x86/cpu: Ensure that CPU info updates are propagated on UP

The boot sequence evaluates CPUID information twice:

  1) During early boot

  2) When finalizing the early setup right before
     mitigations are selected and alternatives are patched.

In both cases the evaluation is stored in boot_cpu_data, but on UP the
copying of boot_cpu_data to the per CPU info of the boot CPU happens
between #1 and #2. So any update which happens in #2 is never propagated to
the per CPU info instance.

Consolidate the whole logic and copy boot_cpu_data right before applying
alternatives as that's the point where boot_cpu_data is in it's final
state and not supposed to change anymore.

This also removes the voodoo mb() from smp_prepare_cpus_common() which
had absolutely no purpose.

Fixes: 71eb489 ("x86/percpu: Cure per CPU madness on UP")
Reported-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Tested-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20240322185305.127642785@linutronix.de

arch/x86/kernel/cpu/common.c

@@ -2307,6 +2307,8 @@ void arch_smt_update(void)
 
 void __init arch_cpu_finalize_init(void)
 {
+	struct cpuinfo_x86 *c = this_cpu_ptr(&cpu_info);
+
 	identify_boot_cpu();
 
 	select_idle_routine();
@@ -2345,6 +2347,13 @@ void __init arch_cpu_finalize_init(void)
 	fpu__init_system();
 	fpu__init_cpu();
 
+	/*
+	 * Ensure that access to the per CPU representation has the initial
+	 * boot CPU configuration.
+	 */
+	*c = boot_cpu_data;
+	c->initialized = true;
+
 	alternative_instructions();
 
 	if (IS_ENABLED(CONFIG_X86_64)) {

arch/x86/kernel/setup.c

@@ -1206,16 +1206,6 @@ void __init i386_reserve_resources(void)
 
 #endif /* CONFIG_X86_32 */
 
-#ifndef CONFIG_SMP
-void __init smp_prepare_boot_cpu(void)
-{
-	struct cpuinfo_x86 *c = &cpu_data(0);
-
-	*c = boot_cpu_data;
-	c->initialized = true;
-}
-#endif
-
 static struct notifier_block kernel_offset_notifier = {
 	.notifier_call = dump_kernel_offset
 };

arch/x86/kernel/smpboot.c

@@ -313,14 +313,6 @@ static void notrace start_secondary(void *unused)
 	cpu_startup_entry(CPUHP_AP_ONLINE_IDLE);
 }
 
-static void __init smp_store_boot_cpu_info(void)
-{
-	struct cpuinfo_x86 *c = &cpu_data(0);
-
-	*c = boot_cpu_data;
-	c->initialized = true;
-}
-
 /*
  * The bootstrap kernel entry code has set these up. Save them for
  * a given CPU
@@ -1039,29 +1031,15 @@ static __init void disable_smp(void)
 	cpumask_set_cpu(0, topology_die_cpumask(0));
 }
 
-static void __init smp_cpu_index_default(void)
-{
-	int i;
-	struct cpuinfo_x86 *c;
-
-	for_each_possible_cpu(i) {
-		c = &cpu_data(i);
-		/* mark all to hotplug */
-		c->cpu_index = nr_cpu_ids;
-	}
-}
-
 void __init smp_prepare_cpus_common(void)
 {
 	unsigned int i;
 
-	smp_cpu_index_default();
-
-	/*
-	 * Setup boot CPU information
-	 */
-	smp_store_boot_cpu_info(); /* Final full version of the data */
-	mb();
+	/* Mark all except the boot CPU as hotpluggable */
+	for_each_possible_cpu(i) {
+		if (i)
+			per_cpu(cpu_info.cpu_index, i) = nr_cpu_ids;
+	}
 
 	for_each_possible_cpu(i) {
 		zalloc_cpumask_var(&per_cpu(cpu_sibling_map, i), GFP_KERNEL);