Merge branch 'for-next/vcpu-hotplug' into for-next/core

* for-next/vcpu-hotplug: (21 commits)
  : arm64 support for virtual CPU hotplug (ACPI)
  irqchip/gic-v3: Fix 'broken_rdists' unused warning when !SMP and !ACPI
  arm64: Kconfig: Fix dependencies to enable ACPI_HOTPLUG_CPU
  cpumask: Add enabled cpumask for present CPUs that can be brought online
  arm64: document virtual CPU hotplug's expectations
  arm64: Kconfig: Enable hotplug CPU on arm64 if ACPI_PROCESSOR is enabled.
  arm64: arch_register_cpu() variant to check if an ACPI handle is now available.
  arm64: psci: Ignore DENIED CPUs
  irqchip/gic-v3: Add support for ACPI's disabled but 'online capable' CPUs
  irqchip/gic-v3: Don't return errors from gic_acpi_match_gicc()
  arm64: acpi: Harden get_cpu_for_acpi_id() against missing CPU entry
  arm64: acpi: Move get_cpu_for_acpi_id() to a header
  ACPI: Add post_eject to struct acpi_scan_handler for cpu hotplug
  ACPI: scan: switch to flags for acpi_scan_check_and_detach()
  ACPI: processor: Register deferred CPUs from acpi_processor_get_info()
  ACPI: processor: Add acpi_get_processor_handle() helper
  ACPI: processor: Move checks and availability of acpi_processor earlier
  ACPI: processor: Fix memory leaks in error paths of processor_add()
  ACPI: processor: Return an error if acpi_processor_get_info() fails in processor_add()
  ACPI: processor: Drop duplicated check on _STA (enabled + present)
  cpu: Do not warn on arch_register_cpu() returning -EPROBE_DEFER
  ...

Documentation/ABI/testing/sysfs-devices-system-cpu

@@ -694,3 +694,9 @@ Description:
 		(RO) indicates whether or not the kernel directly supports
 		modifying the crash elfcorehdr for CPU hot un/plug and/or
 		on/offline changes.
+
+What:		/sys/devices/system/cpu/enabled
+Date:		Nov 2022
+Contact:	Linux kernel mailing list <linux-kernel@vger.kernel.org>
+Description:
+		(RO) the list of CPUs that can be brought online.

Documentation/arch/arm64/cpu-hotplug.rst

+.. SPDX-License-Identifier: GPL-2.0
+.. _cpuhp_index:
+
+====================
+CPU Hotplug and ACPI
+====================
+
+CPU hotplug in the arm64 world is commonly used to describe the kernel taking
+CPUs online/offline using PSCI. This document is about ACPI firmware allowing
+CPUs that were not available during boot to be added to the system later.
+
+``possible`` and ``present`` refer to the state of the CPU as seen by linux.
+
+
+CPU Hotplug on physical systems - CPUs not present at boot
+----------------------------------------------------------
+
+Physical systems need to mark a CPU that is ``possible`` but not ``present`` as
+being ``present``. An example would be a dual socket machine, where the package
+in one of the sockets can be replaced while the system is running.
+
+This is not supported.
+
+In the arm64 world CPUs are not a single device but a slice of the system.
+There are no systems that support the physical addition (or removal) of CPUs
+while the system is running, and ACPI is not able to sufficiently describe
+them.
+
+e.g. New CPUs come with new caches, but the platform's cache toplogy is
+described in a static table, the PPTT. How caches are shared between CPUs is
+not discoverable, and must be described by firmware.
+
+e.g. The GIC redistributor for each CPU must be accessed by the driver during
+boot to discover the system wide supported features. ACPI's MADT GICC
+structures can describe a redistributor associated with a disabled CPU, but
+can't describe whether the redistributor is accessible, only that it is not
+'always on'.
+
+arm64's ACPI tables assume that everything described is ``present``.
+
+
+CPU Hotplug on virtual systems - CPUs not enabled at boot
+---------------------------------------------------------
+
+Virtual systems have the advantage that all the properties the system will
+ever have can be described at boot. There are no power-domain considerations
+as such devices are emulated.
+
+CPU Hotplug on virtual systems is supported. It is distinct from physical
+CPU Hotplug as all resources are described as ``present``, but CPUs may be
+marked as disabled by firmware. Only the CPU's online/offline behaviour is
+influenced by firmware. An example is where a virtual machine boots with a
+single CPU, and additional CPUs are added once a cloud orchestrator deploys
+the workload.
+
+For a virtual machine, the VMM (e.g. Qemu) plays the part of firmware.
+
+Virtual hotplug is implemented as a firmware policy affecting which CPUs can be
+brought online. Firmware can enforce its policy via PSCI's return codes. e.g.
+``DENIED``.
+
+The ACPI tables must describe all the resources of the virtual machine. CPUs
+that firmware wishes to disable either from boot (or later) should not be
+``enabled`` in the MADT GICC structures, but should have the ``online capable``
+bit set, to indicate they can be enabled later. The boot CPU must be marked as
+``enabled``.  The 'always on' GICR structure must be used to describe the
+redistributors.
+
+CPUs described as ``online capable`` but not ``enabled`` can be set to enabled
+by the DSDT's Processor object's _STA method. On virtual systems the _STA method
+must always report the CPU as ``present``. Changes to the firmware policy can
+be notified to the OS via device-check or eject-request.
+
+CPUs described as ``enabled`` in the static table, should not have their _STA
+modified dynamically by firmware. Soft-restart features such as kexec will
+re-read the static properties of the system from these static tables, and
+may malfunction if these no longer describe the running system. Linux will
+re-discover the dynamic properties of the system from the _STA method later
+during boot.

Documentation/arch/arm64/index.rst

@@ -13,6 +13,7 @@ ARM64 Architecture
     asymmetric-32bit
     booting
     cpu-feature-registers
+    cpu-hotplug
     elf_hwcaps
     hugetlbpage
     kdump

arch/arm64/Kconfig

@@ -5,6 +5,7 @@ config ARM64
 	select ACPI_CCA_REQUIRED if ACPI
 	select ACPI_GENERIC_GSI if ACPI
 	select ACPI_GTDT if ACPI
+	select ACPI_HOTPLUG_CPU if ACPI_PROCESSOR && HOTPLUG_CPU
 	select ACPI_IORT if ACPI
 	select ACPI_REDUCED_HARDWARE_ONLY if ACPI
 	select ACPI_MCFG if (ACPI && PCI)

arch/arm64/include/asm/acpi.h

@@ -119,6 +119,18 @@ static inline u32 get_acpi_id_for_cpu(unsigned int cpu)
 	return	acpi_cpu_get_madt_gicc(cpu)->uid;
 }
 
+static inline int get_cpu_for_acpi_id(u32 uid)
+{
+	int cpu;
+
+	for (cpu = 0; cpu < nr_cpu_ids; cpu++)
+		if (acpi_cpu_get_madt_gicc(cpu) &&
+		    uid == get_acpi_id_for_cpu(cpu))
+			return cpu;
+
+	return -EINVAL;
+}
+
 static inline void arch_fix_phys_package_id(int num, u32 slot) { }
 void __init acpi_init_cpus(void);
 int apei_claim_sea(struct pt_regs *regs);

arch/arm64/kernel/acpi.c

@@ -30,6 +30,7 @@
 #include <linux/pgtable.h>
 
 #include <acpi/ghes.h>
+#include <acpi/processor.h>
 #include <asm/cputype.h>
 #include <asm/cpu_ops.h>
 #include <asm/daifflags.h>
@@ -438,3 +439,24 @@ void arch_reserve_mem_area(acpi_physical_address addr, size_t size)
 {
 	memblock_mark_nomap(addr, size);
 }
+
+#ifdef CONFIG_ACPI_HOTPLUG_CPU
+int acpi_map_cpu(acpi_handle handle, phys_cpuid_t physid, u32 apci_id,
+		 int *pcpu)
+{
+	/* If an error code is passed in this stub can't fix it */
+	if (*pcpu < 0) {
+		pr_warn_once("Unable to map CPU to valid ID\n");
+		return *pcpu;
+	}
+
+	return 0;
+}
+EXPORT_SYMBOL(acpi_map_cpu);
+
+int acpi_unmap_cpu(int cpu)
+{
+	return 0;
+}
+EXPORT_SYMBOL(acpi_unmap_cpu);
+#endif /* CONFIG_ACPI_HOTPLUG_CPU */

arch/arm64/kernel/acpi_numa.c

@@ -34,17 +34,6 @@ int __init acpi_numa_get_nid(unsigned int cpu)
 	return acpi_early_node_map[cpu];
 }
 
-static inline int get_cpu_for_acpi_id(u32 uid)
-{
-	int cpu;
-
-	for (cpu = 0; cpu < nr_cpu_ids; cpu++)
-		if (uid == get_acpi_id_for_cpu(cpu))
-			return cpu;
-
-	return -EINVAL;
-}
-
 static int __init acpi_parse_gicc_pxm(union acpi_subtable_headers *header,
 				      const unsigned long end)
 {

arch/arm64/kernel/psci.c

@@ -40,7 +40,7 @@ static int cpu_psci_cpu_boot(unsigned int cpu)
 {
 	phys_addr_t pa_secondary_entry = __pa_symbol(secondary_entry);
 	int err = psci_ops.cpu_on(cpu_logical_map(cpu), pa_secondary_entry);
-	if (err)
+	if (err && err != -EPERM)
 		pr_err("failed to boot CPU%d (%d)\n", cpu, err);
 
 	return err;

arch/arm64/kernel/smp.c

@@ -129,6 +129,7 @@ int __cpu_up(unsigned int cpu, struct task_struct *idle)
 	/* Now bring the CPU into our world */
 	ret = boot_secondary(cpu, idle);
 	if (ret) {
+		if (ret != -EPERM)
 			pr_err("CPU%u: failed to boot: %d\n", cpu, ret);
 		return ret;
 	}
@@ -507,6 +508,59 @@ static int __init smp_cpu_setup(int cpu)
 static bool bootcpu_valid __initdata;
 static unsigned int cpu_count = 1;
 
+int arch_register_cpu(int cpu)
+{
+	acpi_handle acpi_handle = acpi_get_processor_handle(cpu);
+	struct cpu *c = &per_cpu(cpu_devices, cpu);
+
+	if (!acpi_disabled && !acpi_handle &&
+	    IS_ENABLED(CONFIG_ACPI_HOTPLUG_CPU))
+		return -EPROBE_DEFER;
+
+#ifdef CONFIG_ACPI_HOTPLUG_CPU
+	/* For now block anything that looks like physical CPU Hotplug */
+	if (invalid_logical_cpuid(cpu) || !cpu_present(cpu)) {
+		pr_err_once("Changing CPU present bit is not supported\n");
+		return -ENODEV;
+	}
+#endif
+
+	/*
+	 * Availability of the acpi handle is sufficient to establish
+	 * that _STA has aleady been checked. No need to recheck here.
+	 */
+	c->hotpluggable = arch_cpu_is_hotpluggable(cpu);
+
+	return register_cpu(c, cpu);
+}
+
+#ifdef CONFIG_ACPI_HOTPLUG_CPU
+void arch_unregister_cpu(int cpu)
+{
+	acpi_handle acpi_handle = acpi_get_processor_handle(cpu);
+	struct cpu *c = &per_cpu(cpu_devices, cpu);
+	acpi_status status;
+	unsigned long long sta;
+
+	if (!acpi_handle) {
+		pr_err_once("Removing a CPU without associated ACPI handle\n");
+		return;
+	}
+
+	status = acpi_evaluate_integer(acpi_handle, "_STA", NULL, &sta);
+	if (ACPI_FAILURE(status))
+		return;
+
+	/* For now do not allow anything that looks like physical CPU HP */
+	if (cpu_present(cpu) && !(sta & ACPI_STA_DEVICE_PRESENT)) {
+		pr_err_once("Changing CPU present bit is not supported\n");
+		return;
+	}
+
+	unregister_cpu(c);
+}
+#endif /* CONFIG_ACPI_HOTPLUG_CPU */
+
 #ifdef CONFIG_ACPI
 static struct acpi_madt_generic_interrupt cpu_madt_gicc[NR_CPUS];
 
@@ -527,7 +581,8 @@ acpi_map_gic_cpu_interface(struct acpi_madt_generic_interrupt *processor)
 {
 	u64 hwid = processor->arm_mpidr;
 
-	if (!acpi_gicc_is_usable(processor)) {
+	if (!(processor->flags &
+	      (ACPI_MADT_ENABLED | ACPI_MADT_GICC_ONLINE_CAPABLE))) {
 		pr_debug("skipping disabled CPU entry with 0x%llx MPIDR\n", hwid);
 		return;
 	}

drivers/acpi/acpi_processor.c

@@ -35,6 +35,17 @@ EXPORT_PER_CPU_SYMBOL(processors);
 struct acpi_processor_errata errata __read_mostly;
 EXPORT_SYMBOL_GPL(errata);
 
+acpi_handle acpi_get_processor_handle(int cpu)
+{
+	struct acpi_processor *pr;
+
+	pr = per_cpu(processors, cpu);
+	if (pr)
+		return pr->handle;
+
+	return NULL;
+}
+
 static int acpi_processor_errata_piix4(struct pci_dev *dev)
 {
 	u8 value1 = 0;
@@ -183,20 +194,44 @@ static void __init acpi_pcc_cpufreq_init(void) {}
 #endif /* CONFIG_X86 */
 
 /* Initialization */
+static DEFINE_PER_CPU(void *, processor_device_array);
+
+static int acpi_processor_set_per_cpu(struct acpi_processor *pr,
+				      struct acpi_device *device)
+{
+	BUG_ON(pr->id >= nr_cpu_ids);
+
+	/*
+	 * Buggy BIOS check.
+	 * ACPI id of processors can be reported wrongly by the BIOS.
+	 * Don't trust it blindly
+	 */
+	if (per_cpu(processor_device_array, pr->id) != NULL &&
+	    per_cpu(processor_device_array, pr->id) != device) {
+		dev_warn(&device->dev,
+			 "BIOS reported wrong ACPI id %d for the processor\n",
+			 pr->id);
+		return -EINVAL;
+	}
+	/*
+	 * processor_device_array is not cleared on errors to allow buggy BIOS
+	 * checks.
+	 */
+	per_cpu(processor_device_array, pr->id) = device;
+	per_cpu(processors, pr->id) = pr;
+
+	return 0;
+}
+
 #ifdef CONFIG_ACPI_HOTPLUG_CPU
-static int acpi_processor_hotadd_init(struct acpi_processor *pr)
+static int acpi_processor_hotadd_init(struct acpi_processor *pr,
+				      struct acpi_device *device)
 {
-	unsigned long long sta;
-	acpi_status status;
 	int ret;
 
 	if (invalid_phys_cpuid(pr->phys_id))
 		return -ENODEV;
 
-	status = acpi_evaluate_integer(pr->handle, "_STA", NULL, &sta);
-	if (ACPI_FAILURE(status) || !(sta & ACPI_STA_DEVICE_PRESENT))
-		return -ENODEV;
-
 	cpu_maps_update_begin();
 	cpus_write_lock();
 
@@ -204,19 +239,26 @@ static int acpi_processor_hotadd_init(struct acpi_processor *pr)
 	if (ret)
 		goto out;
 
+	ret = acpi_processor_set_per_cpu(pr, device);
+	if (ret) {
+		acpi_unmap_cpu(pr->id);
+		goto out;
+	}
+
 	ret = arch_register_cpu(pr->id);
 	if (ret) {
+		/* Leave the processor device array in place to detect buggy bios */
+		per_cpu(processors, pr->id) = NULL;
 		acpi_unmap_cpu(pr->id);
 		goto out;
 	}
 
 	/*
-	 * CPU got hot-added, but cpu_data is not initialized yet.  Set a flag
-	 * to delay cpu_idle/throttling initialization and do it when the CPU
-	 * gets online for the first time.
+	 * CPU got hot-added, but cpu_data is not initialized yet. Do
+	 * cpu_idle/throttling initialization when the CPU gets online for
+	 * the first time.
 	 */
 	pr_info("CPU%d has been hot-added\n", pr->id);
-	pr->flags.need_hotplug_init = 1;
 
 out:
 	cpus_write_unlock();
@@ -224,7 +266,8 @@ static int acpi_processor_hotadd_init(struct acpi_processor *pr)
 	return ret;
 }
 #else
-static inline int acpi_processor_hotadd_init(struct acpi_processor *pr)
+static inline int acpi_processor_hotadd_init(struct acpi_processor *pr,
+					     struct acpi_device *device)
 {
 	return -ENODEV;
 }
@@ -239,6 +282,7 @@ static int acpi_processor_get_info(struct acpi_device *device)
 	acpi_status status = AE_OK;
 	static int cpu0_initialized;
 	unsigned long long value;
+	int ret;
 
 	acpi_processor_errata();
 
@@ -315,19 +359,19 @@ static int acpi_processor_get_info(struct acpi_device *device)
 	}
 
 	/*
-	 *  Extra Processor objects may be enumerated on MP systems with
-	 *  less than the max # of CPUs. They should be ignored _iff
-	 *  they are physically not present.
-	 *
-	 *  NOTE: Even if the processor has a cpuid, it may not be present
-	 *  because cpuid <-> apicid mapping is persistent now.
+	 *  This code is not called unless we know the CPU is present and
+	 *  enabled. The two paths are:
+	 *  a) Initially present CPUs on architectures that do not defer
+	 *     their arch_register_cpu() calls until this point.
+	 *  b) Hotplugged CPUs (enabled bit in _STA has transitioned from not
+	 *     enabled to enabled)
 	 */
-	if (invalid_logical_cpuid(pr->id) || !cpu_present(pr->id)) {
-		int ret = acpi_processor_hotadd_init(pr);
-
+	if (!get_cpu_device(pr->id))
+		ret = acpi_processor_hotadd_init(pr, device);
+	else
+		ret = acpi_processor_set_per_cpu(pr, device);
 	if (ret)
 		return ret;
-	}
 
 	/*
 	 * On some boxes several processors use the same processor bus id.
@@ -372,8 +416,6 @@ static int acpi_processor_get_info(struct acpi_device *device)
  * (cpu_data(cpu)) values, like CPU feature flags, family, model, etc.
  * Such things have to be put in and set up by the processor driver's .probe().
  */
-static DEFINE_PER_CPU(void *, processor_device_array);
-
 static int acpi_processor_add(struct acpi_device *device,
 					const struct acpi_device_id *id)
 {
@@ -400,39 +442,17 @@ static int acpi_processor_add(struct acpi_device *device,
 
 	result = acpi_processor_get_info(device);
 	if (result) /* Processor is not physically present or unavailable */
-		return 0;
-
-	BUG_ON(pr->id >= nr_cpu_ids);
-
-	/*
-	 * Buggy BIOS check.
-	 * ACPI id of processors can be reported wrongly by the BIOS.
-	 * Don't trust it blindly
-	 */
-	if (per_cpu(processor_device_array, pr->id) != NULL &&
-	    per_cpu(processor_device_array, pr->id) != device) {
-		dev_warn(&device->dev,
-			"BIOS reported wrong ACPI id %d for the processor\n",
-			pr->id);
-		/* Give up, but do not abort the namespace scan. */
-		goto err;
-	}
-	/*
-	 * processor_device_array is not cleared on errors to allow buggy BIOS
-	 * checks.
-	 */
-	per_cpu(processor_device_array, pr->id) = device;
-	per_cpu(processors, pr->id) = pr;
+		goto err_clear_driver_data;
 
 	dev = get_cpu_device(pr->id);
 	if (!dev) {
 		result = -ENODEV;
-		goto err;
+		goto err_clear_per_cpu;
 	}
 
 	result = acpi_bind_one(dev, device);
 	if (result)
-		goto err;
+		goto err_clear_per_cpu;
 
 	pr->dev = dev;
 
@@ -443,10 +463,11 @@ static int acpi_processor_add(struct acpi_device *device,
 	dev_err(dev, "Processor driver could not be attached\n");
 	acpi_unbind_one(dev);
 
- err:
-	free_cpumask_var(pr->throttling.shared_cpu_map);
-	device->driver_data = NULL;
+ err_clear_per_cpu:
 	per_cpu(processors, pr->id) = NULL;
+ err_clear_driver_data:
+	device->driver_data = NULL;
+	free_cpumask_var(pr->throttling.shared_cpu_map);
  err_free_pr:
 	kfree(pr);
 	return result;
@@ -454,7 +475,7 @@ static int acpi_processor_add(struct acpi_device *device,
 
 #ifdef CONFIG_ACPI_HOTPLUG_CPU
 /* Removal */
-static void acpi_processor_remove(struct acpi_device *device)
+static void acpi_processor_post_eject(struct acpi_device *device)
 {
 	struct acpi_processor *pr;
 
@@ -476,10 +497,6 @@ static void acpi_processor_remove(struct acpi_device *device)
 	device_release_driver(pr->dev);
 	acpi_unbind_one(pr->dev);
 
-	/* Clean up. */
-	per_cpu(processor_device_array, pr->id) = NULL;
-	per_cpu(processors, pr->id) = NULL;
-
 	cpu_maps_update_begin();
 	cpus_write_lock();
 
@@ -487,6 +504,10 @@ static void acpi_processor_remove(struct acpi_device *device)
 	arch_unregister_cpu(pr->id);
 	acpi_unmap_cpu(pr->id);
 
+	/* Clean up. */
+	per_cpu(processor_device_array, pr->id) = NULL;
+	per_cpu(processors, pr->id) = NULL;
+
 	cpus_write_unlock();
 	cpu_maps_update_done();
 
@@ -622,7 +643,7 @@ static struct acpi_scan_handler processor_handler = {
 	.ids = processor_device_ids,
 	.attach = acpi_processor_add,
 #ifdef CONFIG_ACPI_HOTPLUG_CPU
-	.detach = acpi_processor_remove,
+	.post_eject = acpi_processor_post_eject,
 #endif
 	.hotplug = {
 		.enabled = true,

drivers/acpi/processor_core.c

@@ -90,7 +90,8 @@ static int map_gicc_mpidr(struct acpi_subtable_header *entry,
 	struct acpi_madt_generic_interrupt *gicc =
 	    container_of(entry, struct acpi_madt_generic_interrupt, header);
 
-	if (!acpi_gicc_is_usable(gicc))
+	if (!(gicc->flags &
+	      (ACPI_MADT_ENABLED | ACPI_MADT_GICC_ONLINE_CAPABLE)))
 		return -ENODEV;
 
 	/* device_declaration means Device object in DSDT, in the

drivers/acpi/processor_driver.c

@@ -33,7 +33,6 @@ MODULE_AUTHOR("Paul Diefenbaugh");
 MODULE_DESCRIPTION("ACPI Processor Driver");
 MODULE_LICENSE("GPL");
 
-static int acpi_processor_start(struct device *dev);
 static int acpi_processor_stop(struct device *dev);
 
 static const struct acpi_device_id processor_device_ids[] = {
@@ -47,7 +46,6 @@ static struct device_driver acpi_processor_driver = {
 	.name = "processor",
 	.bus = &cpu_subsys,
 	.acpi_match_table = processor_device_ids,
-	.probe = acpi_processor_start,
 	.remove = acpi_processor_stop,
 };
 
@@ -115,12 +113,9 @@ static int acpi_soft_cpu_online(unsigned int cpu)
 	 * CPU got physically hotplugged and onlined for the first time:
 	 * Initialize missing things.
 	 */
-	if (pr->flags.need_hotplug_init) {
+	if (!pr->flags.previously_online) {
 		int ret;
 
-		pr_info("Will online and init hotplugged CPU: %d\n",
-			pr->id);
-		pr->flags.need_hotplug_init = 0;
 		ret = __acpi_processor_start(device);
 		WARN(ret, "Failed to start CPU: %d\n", pr->id);
 	} else {
@@ -167,9 +162,6 @@ static int __acpi_processor_start(struct acpi_device *device)
 	if (!pr)
 		return -ENODEV;
 
-	if (pr->flags.need_hotplug_init)
-		return 0;
-
 	result = acpi_cppc_processor_probe(pr);
 	if (result && !IS_ENABLED(CONFIG_ACPI_CPU_FREQ_PSS))
 		dev_dbg(&device->dev, "CPPC data invalid or not present\n");
@@ -185,32 +177,21 @@ static int __acpi_processor_start(struct acpi_device *device)
 
 	status = acpi_install_notify_handler(device->handle, ACPI_DEVICE_NOTIFY,
 					     acpi_processor_notify, device);
-	if (ACPI_SUCCESS(status))
+	if (!ACPI_SUCCESS(status)) {
+		result = -ENODEV;
+		goto err_thermal_exit;
+	}
+	pr->flags.previously_online = 1;
+
 	return 0;
 
-	result = -ENODEV;
+err_thermal_exit:
 	acpi_processor_thermal_exit(pr, device);
-
 err_power_exit:
 	acpi_processor_power_exit(pr);
 	return result;
 }
 
-static int acpi_processor_start(struct device *dev)
-{
-	struct acpi_device *device = ACPI_COMPANION(dev);
-	int ret;
-
-	if (!device)
-		return -ENODEV;
-
-	/* Protect against concurrent CPU hotplug operations */
-	cpu_hotplug_disable();
-	ret = __acpi_processor_start(device);
-	cpu_hotplug_enable();
-	return ret;
-}
-
 static int acpi_processor_stop(struct device *dev)
 {
 	struct acpi_device *device = ACPI_COMPANION(dev);
@@ -279,7 +260,7 @@ static int __init acpi_processor_driver_init(void)
 	if (result < 0)
 		return result;
 
-	result = cpuhp_setup_state_nocalls(CPUHP_AP_ONLINE_DYN,
+	result = cpuhp_setup_state(CPUHP_AP_ONLINE_DYN,
 				   "acpi/cpu-drv:online",
 				   acpi_soft_cpu_online, NULL);
 	if (result < 0)

drivers/acpi/scan.c

@@ -243,13 +243,17 @@ static int acpi_scan_try_to_offline(struct acpi_device *device)
 	return 0;
 }
 
-static int acpi_scan_check_and_detach(struct acpi_device *adev, void *check)
+#define ACPI_SCAN_CHECK_FLAG_STATUS	BIT(0)
+#define ACPI_SCAN_CHECK_FLAG_EJECT	BIT(1)
+
+static int acpi_scan_check_and_detach(struct acpi_device *adev, void *p)
 {
 	struct acpi_scan_handler *handler = adev->handler;
+	uintptr_t flags = (uintptr_t)p;
 
-	acpi_dev_for_each_child_reverse(adev, acpi_scan_check_and_detach, check);
+	acpi_dev_for_each_child_reverse(adev, acpi_scan_check_and_detach, p);
 
-	if (check) {
+	if (flags & ACPI_SCAN_CHECK_FLAG_STATUS) {
 		acpi_bus_get_status(adev);
 		/*
 		 * Skip devices that are still there and take the enabled
@@ -269,8 +273,6 @@ static int acpi_scan_check_and_detach(struct acpi_device *adev, void *check)
 	if (handler) {
 		if (handler->detach)
 			handler->detach(adev);
-
-		adev->handler = NULL;
 	} else {
 		device_release_driver(&adev->dev);
 	}
@@ -280,6 +282,28 @@ static int acpi_scan_check_and_detach(struct acpi_device *adev, void *check)
 	 */
 	acpi_device_set_power(adev, ACPI_STATE_D3_COLD);
 	adev->flags.initialized = false;
+
+	/* For eject this is deferred to acpi_bus_post_eject() */
+	if (!(flags & ACPI_SCAN_CHECK_FLAG_EJECT)) {
+		adev->handler = NULL;
+		acpi_device_clear_enumerated(adev);
+	}
+	return 0;
+}
+
+static int acpi_bus_post_eject(struct acpi_device *adev, void *not_used)
+{
+	struct acpi_scan_handler *handler = adev->handler;
+
+	acpi_dev_for_each_child_reverse(adev, acpi_bus_post_eject, NULL);
+
+	if (handler) {
+		if (handler->post_eject)
+			handler->post_eject(adev);
+
+		adev->handler = NULL;
+	}
+
 	acpi_device_clear_enumerated(adev);
 
 	return 0;
@@ -287,7 +311,9 @@ static int acpi_scan_check_and_detach(struct acpi_device *adev, void *check)
 
 static void acpi_scan_check_subtree(struct acpi_device *adev)
 {
-	acpi_scan_check_and_detach(adev, (void *)true);
+	uintptr_t flags = ACPI_SCAN_CHECK_FLAG_STATUS;
+
+	acpi_scan_check_and_detach(adev, (void *)flags);
 }
 
 static int acpi_scan_hot_remove(struct acpi_device *device)
@@ -295,6 +321,7 @@ static int acpi_scan_hot_remove(struct acpi_device *device)
 	acpi_handle handle = device->handle;
 	unsigned long long sta;
 	acpi_status status;
+	uintptr_t flags = ACPI_SCAN_CHECK_FLAG_EJECT;
 
 	if (device->handler && device->handler->hotplug.demand_offline) {
 		if (!acpi_scan_is_offline(device, true))
@@ -307,7 +334,7 @@ static int acpi_scan_hot_remove(struct acpi_device *device)
 
 	acpi_handle_debug(handle, "Ejecting\n");
 
-	acpi_bus_trim(device);
+	acpi_scan_check_and_detach(device, (void *)flags);
 
 	acpi_evaluate_lck(handle, 0);
 	/*
@@ -330,6 +357,8 @@ static int acpi_scan_hot_remove(struct acpi_device *device)
 	} else if (sta & ACPI_STA_DEVICE_ENABLED) {
 		acpi_handle_warn(handle,
 			"Eject incomplete - status 0x%llx\n", sta);
+	} else {
+		acpi_bus_post_eject(device, NULL);
 	}
 
 	return 0;
@@ -2596,7 +2625,9 @@ EXPORT_SYMBOL(acpi_bus_scan);
  */
 void acpi_bus_trim(struct acpi_device *adev)
 {
-	acpi_scan_check_and_detach(adev, NULL);
+	uintptr_t flags = 0;
+
+	acpi_scan_check_and_detach(adev, (void *)flags);
 }
 EXPORT_SYMBOL_GPL(acpi_bus_trim);
 

drivers/base/cpu.c

@@ -95,6 +95,7 @@ void unregister_cpu(struct cpu *cpu)
 {
 	int logical_cpu = cpu->dev.id;
 
+	set_cpu_enabled(logical_cpu, false);
 	unregister_cpu_under_node(logical_cpu, cpu_to_node(logical_cpu));
 
 	device_unregister(&cpu->dev);
@@ -273,6 +274,13 @@ static ssize_t print_cpus_offline(struct device *dev,
 }
 static DEVICE_ATTR(offline, 0444, print_cpus_offline, NULL);
 
+static ssize_t print_cpus_enabled(struct device *dev,
+				  struct device_attribute *attr, char *buf)
+{
+	return sysfs_emit(buf, "%*pbl\n", cpumask_pr_args(cpu_enabled_mask));
+}
+static DEVICE_ATTR(enabled, 0444, print_cpus_enabled, NULL);
+
 static ssize_t print_cpus_isolated(struct device *dev,
 				  struct device_attribute *attr, char *buf)
 {
@@ -413,6 +421,7 @@ int register_cpu(struct cpu *cpu, int num)
 	register_cpu_under_node(num, cpu_to_node(num));
 	dev_pm_qos_expose_latency_limit(&cpu->dev,
 					PM_QOS_RESUME_LATENCY_NO_CONSTRAINT);
+	set_cpu_enabled(num, true);
 
 	return 0;
 }
@@ -494,6 +503,7 @@ static struct attribute *cpu_root_attrs[] = {
 	&cpu_attrs[2].attr.attr,
 	&dev_attr_kernel_max.attr,
 	&dev_attr_offline.attr,
+	&dev_attr_enabled.attr,
 	&dev_attr_isolated.attr,
 #ifdef CONFIG_NO_HZ_FULL
 	&dev_attr_nohz_full.attr,
@@ -558,7 +568,7 @@ static void __init cpu_dev_register_generic(void)
 
 	for_each_present_cpu(i) {
 		ret = arch_register_cpu(i);
-		if (ret)
+		if (ret && ret != -EPROBE_DEFER)
 			pr_warn("register_cpu %d failed (%d)\n", i, ret);
 	}
 }

drivers/irqchip/irq-gic-v3.c

@@ -47,6 +47,8 @@ static u8 dist_prio_nmi __ro_after_init = GICV3_PRIO_NMI;
 
 #define GIC_IRQ_TYPE_PARTITION	(GIC_IRQ_TYPE_LPI + 1)
 
+static struct cpumask broken_rdists __read_mostly __maybe_unused;
+
 struct redist_region {
 	void __iomem		*redist_base;
 	phys_addr_t		phys_base;
@@ -1317,6 +1319,18 @@ static void gic_cpu_init(void)
 #define MPIDR_TO_SGI_RS(mpidr)	(MPIDR_RS(mpidr) << ICC_SGI1R_RS_SHIFT)
 #define MPIDR_TO_SGI_CLUSTER_ID(mpidr)	((mpidr) & ~0xFUL)
 
+/*
+ * gic_starting_cpu() is called after the last point where cpuhp is allowed
+ * to fail. So pre check for problems earlier.
+ */
+static int gic_check_rdist(unsigned int cpu)
+{
+	if (cpumask_test_cpu(cpu, &broken_rdists))
+		return -EINVAL;
+
+	return 0;
+}
+
 static int gic_starting_cpu(unsigned int cpu)
 {
 	gic_cpu_init();
@@ -1408,6 +1422,10 @@ static void __init gic_smp_init(void)
 	};
 	int base_sgi;
 
+	cpuhp_setup_state_nocalls(CPUHP_BP_PREPARE_DYN,
+				  "irqchip/arm/gicv3:checkrdist",
+				  gic_check_rdist, NULL);
+
 	cpuhp_setup_state_nocalls(CPUHP_AP_IRQ_GIC_STARTING,
 				  "irqchip/arm/gicv3:starting",
 				  gic_starting_cpu, NULL);
@@ -2360,8 +2378,24 @@ gic_acpi_parse_madt_gicc(union acpi_subtable_headers *header,
 	u32 size = reg == GIC_PIDR2_ARCH_GICv4 ? SZ_64K * 4 : SZ_64K * 2;
 	void __iomem *redist_base;
 
-	if (!acpi_gicc_is_usable(gicc))
+	/* Neither enabled or online capable means it doesn't exist, skip it */
+	if (!(gicc->flags & (ACPI_MADT_ENABLED | ACPI_MADT_GICC_ONLINE_CAPABLE)))
+		return 0;
+
+	/*
+	 * Capable but disabled CPUs can be brought online later. What about
+	 * the redistributor? ACPI doesn't want to say!
+	 * Virtual hotplug systems can use the MADT's "always-on" GICR entries.
+	 * Otherwise, prevent such CPUs from being brought online.
+	 */
+	if (!(gicc->flags & ACPI_MADT_ENABLED)) {
+		int cpu = get_cpu_for_acpi_id(gicc->uid);
+
+		pr_warn("CPU %u's redistributor is inaccessible: this CPU can't be brought online\n", cpu);
+		if (cpu >= 0)
+			cpumask_set_cpu(cpu, &broken_rdists);
 		return 0;
+	}
 
 	redist_base = ioremap(gicc->gicr_base_address, size);
 	if (!redist_base)
@@ -2408,21 +2442,15 @@ static int __init gic_acpi_match_gicc(union acpi_subtable_headers *header,
 
 	/*
 	 * If GICC is enabled and has valid gicr base address, then it means
-	 * GICR base is presented via GICC
+	 * GICR base is presented via GICC. The redistributor is only known to
+	 * be accessible if the GICC is marked as enabled. If this bit is not
+	 * set, we'd need to add the redistributor at runtime, which isn't
+	 * supported.
 	 */
-	if (acpi_gicc_is_usable(gicc) && gicc->gicr_base_address) {
+	if (gicc->flags & ACPI_MADT_ENABLED && gicc->gicr_base_address)
 		acpi_data.enabled_rdists++;
-		return 0;
-	}
 
-	/*
-	 * It's perfectly valid firmware can pass disabled GICC entry, driver
-	 * should not treat as errors, skip the entry instead of probe fail.
-	 */
-	if (!acpi_gicc_is_usable(gicc))
 	return 0;
-
-	return -ENODEV;
 }
 
 static int __init gic_acpi_count_gicr_regions(void)
@@ -2478,7 +2506,8 @@ static int __init gic_acpi_parse_virt_madt_gicc(union acpi_subtable_headers *hea
 	int maint_irq_mode;
 	static int first_madt = true;
 
-	if (!acpi_gicc_is_usable(gicc))
+	if (!(gicc->flags &
+	      (ACPI_MADT_ENABLED | ACPI_MADT_GICC_ONLINE_CAPABLE)))
 		return 0;
 
 	maint_irq_mode = (gicc->flags & ACPI_MADT_VGIC_IRQ_MODE) ?

include/acpi/acpi_bus.h

@@ -134,6 +134,7 @@ struct acpi_scan_handler {
 	bool (*match)(const char *idstr, const struct acpi_device_id **matchid);
 	int (*attach)(struct acpi_device *dev, const struct acpi_device_id *id);
 	void (*detach)(struct acpi_device *dev);
+	void (*post_eject)(struct acpi_device *dev);
 	void (*bind)(struct device *phys_dev);
 	void (*unbind)(struct device *phys_dev);
 	struct acpi_hotplug_profile hotplug;

include/acpi/processor.h

@@ -217,7 +217,7 @@ struct acpi_processor_flags {
 	u8 has_lpi:1;
 	u8 power_setup_done:1;
 	u8 bm_rld_set:1;
-	u8 need_hotplug_init:1;
+	u8 previously_online:1;
 };
 
 struct acpi_processor {

include/linux/acpi.h

@@ -237,11 +237,6 @@ acpi_table_parse_cedt(enum acpi_cedt_type id,
 int acpi_parse_mcfg (struct acpi_table_header *header);
 void acpi_table_print_madt_entry (struct acpi_subtable_header *madt);
 
-static inline bool acpi_gicc_is_usable(struct acpi_madt_generic_interrupt *gicc)
-{
-	return gicc->flags & ACPI_MADT_ENABLED;
-}
-
 #if defined(CONFIG_X86) || defined(CONFIG_LOONGARCH)
 void acpi_numa_processor_affinity_init (struct acpi_srat_cpu_affinity *pa);
 #else
@@ -304,6 +299,8 @@ int acpi_map_cpu(acpi_handle handle, phys_cpuid_t physid, u32 acpi_id,
 int acpi_unmap_cpu(int cpu);
 #endif /* CONFIG_ACPI_HOTPLUG_CPU */
 
+acpi_handle acpi_get_processor_handle(int cpu);
+
 #ifdef CONFIG_ACPI_HOTPLUG_IOAPIC
 int acpi_get_ioapic_id(acpi_handle handle, u32 gsi_base, u64 *phys_addr);
 #endif
@@ -1076,6 +1073,11 @@ static inline bool acpi_sleep_state_supported(u8 sleep_state)
 	return false;
 }
 
+static inline acpi_handle acpi_get_processor_handle(int cpu)
+{
+	return NULL;
+}
+
 #endif	/* !CONFIG_ACPI */
 
 extern void arch_post_acpi_subsys_init(void);

include/linux/cpumask.h

@@ -93,6 +93,7 @@ static inline void set_nr_cpu_ids(unsigned int nr)
  *
  *     cpu_possible_mask- has bit 'cpu' set iff cpu is populatable
  *     cpu_present_mask - has bit 'cpu' set iff cpu is populated
+ *     cpu_enabled_mask  - has bit 'cpu' set iff cpu can be brought online
  *     cpu_online_mask  - has bit 'cpu' set iff cpu available to scheduler
  *     cpu_active_mask  - has bit 'cpu' set iff cpu available to migration
  *
@@ -125,11 +126,13 @@ static inline void set_nr_cpu_ids(unsigned int nr)
 
 extern struct cpumask __cpu_possible_mask;
 extern struct cpumask __cpu_online_mask;
+extern struct cpumask __cpu_enabled_mask;
 extern struct cpumask __cpu_present_mask;
 extern struct cpumask __cpu_active_mask;
 extern struct cpumask __cpu_dying_mask;
 #define cpu_possible_mask ((const struct cpumask *)&__cpu_possible_mask)
 #define cpu_online_mask   ((const struct cpumask *)&__cpu_online_mask)
+#define cpu_enabled_mask   ((const struct cpumask *)&__cpu_enabled_mask)
 #define cpu_present_mask  ((const struct cpumask *)&__cpu_present_mask)
 #define cpu_active_mask   ((const struct cpumask *)&__cpu_active_mask)
 #define cpu_dying_mask    ((const struct cpumask *)&__cpu_dying_mask)
@@ -1075,6 +1078,7 @@ extern const DECLARE_BITMAP(cpu_all_bits, NR_CPUS);
 #else
 #define for_each_possible_cpu(cpu) for_each_cpu((cpu), cpu_possible_mask)
 #define for_each_online_cpu(cpu)   for_each_cpu((cpu), cpu_online_mask)
+#define for_each_enabled_cpu(cpu)   for_each_cpu((cpu), cpu_enabled_mask)
 #define for_each_present_cpu(cpu)  for_each_cpu((cpu), cpu_present_mask)
 #endif
 
@@ -1092,6 +1096,15 @@ set_cpu_possible(unsigned int cpu, bool possible)
 		cpumask_clear_cpu(cpu, &__cpu_possible_mask);
 }
 
+static inline void
+set_cpu_enabled(unsigned int cpu, bool can_be_onlined)
+{
+	if (can_be_onlined)
+		cpumask_set_cpu(cpu, &__cpu_enabled_mask);
+	else
+		cpumask_clear_cpu(cpu, &__cpu_enabled_mask);
+}
+
 static inline void
 set_cpu_present(unsigned int cpu, bool present)
 {
@@ -1173,6 +1186,7 @@ static __always_inline unsigned int num_online_cpus(void)
 	return raw_atomic_read(&__num_online_cpus);
 }
 #define num_possible_cpus()	cpumask_weight(cpu_possible_mask)
+#define num_enabled_cpus()	cpumask_weight(cpu_enabled_mask)
 #define num_present_cpus()	cpumask_weight(cpu_present_mask)
 #define num_active_cpus()	cpumask_weight(cpu_active_mask)
 
@@ -1181,6 +1195,11 @@ static inline bool cpu_online(unsigned int cpu)
 	return cpumask_test_cpu(cpu, cpu_online_mask);
 }
 
+static inline bool cpu_enabled(unsigned int cpu)
+{
+	return cpumask_test_cpu(cpu, cpu_enabled_mask);
+}
+
 static inline bool cpu_possible(unsigned int cpu)
 {
 	return cpumask_test_cpu(cpu, cpu_possible_mask);
@@ -1205,6 +1224,7 @@ static inline bool cpu_dying(unsigned int cpu)
 
 #define num_online_cpus()	1U
 #define num_possible_cpus()	1U
+#define num_enabled_cpus()	1U
 #define num_present_cpus()	1U
 #define num_active_cpus()	1U
 
@@ -1218,6 +1238,11 @@ static inline bool cpu_possible(unsigned int cpu)
 	return cpu == 0;
 }
 
+static inline bool cpu_enabled(unsigned int cpu)
+{
+	return cpu == 0;
+}
+
 static inline bool cpu_present(unsigned int cpu)
 {
 	return cpu == 0;

kernel/cpu.c

@@ -3069,6 +3069,9 @@ EXPORT_SYMBOL(__cpu_possible_mask);
 struct cpumask __cpu_online_mask __read_mostly;
 EXPORT_SYMBOL(__cpu_online_mask);
 
+struct cpumask __cpu_enabled_mask __read_mostly;
+EXPORT_SYMBOL(__cpu_enabled_mask);
+
 struct cpumask __cpu_present_mask __read_mostly;
 EXPORT_SYMBOL(__cpu_present_mask);