[PATCH v2 4/5] intel_idle: Add no_ibrs module parameter to force disable IBRS

[Waiman Long]

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 "no_ibrs" 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 no_ibrs
option will increase the IRQ response latency as IRQ will now be
disabled.

Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
---
 drivers/idle/intel_idle.c | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/drivers/idle/intel_idle.c b/drivers/idle/intel_idle.c
index 07fa23707b3c..366dacccc971 100644
--- a/drivers/idle/intel_idle.c
+++ b/drivers/idle/intel_idle.c
@@ -69,6 +69,7 @@ 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 no_ibrs __read_mostly;
 
 static struct cpuidle_device __percpu *intel_idle_cpuidle_devices;
 
@@ -1907,12 +1908,15 @@ static void __init intel_idle_init_cstates_icpu(struct cpuidle_driver *drv)
 			WARN_ON_ONCE(state->flags & CPUIDLE_FLAG_IRQ_ENABLE);
 			state->enter = intel_idle_xstate;
 		} else if (cpu_feature_enabled(X86_FEATURE_KERNEL_IBRS) &&
-			   state->flags & CPUIDLE_FLAG_IBRS) {
+			  ((state->flags & CPUIDLE_FLAG_IBRS) || no_ibrs)) {
 			/*
 			 * IBRS mitigation requires that C-states are entered
 			 * with interrupts disabled.
 			 */
-			WARN_ON_ONCE(state->flags & CPUIDLE_FLAG_IRQ_ENABLE);
+			if (no_ibrs && (state->flags & CPUIDLE_FLAG_IRQ_ENABLE))
+				state->flags &= ~CPUIDLE_FLAG_IRQ_ENABLE;
+			else
+				WARN_ON_ONCE(state->flags & CPUIDLE_FLAG_IRQ_ENABLE);
 			state->enter = intel_idle_ibrs;
 		} else if (state->flags & CPUIDLE_FLAG_IRQ_ENABLE) {
 			state->enter = intel_idle_irq;
@@ -2165,3 +2169,9 @@ 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(no_ibrs, bool, 0444);
+MODULE_PARM_DESC(no_ibrs, "Disable IBRS when idle");
-- 
2.31.1