diff options
Diffstat (limited to 'Documentation/admin-guide/pm/intel_idle.rst')
| -rw-r--r-- | Documentation/admin-guide/pm/intel_idle.rst | 33 |
1 files changed, 26 insertions, 7 deletions
diff --git a/Documentation/admin-guide/pm/intel_idle.rst b/Documentation/admin-guide/pm/intel_idle.rst index 89309e1b0e48..39bd6ecce7de 100644 --- a/Documentation/admin-guide/pm/intel_idle.rst +++ b/Documentation/admin-guide/pm/intel_idle.rst @@ -20,8 +20,8 @@ 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 :doc:`cpuidle` if you -have not done that yet.] +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 @@ -53,7 +53,8 @@ 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 :doc:`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 @@ -98,7 +99,8 @@ 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 :doc:`cpuidle`). This basically means that +: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). @@ -168,7 +170,7 @@ 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 four module parameters recognized by ``intel_idle`` +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). @@ -186,7 +188,8 @@ 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 :doc:`cpuidle`). +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`` @@ -202,7 +205,8 @@ 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 :doc:`cpuidle`). +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 @@ -212,6 +216,21 @@ 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: |