x86/smpboot: Enable split CPU startup

The x86 CPU bringup state currently does AP wake-up, wait for AP to
respond and then release it for full bringup.

It is safe to be split into a wake-up and and a separate wait+release
state.

Provide the required functions and enable the split CPU bringup, which
prepares for parallel bringup, where the bringup of the non-boot CPUs takes
two iterations: One to prepare and wake all APs and the second to wait and
release them. Depending on timing this can eliminate the wait time
completely.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Michael Kelley <mikelley@microsoft.com>
Tested-by: Oleksandr Natalenko <oleksandr@natalenko.name>
Tested-by: Helge Deller <deller@gmx.de> # parisc
Tested-by: Guilherme G. Piccoli <gpiccoli@igalia.com> # Steam Deck
Link: https://lore.kernel.org/r/20230512205257.133453992@linutronix.de

arch/x86/Kconfig

@@ -274,8 +274,8 @@ config X86
 	select HAVE_UNSTABLE_SCHED_CLOCK
 	select HAVE_USER_RETURN_NOTIFIER
 	select HAVE_GENERIC_VDSO
-	select HOTPLUG_CORE_SYNC_FULL		if SMP
 	select HOTPLUG_SMT			if SMP
+	select HOTPLUG_SPLIT_STARTUP		if SMP
 	select IRQ_FORCED_THREADING
 	select NEED_PER_CPU_EMBED_FIRST_CHUNK
 	select NEED_PER_CPU_PAGE_FIRST_CHUNK

arch/x86/include/asm/smp.h

@@ -40,7 +40,7 @@ struct smp_ops {
 
 	void (*cleanup_dead_cpu)(unsigned cpu);
 	void (*poll_sync_state)(void);
-	int (*cpu_up)(unsigned cpu, struct task_struct *tidle);
+	int (*kick_ap_alive)(unsigned cpu, struct task_struct *tidle);
 	int (*cpu_disable)(void);
 	void (*cpu_die)(unsigned int cpu);
 	void (*play_dead)(void);
@@ -80,11 +80,6 @@ static inline void smp_cpus_done(unsigned int max_cpus)
 	smp_ops.smp_cpus_done(max_cpus);
 }
 
-static inline int __cpu_up(unsigned int cpu, struct task_struct *tidle)
-{
-	return smp_ops.cpu_up(cpu, tidle);
-}
-
 static inline int __cpu_disable(void)
 {
 	return smp_ops.cpu_disable();
@@ -124,7 +119,7 @@ void native_smp_prepare_cpus(unsigned int max_cpus);
 void calculate_max_logical_packages(void);
 void native_smp_cpus_done(unsigned int max_cpus);
 int common_cpu_up(unsigned int cpunum, struct task_struct *tidle);
-int native_cpu_up(unsigned int cpunum, struct task_struct *tidle);
+int native_kick_ap(unsigned int cpu, struct task_struct *tidle);
 int native_cpu_disable(void);
 void __noreturn hlt_play_dead(void);
 void native_play_dead(void);

arch/x86/kernel/smp.c

@@ -268,7 +268,7 @@ struct smp_ops smp_ops = {
 #endif
 	.smp_send_reschedule	= native_smp_send_reschedule,
 
-	.cpu_up			= native_cpu_up,
+	.kick_ap_alive		= native_kick_ap,
 	.cpu_disable		= native_cpu_disable,
 	.play_dead		= native_play_dead,
 

arch/x86/kernel/smpboot.c

@@ -1052,7 +1052,7 @@ static int do_boot_cpu(int apicid, int cpu, struct task_struct *idle)
 	return ret;
 }
 
-static int native_kick_ap(unsigned int cpu, struct task_struct *tidle)
+int native_kick_ap(unsigned int cpu, struct task_struct *tidle)
 {
 	int apicid = apic->cpu_present_to_apicid(cpu);
 	int err;
@@ -1088,15 +1088,15 @@ static int native_kick_ap(unsigned int cpu, struct task_struct *tidle)
 	return err;
 }
 
-int native_cpu_up(unsigned int cpu, struct task_struct *tidle)
+int arch_cpuhp_kick_ap_alive(unsigned int cpu, struct task_struct *tidle)
 {
-	return native_kick_ap(cpu, tidle);
+	return smp_ops.kick_ap_alive(cpu, tidle);
 }
 
 void arch_cpuhp_cleanup_kick_cpu(unsigned int cpu)
 {
 	/* Cleanup possible dangling ends... */
-	if (smp_ops.cpu_up == native_cpu_up && x86_platform.legacy.warm_reset)
+	if (smp_ops.kick_ap_alive == native_kick_ap && x86_platform.legacy.warm_reset)
 		smpboot_restore_warm_reset_vector();
 }
 

arch/x86/xen/smp_pv.c

@@ -314,7 +314,7 @@ cpu_initialize_context(unsigned int cpu, struct task_struct *idle)
 	return 0;
 }
 
-static int xen_pv_cpu_up(unsigned int cpu, struct task_struct *idle)
+static int xen_pv_kick_ap(unsigned int cpu, struct task_struct *idle)
 {
 	int rc;
 
@@ -438,7 +438,7 @@ static const struct smp_ops xen_smp_ops __initconst = {
 	.smp_prepare_cpus = xen_pv_smp_prepare_cpus,
 	.smp_cpus_done = xen_smp_cpus_done,
 
-	.cpu_up = xen_pv_cpu_up,
+	.kick_ap_alive = xen_pv_kick_ap,
 	.cpu_die = xen_pv_cpu_die,
 	.cleanup_dead_cpu = xen_pv_cleanup_dead_cpu,
 	.poll_sync_state = xen_pv_poll_sync_state,