x86/idle: Select idle routine only once

The idle routine selection is done on every CPU bringup operation and
has a guard in place which is effective after the first invocation,
which is a pointless exercise.

Invoke it once on the boot CPU and mark the related functions __init.
The guard check has to stay as xen_set_default_idle() runs early.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/87edcu6vaq.ffs@tglx

arch/x86/include/asm/processor.h

@@ -558,7 +558,7 @@ static inline void load_sp0(unsigned long sp0)
 
 unsigned long __get_wchan(struct task_struct *p);
 
-extern void select_idle_routine(const struct cpuinfo_x86 *c);
+extern void select_idle_routine(void);
 extern void amd_e400_c1e_apic_setup(void);
 
 extern unsigned long		boot_option_idle_override;

arch/x86/kernel/cpu/common.c

@@ -1938,8 +1938,6 @@ static void identify_cpu(struct cpuinfo_x86 *c)
 	/* Init Machine Check Exception if available. */
 	mcheck_cpu_init(c);
 
-	select_idle_routine(c);
-
 #ifdef CONFIG_NUMA
 	numa_add_cpu(smp_processor_id());
 #endif
@@ -2344,6 +2342,8 @@ void __init arch_cpu_finalize_init(void)
 {
 	identify_boot_cpu();
 
+	select_idle_routine();
+
 	/*
 	 * identify_boot_cpu() initialized SMT support information, let the
 	 * core code know.

arch/x86/kernel/process.c

@@ -853,8 +853,9 @@ void __noreturn stop_this_cpu(void *dummy)
  * Do not prefer MWAIT if MONITOR instruction has a bug or idle=nomwait
  * is passed to kernel commandline parameter.
  */
-static bool prefer_mwait_c1_over_halt(const struct cpuinfo_x86 *c)
+static __init bool prefer_mwait_c1_over_halt(void)
 {
+	const struct cpuinfo_x86 *c = &boot_cpu_data;
 	u32 eax, ebx, ecx, edx;
 
 	/* If override is enforced on the command line, fall back to HALT. */
@@ -908,18 +909,19 @@ static __cpuidle void mwait_idle(void)
 	__current_clr_polling();
 }
 
-void select_idle_routine(const struct cpuinfo_x86 *c)
+void __init select_idle_routine(void)
 {
 	if (boot_option_idle_override == IDLE_POLL) {
 		if (IS_ENABLED(CONFIG_SMP) && smp_num_siblings > 1)
 			pr_warn_once("WARNING: polling idle and HT enabled, performance may degrade\n");
 		return;
 	}
 
+	/* Required to guard against xen_set_default_idle() */
 	if (x86_idle_set())
 		return;
 
-	if (prefer_mwait_c1_over_halt(c)) {
+	if (prefer_mwait_c1_over_halt()) {
 		pr_info("using mwait in idle threads\n");
 		static_call_update(x86_idle, mwait_idle);
 	} else if (cpu_feature_enabled(X86_FEATURE_TDX_GUEST)) {