x86/topology: Remove CPU0 hotplug option

This was introduced together with commit e1c467e69040 ("x86, hotplug: Wake
up CPU0 via NMI instead of INIT, SIPI, SIPI") to eventually support
physical hotplug of CPU0:

 "We'll change this code in the future to wake up hard offlined CPU0 if
  real platform and request are available."

11 years later this has not happened and physical hotplug is not officially
supported. Remove the cruft.

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/20230512205255.715707999@linutronix.de

1 files changed, 4 insertions, 94 deletions

diff --git a/arch/x86/kernel/topology.c b/arch/x86/kernel/topology.c
index 1b83377274b8..ca004e2e4469 100644
--- a/arch/x86/kernel/topology.c
+++ b/arch/x86/kernel/topology.c
@@ -38,102 +38,12 @@
 static DEFINE_PER_CPU(struct x86_cpu, cpu_devices);
 
 #ifdef CONFIG_HOTPLUG_CPU
-
-#ifdef CONFIG_BOOTPARAM_HOTPLUG_CPU0
-static int cpu0_hotpluggable = 1;
-#else
-static int cpu0_hotpluggable;
-static int __init enable_cpu0_hotplug(char *str)
-{
-	cpu0_hotpluggable = 1;
-	return 1;
-}
-
-__setup("cpu0_hotplug", enable_cpu0_hotplug);
-#endif
-
-#ifdef CONFIG_DEBUG_HOTPLUG_CPU0
-/*
- * This function offlines a CPU as early as possible and allows userspace to
- * boot up without the CPU. The CPU can be onlined back by user after boot.
- *
- * This is only called for debugging CPU offline/online feature.
- */
-int _debug_hotplug_cpu(int cpu, int action)
+int arch_register_cpu(int cpu)
 {
-	int ret;
-
-	if (!cpu_is_hotpluggable(cpu))
-		return -EINVAL;
+	struct x86_cpu *xc = per_cpu_ptr(&cpu_devices, cpu);
 
-	switch (action) {
-	case 0:
-		ret = remove_cpu(cpu);
-		if (!ret)
-			pr_info("DEBUG_HOTPLUG_CPU0: CPU %u is now offline\n", cpu);
-		else
-			pr_debug("Can't offline CPU%d.\n", cpu);
-		break;
-	case 1:
-		ret = add_cpu(cpu);
-		if (ret)
-			pr_debug("Can't online CPU%d.\n", cpu);
-
-		break;
-	default:
-		ret = -EINVAL;
-	}
-
-	return ret;
-}
-
-static int __init debug_hotplug_cpu(void)
-{
-	_debug_hotplug_cpu(0, 0);
-	return 0;
-}
-
-late_initcall_sync(debug_hotplug_cpu);
-#endif /* CONFIG_DEBUG_HOTPLUG_CPU0 */
-
-int arch_register_cpu(int num)
-{
-	struct cpuinfo_x86 *c = &cpu_data(num);
-
-	/*
-	 * Currently CPU0 is only hotpluggable on Intel platforms. Other
-	 * vendors can add hotplug support later.
-	 * Xen PV guests don't support CPU0 hotplug at all.
-	 */
-	if (c->x86_vendor != X86_VENDOR_INTEL ||
-	    cpu_feature_enabled(X86_FEATURE_XENPV))
-		cpu0_hotpluggable = 0;
-
-	/*
-	 * Two known BSP/CPU0 dependencies: Resume from suspend/hibernate
-	 * depends on BSP. PIC interrupts depend on BSP.
-	 *
-	 * If the BSP dependencies are under control, one can tell kernel to
-	 * enable BSP hotplug. This basically adds a control file and
-	 * one can attempt to offline BSP.
-	 */
-	if (num == 0 && cpu0_hotpluggable) {
-		unsigned int irq;
-		/*
-		 * We won't take down the boot processor on i386 if some
-		 * interrupts only are able to be serviced by the BSP in PIC.
-		 */
-		for_each_active_irq(irq) {
-			if (!IO_APIC_IRQ(irq) && irq_has_action(irq)) {
-				cpu0_hotpluggable = 0;
-				break;
-			}
-		}
-	}
-	if (num || cpu0_hotpluggable)
-		per_cpu(cpu_devices, num).cpu.hotpluggable = 1;
-
-	return register_cpu(&per_cpu(cpu_devices, num).cpu, num);
+	xc->cpu.hotpluggable = cpu > 0;
+	return register_cpu(&xc->cpu, cpu);
 }
 EXPORT_SYMBOL(arch_register_cpu);