[PATCH v6 4/4] intel_idle: Add ibrs_off module parameter to force disable IBRS

Waiman Long
2023-07-27T18:48:07Z

Commit bf5835bcdb96 ("intel_idle: Disable IBRS during long idle")
disables IBRS when the cstate is 6 or lower. However, there are
some use cases where a customer may want to use max_cstate=1 to
lower latency. Such use cases will suffer from the performance
degradation caused by the enabling of IBRS in the sibling idle thread.
Add a "ibrs_off" module parameter to force disable IBRS and the
CPUIDLE_FLAG_IRQ_ENABLE flag if set.

In the case of a Skylake server with max_cstate=1, this new ibrs_off
option will likely increase the IRQ response latency as IRQ will now
be disabled.

When running SPECjbb2015 with cstates set to C1 on a Skylake system.

First test when the kernel is booted with: "intel_idle.ibrs_off"
max-jOPS = 117828, critical-jOPS = 66047

Then retest when the kernel is booted without the "intel_idle.ibrs_off"
added.
max-jOPS = 116408, critical-jOPS = 58958

That means booting with "intel_idle.ibrs_off" improves performance by:
max-jOPS: 1.2%, which could be considered noise range.
critical-jOPS: 12%, which is definitely a solid improvement.

The admin-guide/pm/intel_idle.rst file is updated to add a description
about the new "ibrs_off" module parameter.

Signed-off-by: Waiman Long <longman@redhat.com>
Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
---
Documentation/admin-guide/pm/intel_idle.rst | 17 ++++++++++++++++-
drivers/idle/intel_idle.c | 11 ++++++++++-
2 files changed, 26 insertions(+), 2 deletions(-)

diff --git a/Documentation/admin-guide/pm/intel_idle.rst b/Documentation/admin-guide/pm/intel_idle.rst
index b799a43da62e..39bd6ecce7de 100644
--- a/Documentation/admin-guide/pm/intel_idle.rst
+++ b/Documentation/admin-guide/pm/intel_idle.rst
@@ -170,7 +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).

@@ -216,6 +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:

diff --git a/drivers/idle/intel_idle.c b/drivers/idle/intel_idle.c
index d286f31a5b96..18fa47370bde 100644
--- a/drivers/idle/intel_idle.c
+++ b/drivers/idle/intel_idle.c
@@ -68,6 +68,7 @@ static int max_cstate = CPUIDLE_STATE_MAX - 1;
static unsigned int disabled_states_mask __read_mostly;
static unsigned int preferred_states_mask __read_mostly;
static bool force_irq_on __read_mostly;
+static bool ibrs_off __read_mostly;

static struct cpuidle_device __percpu *intel_idle_cpuidle_devices;

@@ -1852,11 +1853,13 @@ static void state_update_enter_method(struct cpuidle_state *state, int cstate)
}

if (cpu_feature_enabled(X86_FEATURE_KERNEL_IBRS) &&
- state->flags & CPUIDLE_FLAG_IBRS) {
+ ((state->flags & CPUIDLE_FLAG_IBRS) || ibrs_off)) {
/*
* IBRS mitigation requires that C-states are entered
* with interrupts disabled.
*/
+ if (ibrs_off && (state->flags & CPUIDLE_FLAG_IRQ_ENABLE))
+ state->flags &= ~CPUIDLE_FLAG_IRQ_ENABLE;
WARN_ON_ONCE(state->flags & CPUIDLE_FLAG_IRQ_ENABLE);
state->enter = intel_idle_ibrs;
return;
@@ -2175,3 +2178,9 @@ MODULE_PARM_DESC(preferred_cstates, "Mask of preferred idle states");
* 'CPUIDLE_FLAG_INIT_XSTATE' and 'CPUIDLE_FLAG_IBRS' flags.
*/
module_param(force_irq_on, bool, 0444);
+/*
+ * Force the disabling of IBRS when X86_FEATURE_KERNEL_IBRS is on and
+ * CPUIDLE_FLAG_IRQ_ENABLE isn't set.
+ */
+module_param(ibrs_off, bool, 0444);
+MODULE_PARM_DESC(ibrs_off, "Disable IBRS when idle");
--
2.31.1