[PATCH] x86: intel_epb: Add earlyparam option to keep bias at performance

[Jack Allister <jalliste@amazon.com>]

There are certain scenarios where it may be intentional that the EPB was
set at to 0/ENERGY_PERF_BIAS_PERFORMANCE on kernel boot. For example, in
data centers a kexec/live-update of the kernel may be performed regularly.

Usually this live-update is time critical and defaulting of the bias back
to ENERGY_PERF_BIAS_NORMAL may actually be detrimental to the overall
update time if processors' time to ramp up/boost are affected.

This patch introduces a kernel command line "intel_epb_keep_performance"
which will leave the EPB at performance if during the restoration code path
it is detected as such.

Signed-off-by: Jack Allister <jalliste@amazon.com>
Cc: Paul Durrant <pdurrant@amazon.com>
Cc: Jue Wang <juew@amazon.com>
Cc: Usama Arif <usama.arif@bytedance.com>
---
 arch/x86/kernel/cpu/intel_epb.c | 19 ++++++++++++++++---
 1 file changed, 16 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kernel/cpu/intel_epb.c b/arch/x86/kernel/cpu/intel_epb.c
index e4c3ba91321c..0c7dd092f723 100644
--- a/arch/x86/kernel/cpu/intel_epb.c
+++ b/arch/x86/kernel/cpu/intel_epb.c
@@ -50,7 +50,8 @@
  * the OS will do that anyway.  That sometimes is problematic, as it may cause
  * the system battery to drain too fast, for example, so it is better to adjust
  * it on CPU bring-up and if the initial EPB value for a given CPU is 0, the
- * kernel changes it to 6 ('normal').
+ * kernel changes it to 6 ('normal'). This however is overridable via
+ * intel_epb_keep_performance if required.
  */
 
 static DEFINE_PER_CPU(u8, saved_epb);
@@ -75,6 +76,8 @@ static u8 energ_perf_values[] = {
 	[EPB_INDEX_POWERSAVE] = ENERGY_PERF_BIAS_POWERSAVE,
 };
 
+static bool intel_epb_keep_performance __read_mostly;
+
 static int intel_epb_save(void)
 {
 	u64 epb;
@@ -107,8 +110,12 @@ static void intel_epb_restore(void)
 		 */
 		val = epb & EPB_MASK;
 		if (val == ENERGY_PERF_BIAS_PERFORMANCE) {
-			val = energ_perf_values[EPB_INDEX_NORMAL];
-			pr_warn_once("ENERGY_PERF_BIAS: Set to 'normal', was 'performance'\n");
+			if (!intel_epb_keep_performance) {
+				val = energ_perf_values[EPB_INDEX_NORMAL];
+				pr_warn_once("ENERGY_PERF_BIAS: Set to 'normal', was 'performance'\n");
+			} else {
+				pr_warn_once("ENERGY_PERF_BIAS: Kept at 'performance', no change\n");
+			}
 		}
 	}
 	wrmsrl(MSR_IA32_ENERGY_PERF_BIAS, (epb & ~EPB_MASK) | val);
@@ -213,6 +220,12 @@ static const struct x86_cpu_id intel_epb_normal[] = {
 	{}
 };
 
+static __init int intel_epb_keep_performance_setup(char *str)
+{
+	return kstrtobool(str, &intel_epb_keep_performance);
+}
+early_param("intel_epb_keep_performance", intel_epb_keep_performance_setup);
+
 static __init int intel_epb_init(void)
 {
 	const struct x86_cpu_id *id = x86_match_cpu(intel_epb_normal);
-- 
2.40.1