mwait_idle_cpu_init() is only called for APs, yet MSR writes will
typically need re-doing post-S3 even for the BSP. When multiple cores /
threads are present (and to come back online) in a package, for package
scope MSRs this may be covered by APs doing the writes, but we can't rely
on that.

Signed-off-by: Jan Beulich <jbeulich@suse.com>

--- a/xen/arch/x86/acpi/power.c
+++ b/xen/arch/x86/acpi/power.c
@@ -28,6 +28,7 @@
 #include <asm/io_apic.h>
 #include <asm/irq.h>
 #include <asm/microcode.h>
+#include <asm/mwait.h>
 #include <asm/prot-key.h>
 #include <asm/spec_ctrl.h>
 #include <asm/tboot.h>
@@ -299,6 +300,7 @@ static int enter_state(u32 state)
     acpi_sleep_post(state);
     if ( hvm_cpu_up() )
         BUG();
+    mwait_idle_resume();
     cpufreq_add_cpu(0);
 
  enable_cpu:
--- a/xen/arch/x86/cpu/mwait-idle.c
+++ b/xen/arch/x86/cpu/mwait-idle.c
@@ -1680,6 +1680,28 @@ static int __init mwait_idle_probe(void)
 	return 0;
 }
 
+static void mwait_idle_cpu_tweak(unsigned int cpu)
+{
+	if (icpu->auto_demotion_disable_flags)
+		on_selected_cpus(cpumask_of(cpu), auto_demotion_disable, NULL, 1);
+
+	if (icpu->byt_auto_demotion_disable_flag)
+		on_selected_cpus(cpumask_of(cpu), byt_auto_demotion_disable, NULL, 1);
+
+	switch (icpu->c1e_promotion) {
+	case C1E_PROMOTION_DISABLE:
+		on_selected_cpus(cpumask_of(cpu), c1e_promotion_disable, NULL, 1);
+		break;
+
+	case C1E_PROMOTION_ENABLE:
+		on_selected_cpus(cpumask_of(cpu), c1e_promotion_enable, NULL, 1);
+		break;
+
+	case C1E_PROMOTION_PRESERVE:
+		break;
+	}
+}
+
 static int cf_check mwait_idle_cpu_init(
     struct notifier_block *nfb, unsigned long action, void *hcpu)
 {
@@ -1762,24 +1784,7 @@ static int cf_check mwait_idle_cpu_init(
 		dev->count++;
 	}
 
-	if (icpu->auto_demotion_disable_flags)
-		on_selected_cpus(cpumask_of(cpu), auto_demotion_disable, NULL, 1);
-
-	if (icpu->byt_auto_demotion_disable_flag)
-		on_selected_cpus(cpumask_of(cpu), byt_auto_demotion_disable, NULL, 1);
-
-	switch (icpu->c1e_promotion) {
-	case C1E_PROMOTION_DISABLE:
-		on_selected_cpus(cpumask_of(cpu), c1e_promotion_disable, NULL, 1);
-		break;
-
-	case C1E_PROMOTION_ENABLE:
-		on_selected_cpus(cpumask_of(cpu), c1e_promotion_enable, NULL, 1);
-		break;
-
-	case C1E_PROMOTION_PRESERVE:
-		break;
-	}
+	mwait_idle_cpu_tweak(cpu);
 
 	return NOTIFY_DONE;
 }
@@ -1811,6 +1816,14 @@ int __init mwait_idle_init(struct notifi
 	return err;
 }
 
+void mwait_idle_resume(void)
+{
+	if (!icpu)
+		return;
+
+	mwait_idle_cpu_tweak(smp_processor_id());
+}
+
 /* Helper function for HPET. */
 bool __init mwait_pc10_supported(void)
 {
--- a/xen/arch/x86/include/asm/mwait.h
+++ b/xen/arch/x86/include/asm/mwait.h
@@ -14,9 +14,12 @@
 #define MWAIT_ECX_INTERRUPT_BREAK	0x1
 
 void mwait_idle_with_hints(unsigned int eax, unsigned int ecx);
+
 #ifdef CONFIG_INTEL
+void mwait_idle_resume(void);
 bool mwait_pc10_supported(void);
 #else
+static inline void mwait_idle_resume(void) {}
 static inline bool mwait_pc10_supported(void)
 {
     return false;