x86/smpboot: Split up native_cpu_up() into separate phases and document them

There are four logical parts to what native_cpu_up() does on the BSP (or
on the controlling CPU for a later hotplug):

 1) Wake the AP by sending the INIT/SIPI/SIPI sequence.

 2) Wait for the AP to make it as far as wait_for_master_cpu() which
    sets that CPU's bit in cpu_initialized_mask, then sets the bit in
    cpu_callout_mask to let the AP proceed through cpu_init().

 3) Wait for the AP to finish cpu_init() and get as far as the
    smp_callin() call, which sets that CPU's bit in cpu_callin_mask.

 4) Perform the TSC synchronization and wait for the AP to actually
    mark itself online in cpu_online_mask.

In preparation to allow these phases to operate in parallel on multiple
APs, split them out into separate functions and document the interactions
a little more clearly in both the BP and AP code paths.

No functional change intended.
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
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/20230512205255.928917242@linutronix.de

arch/x86/kernel/smpboot.c

@@ -193,6 +193,10 @@ static void smp_callin(void)
 
 	wmb();
 
+	/*
+	 * This runs the AP through all the cpuhp states to its target
+	 * state CPUHP_ONLINE.
+	 */
 	notify_cpu_starting(cpuid);
 
 	/*
@@ -233,12 +237,28 @@ static void notrace start_secondary(void *unused)
 	load_cr3(swapper_pg_dir);
 	__flush_tlb_all();
 #endif
+	/*
+	 * Sync point with wait_cpu_initialized(). Before proceeding through
+	 * cpu_init(), the AP will call wait_for_master_cpu() which sets its
+	 * own bit in cpu_initialized_mask and then waits for the BSP to set
+	 * its bit in cpu_callout_mask to release it.
+	 */
 	cpu_init_secondary();
 	rcu_cpu_starting(raw_smp_processor_id());
 	x86_cpuinit.early_percpu_clock_init();
+
+	/*
+	 * Sync point with wait_cpu_callin(). The AP doesn't wait here
+	 * but just sets the bit to let the controlling CPU (BSP) know that
+	 * it's got this far.
+	 */
 	smp_callin();
 
-	/* Check TSC synchronization with the control CPU: */
+	/*
+	 * Check TSC synchronization with the control CPU, which will do
+	 * its part of this from wait_cpu_online(), making it an implicit
+	 * synchronization point.
+	 */
 	check_tsc_sync_target();
 
 	/*
@@ -257,6 +277,7 @@ static void notrace start_secondary(void *unused)
 	 * half valid vector space.
 	 */
 	lock_vector_lock();
+	/* Sync point with do_wait_cpu_online() */
 	set_cpu_online(smp_processor_id(), true);
 	lapic_online();
 	unlock_vector_lock();
@@ -979,17 +1000,13 @@ int common_cpu_up(unsigned int cpu, struct task_struct *idle)
 /*
  * NOTE - on most systems this is a PHYSICAL apic ID, but on multiquad
  * (ie clustered apic addressing mode), this is a LOGICAL apic ID.
- * Returns zero if CPU booted OK, else error code from
+ * Returns zero if startup was successfully sent, else error code from
  * ->wakeup_secondary_cpu.
  */
 static int do_boot_cpu(int apicid, int cpu, struct task_struct *idle)
 {
-	/* start_ip had better be page-aligned! */
 	unsigned long start_ip = real_mode_header->trampoline_start;
 
-	unsigned long boot_error = 0;
-	unsigned long timeout;
-
 #ifdef CONFIG_X86_64
 	/* If 64-bit wakeup method exists, use the 64-bit mode trampoline IP */
 	if (apic->wakeup_secondary_cpu_64)
@@ -1046,60 +1063,89 @@ static int do_boot_cpu(int apicid, int cpu, struct task_struct *idle)
 	 * - Use an INIT boot APIC message
 	 */
 	if (apic->wakeup_secondary_cpu_64)
-		boot_error = apic->wakeup_secondary_cpu_64(apicid, start_ip);
+		return apic->wakeup_secondary_cpu_64(apicid, start_ip);
 	else if (apic->wakeup_secondary_cpu)
-		boot_error = apic->wakeup_secondary_cpu(apicid, start_ip);
-	else
-		boot_error = wakeup_secondary_cpu_via_init(apicid, start_ip);
+		return apic->wakeup_secondary_cpu(apicid, start_ip);
+
+	return wakeup_secondary_cpu_via_init(apicid, start_ip);
+}
+
+static int wait_cpu_cpumask(unsigned int cpu, const struct cpumask *mask)
+{
+	unsigned long timeout;
 
-	if (!boot_error) {
 	/*
-		 * Wait 10s total for first sign of life from AP
+	 * Wait up to 10s for the CPU to report in.
 	 */
-		boot_error = -1;
 	timeout = jiffies + 10*HZ;
 	while (time_before(jiffies, timeout)) {
-			if (cpumask_test_cpu(cpu, cpu_initialized_mask)) {
-				/*
-				 * Tell AP to proceed with initialization
-				 */
-				cpumask_set_cpu(cpu, cpu_callout_mask);
-				boot_error = 0;
-				break;
-			}
+		if (cpumask_test_cpu(cpu, mask))
+			return 0;
+
 		schedule();
 	}
-	}
+	return -1;
+}
 
-	if (!boot_error) {
-		/*
-		 * Wait till AP completes initial initialization
+/*
+ * Bringup step two: Wait for the target AP to reach cpu_init_secondary()
+ * and thus wait_for_master_cpu(), then set cpu_callout_mask to allow it
+ * to proceed.  The AP will then proceed past setting its 'callin' bit
+ * and end up waiting in check_tsc_sync_target() until we reach
+ * do_wait_cpu_online() to tend to it.
  */
-		while (!cpumask_test_cpu(cpu, cpu_callin_mask)) {
+static int wait_cpu_initialized(unsigned int cpu)
+{
 	/*
-			 * Allow other tasks to run while we wait for the
-			 * AP to come online. This also gives a chance
-			 * for the MTRR work(triggered by the AP coming online)
-			 * to be completed in the stop machine context.
+	 * Wait for first sign of life from AP.
 	 */
+	if (wait_cpu_cpumask(cpu, cpu_initialized_mask))
+		return -1;
+
+	cpumask_set_cpu(cpu, cpu_callout_mask);
+	return 0;
+}
+
+/*
+ * Bringup step three: Wait for the target AP to reach smp_callin().
+ * The AP is not waiting for us here so we don't need to parallelise
+ * this step. Not entirely clear why we care about this, since we just
+ * proceed directly to TSC synchronization which is the next sync
+ * point with the AP anyway.
+ */
+static void wait_cpu_callin(unsigned int cpu)
+{
+	while (!cpumask_test_cpu(cpu, cpu_callin_mask))
 		schedule();
-		}
-	}
+}
+
+/*
+ * Bringup step four: Synchronize the TSC and wait for the target AP
+ * to reach set_cpu_online() in start_secondary().
+ */
+static void wait_cpu_online(unsigned int cpu)
+{
+	unsigned long flags;
 
-	if (x86_platform.legacy.warm_reset) {
 	/*
-		 * Cleanup possible dangling ends...
+	 * Check TSC synchronization with the AP (keep irqs disabled
+	 * while doing so):
 	 */
-		smpboot_restore_warm_reset_vector();
-	}
+	local_irq_save(flags);
+	check_tsc_sync_source(cpu);
+	local_irq_restore(flags);
 
-	return boot_error;
+	/*
+	 * Wait for the AP to mark itself online, so the core caller
+	 * can drop sparse_irq_lock.
+	 */
+	while (!cpu_online(cpu))
+		schedule();
 }
 
-int native_cpu_up(unsigned int cpu, struct task_struct *tidle)
+static int native_kick_ap(unsigned int cpu, struct task_struct *tidle)
 {
 	int apicid = apic->cpu_present_to_apicid(cpu);
-	unsigned long flags;
 	int err;
 
 	lockdep_assert_irqs_enabled();
@@ -1140,25 +1186,33 @@ int native_cpu_up(unsigned int cpu, struct task_struct *tidle)
 		return err;
 
 	err = do_boot_cpu(apicid, cpu, tidle);
-	if (err) {
+	if (err)
 		pr_err("do_boot_cpu failed(%d) to wakeup CPU#%u\n", err, cpu);
+
 	return err;
-	}
+}
 
-	/*
-	 * Check TSC synchronization with the AP (keep irqs disabled
-	 * while doing so):
-	 */
-	local_irq_save(flags);
-	check_tsc_sync_source(cpu);
-	local_irq_restore(flags);
+int native_cpu_up(unsigned int cpu, struct task_struct *tidle)
+{
+	int ret;
 
-	while (!cpu_online(cpu)) {
-		cpu_relax();
-		touch_nmi_watchdog();
-	}
+	ret = native_kick_ap(cpu, tidle);
+	if (ret)
+		goto out;
 
-	return 0;
+	ret = wait_cpu_initialized(cpu);
+	if (ret)
+		goto out;
+
+	wait_cpu_callin(cpu);
+	wait_cpu_online(cpu);
+
+out:
+	/* Cleanup possible dangling ends... */
+	if (x86_platform.legacy.warm_reset)
+		smpboot_restore_warm_reset_vector();
+
+	return ret;
 }
 
 /**