``intel_idle`` CPU Idle Time Management Driver
==============================================

:Copyright: |copy| 2020 Intel Corporation

:Author: Rafael J. Wysocki <rafael.j.wysocki@intel.com>


General Information
===================

``intel_idle`` is a part of the
:doc:`CPU idle time management subsystem <cpuidle>` in the Linux kernel
(``CPUIdle``).  It is the default CPU idle time management driver for the
Nehalem and later generations of Intel processors, but the level of support for
a particular processor model in it depends on whether or not it recognizes that
processor model and may also depend on information coming from the platform
firmware.  [To understand ``intel_idle`` it is necessary to know how ``CPUIdle``
works in general, so this is the time to get familiar with
Documentation/admin-guide/pm/cpuidle.rst if you have not done that yet.]

``intel_idle`` uses the ``MWAIT`` instruction to inform the processor that the
logical CPU executing it is idle and so it may be possible to put some of the
processor's functional blocks into low-power states.  That instruction takes two
arguments (passed in the ``EAX`` and ``ECX`` registers of the target CPU), the
first of which, referred to as a *hint*, can be used by the processor to
determine what can be done (for details refer to Intel Software Developer’s
Manual [1]_).  Accordingly, ``intel_idle`` refuses to work with processors in
which the support for the ``MWAIT`` instruction has been disabled (for example,
via the platform firmware configuration menu) or which do not support that
instruction at all.

``intel_idle`` is not modular, so it cannot be unloaded, which means that the
only way to pass early-configuration-time parameters to it is via the kernel
command line.


.. _intel-idle-enumeration-of-states:

Enumeration of Idle States
==========================

Each ``MWAIT`` hint value is interpreted by the processor as a license to
reconfigure itself in a certain way in order to save energy.  The processor
configurations (with reduced power draw) resulting from that are referred to
as C-states (in the ACPI terminology) or idle states.  The list of meaningful
``MWAIT`` hint values and idle states (i.e. low-power configurations of the
processor) corresponding to them depends on the processor model and it may also
depend on the configuration of the platform.

In order to create a list of available idle states required by the ``CPUIdle``
subsystem (see :ref:`idle-states-representation` in
Documentation/admin-guide/pm/cpuidle.rst),
``intel_idle`` can use two sources of information: static tables of idle states
for different processor models included in the driver itself and the ACPI tables
of the system.  The former are always used if the processor model at hand is
recognized by ``intel_idle`` and the latter are used if that is required for
the given processor model (which is the case for all server processor models
recognized by ``intel_idle``) or if the processor model is not recognized.
[There is a module parameter that can be used to make the driver use the ACPI
tables with any processor model recognized by it; see
`below <intel-idle-parameters_>`_.]

If the ACPI tables are going to be used for building the list of available idle
states, ``intel_idle`` first looks for a ``_CST`` object under one of the ACPI
objects corresponding to the CPUs in the system (refer to the ACPI specification
[2]_ for the description of ``_CST`` and its output package).  Because the
``CPUIdle`` subsystem expects that the list of idle states supplied by the
driver will be suitable for all of the CPUs handled by it and ``intel_idle`` is
registered as the ``CPUIdle`` driver for all of the CPUs in the system, the
driver looks for the first ``_CST`` object returning at least one valid idle
state description and such that all of the idle states included in its return
package are of the FFH (Functional Fixed Hardware) type, which means that the
``MWAIT`` instruction is expected to be used to tell the processor that it can
enter one of them.  The return package of that ``_CST`` is then assumed to be
applicable to all of the other CPUs in the system and the idle state
descriptions extracted from it are stored in a preliminary list of idle states
coming from the ACPI tables.  [This step is skipped if ``intel_idle`` is
configured to ignore the ACPI tables; see `below <intel-idle-parameters_>`_.]

Next, the first (index 0) entry in the list of available idle states is
initialized to represent a "polling idle state" (a pseudo-idle state in which
the target CPU continuously fetches and executes instructions), and the
subsequent (real) idle state entries are populated as follows.

If the processor model at hand is recognized by ``intel_idle``, there is a
(static) table of idle state descriptions for it in the driver.  In that case,
the "internal" table is the primary source of information on idle states and the
information from it is copied to the final list of available idle states.  If
using the ACPI tables for the enumeration of idle states is not required
(depending on the processor model), all of the listed idle state are enabled by
default (so all of them will be taken into consideration by ``CPUIdle``
governors during CPU idle state selection).  Otherwise, some of the listed idle
states may not be enabled by default if there are no matching entries in the
preliminary list of idle states coming from the ACPI tables.  In that case user
space still can enable them later (on a per-CPU basis) with the help of
the ``disable`` idle state attribute in ``sysfs`` (see
:ref:`idle-states-representation` in
Documentation/admin-guide/pm/cpuidle.rst).  This basically means that
the idle states "known" to the driver may not be enabled by default if they have
not been exposed by the platform firmware (through the ACPI tables).

If the given processor model is not recognized by ``intel_idle``, but it
supports ``MWAIT``, the preliminary list of idle states coming from the ACPI
tables is used for building the final list that will be supplied to the
``CPUIdle`` core during driver registration.  For each idle state in that list,
the description, ``MWAIT`` hint and exit latency are copied to the corresponding
entry in the final list of idle states.  The name of the idle state represented
by it (to be returned by the ``name`` idle state attribute in ``sysfs``) is
"CX_ACPI", where X is the index of that idle state in the final list (note that
the minimum value of X is 1, because 0 is reserved for the "polling" state), and
its target residency is based on the exit latency value.  Specifically, for
C1-type idle states the exit latency value is also used as the target residency
(for compatibility with the majority of the "internal" tables of idle states for
various processor models recognized by ``intel_idle``) and for the other idle
state types (C2 and C3) the target residency value is 3 times the exit latency
(again, that is because it reflects the target residency to exit latency ratio
in the majority of cases for the processor models recognized by ``intel_idle``).
All of the idle states in the final list are enabled by default in this case.


.. _intel-idle-initialization:

Initialization
==============

The initialization of ``intel_idle`` starts with checking if the kernel command
line options forbid the use of the ``MWAIT`` instruction.  If that is the case,
an error code is returned right away.

The next step is to check whether or not the processor model is known to the
driver, which determines the idle states enumeration method (see
`above <intel-idle-enumeration-of-states_>`_), and whether or not the processor
supports ``MWAIT`` (the initialization fails if that is not the case).  Then,
the ``MWAIT`` support in the processor is enumerated through ``CPUID`` and the
driver initialization fails if the level of support is not as expected (for
example, if the total number of ``MWAIT`` substates returned is 0).

Next, if the driver is not configured to ignore the ACPI tables (see
`below <intel-idle-parameters_>`_), the idle states information provided by the
platform firmware is extracted from them.

Then, ``CPUIdle`` device objects are allocated for all CPUs and the list of
available idle states is created as explained
`above <intel-idle-enumeration-of-states_>`_.

Finally, ``intel_idle`` is registered with the help of cpuidle_register_driver()
as the ``CPUIdle`` driver for all CPUs in the system and a CPU online callback
for configuring individual CPUs is registered via cpuhp_setup_state(), which
(among other things) causes the callback routine to be invoked for all of the
CPUs present in the system at that time (each CPU executes its own instance of
the callback routine).  That routine registers a ``CPUIdle`` device for the CPU
running it (which enables the ``CPUIdle`` subsystem to operate that CPU) and
optionally performs some CPU-specific initialization actions that may be
required for the given processor model.


.. _intel-idle-parameters:

Kernel Command Line Options and Module Parameters
=================================================

The *x86* architecture support code recognizes three kernel command line
options related to CPU idle time management: ``idle=poll``, ``idle=halt``,
and ``idle=nomwait``.  If any of them is present in the kernel command line, the
``MWAIT`` instruction is not allowed to be used, so the initialization of
``intel_idle`` will fail.

Apart from that there are five module parameters recognized by ``intel_idle``
itself that can be set via the kernel command line (they cannot be updated via
sysfs, so that is the only way to change their values).

The ``max_cstate`` parameter value is the maximum idle state index in the list
of idle states supplied to the ``CPUIdle`` core during the registration of the
driver.  It is also the maximum number of regular (non-polling) idle states that
can be used by ``intel_idle``, so the enumeration of idle states is terminated
after finding that number of usable idle states (the other idle states that
potentially might have been used if ``max_cstate`` had been greater are not
taken into consideration at all).  Setting ``max_cstate`` can prevent
``intel_idle`` from exposing idle states that are regarded as "too deep" for
some reason to the ``CPUIdle`` core, but it does so by making them effectively
invisible until the system is shut down and started again which may not always
be desirable.  In practice, it is only really necessary to do that if the idle
states in question cannot be enabled during system startup, because in the
working state of the system the CPU power management quality of service (PM
QoS) feature can be used to prevent ``CPUIdle`` from touching those idle states
even if they have been enumerated (see :ref:`cpu-pm-qos` in
Documentation/admin-guide/pm/cpuidle.rst).
Setting ``max_cstate`` to 0 causes the ``intel_idle`` initialization to fail.

The ``no_acpi`` and ``use_acpi`` module parameters (recognized by ``intel_idle``
if the kernel has been configured with ACPI support) can be set to make the
driver ignore the system's ACPI tables entirely or use them for all of the
recognized processor models, respectively (they both are unset by default and
``use_acpi`` has no effect if ``no_acpi`` is set).

The value of the ``states_off`` module parameter (0 by default) represents a
list of idle states to be disabled by default in the form of a bitmask.

Namely, the positions of the bits that are set in the ``states_off`` value are
the indices of idle states to be disabled by default (as reflected by the names
of the corresponding idle state directories in ``sysfs``, :file:`state0`,
:file:`state1` ... :file:`state<i>` ..., where ``<i>`` is the index of the given
idle state; see :ref:`idle-states-representation` in
Documentation/admin-guide/pm/cpuidle.rst).

For example, if ``states_off`` is equal to 3, the driver will disable idle
states 0 and 1 by default, and if it is equal to 8, idle state 3 will be
disabled by default and so on (bit positions beyond the maximum idle state index
are ignored).

The idle states disabled this way can be enabled (on a per-CPU basis) from user
space via ``sysfs``.

The ``ibrs_off`` module parameter is a boolean flag (defaults to
false). If set, it is used to control if IBRS (Indirect Branch Restricted
Speculation) should be turned off when the CPU enters an idle state.
This flag does not affect CPUs that use Enhanced IBRS which can remain
on with little performance impact.

For some CPUs, IBRS will be selected as mitigation for Spectre v2 and Retbleed
security vulnerabilities by default.  Leaving the IBRS mode on while idling may
have a performance impact on its sibling CPU.  The IBRS mode will be turned off
by default when the CPU enters into a deep idle state, but not in some
shallower ones.  Setting the ``ibrs_off`` module parameter will force the IBRS
mode to off when the CPU is in any one of the available idle states.  This may
help performance of a sibling CPU at the expense of a slightly higher wakeup
latency for the idle CPU.


.. _intel-idle-core-and-package-idle-states:

Core and Package Levels of Idle States
======================================

Typically, in a processor supporting the ``MWAIT`` instruction there are (at
least) two levels of idle states (or C-states).  One level, referred to as
"core C-states", covers individual cores in the processor, whereas the other
level, referred to as "package C-states", covers the entire processor package
and it may also involve other components of the system (GPUs, memory
controllers, I/O hubs etc.).

Some of the ``MWAIT`` hint values allow the processor to use core C-states only
(most importantly, that is the case for the ``MWAIT`` hint value corresponding
to the ``C1`` idle state), but the majority of them give it a license to put
the target core (i.e. the core containing the logical CPU executing ``MWAIT``
with the given hint value) into a specific core C-state and then (if possible)
to enter a specific package C-state at the deeper level.  For example, the
``MWAIT`` hint value representing the ``C3`` idle state allows the processor to
put the target core into the low-power state referred to as "core ``C3``" (or
``CC3``), which happens if all of the logical CPUs (SMT siblings) in that core
have executed ``MWAIT`` with the ``C3`` hint value (or with a hint value
representing a deeper idle state), and in addition to that (in the majority of
cases) it gives the processor a license to put the entire package (possibly
including some non-CPU components such as a GPU or a memory controller) into the
low-power state referred to as "package ``C3``" (or ``PC3``), which happens if
all of the cores have gone into the ``CC3`` state and (possibly) some additional
conditions are satisfied (for instance, if the GPU is covered by ``PC3``, it may
be required to be in a certain GPU-specific low-power state for ``PC3`` to be
reachable).

As a rule, there is no simple way to make the processor use core C-states only
if the conditions for entering the corresponding package C-states are met, so
the logical CPU executing ``MWAIT`` with a hint value that is not core-level
only (like for ``C1``) must always assume that this may cause the processor to
enter a package C-state.  [That is why the exit latency and target residency
values corresponding to the majority of ``MWAIT`` hint values in the "internal"
tables of idle states in ``intel_idle`` reflect the properties of package
C-states.]  If using package C-states is not desirable at all, either
:ref:`PM QoS <cpu-pm-qos>` or the ``max_cstate`` module parameter of
``intel_idle`` described `above <intel-idle-parameters_>`_ must be used to
restrict the range of permissible idle states to the ones with core-level only
``MWAIT`` hint values (like ``C1``).


References
==========

.. [1] *Intel® 64 and IA-32 Architectures Software Developer’s Manual Volume 2B*,
       https://www.intel.com/content/www/us/en/architecture-and-technology/64-ia-32-architectures-software-developer-vol-2b-manual.html

.. [2] *Advanced Configuration and Power Interface (ACPI) Specification*,
       https://uefi.org/specifications