cpu/hotplug: Add support for declaring CPU offlining not supported

The ACPI MADT mailbox wakeup method doesn't allow to offline a CPU after
it has been woken up.

Currently, offlining is prevented based on the confidential computing attribute
which is set for Intel TDX. But TDX is not the only possible user of the wake up
method. The MADT wakeup can be implemented outside of a confidential computing
environment. Offline support is a property of the wakeup method, not the CoCo
implementation.

Introduce cpu_hotplug_disable_offlining() that can be called to indicate that
CPU offlining should be disabled.

This function is going to replace CC_ATTR_HOTPLUG_DISABLED for ACPI MADT wakeup
method.

Signed-off-by: Kirill A. Shutemov <[e-mail]>
Signed-off-by: Borislav Petkov (AMD) <[e-mail]>
Reviewed-by: Thomas Gleixner <[e-mail]>
Tested-by: Tao Liu <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

include/linux/cpuhplock.h

···
 void cpus_read_unlock(void);
 int  cpus_read_trylock(void);
 void lockdep_assert_cpus_held(void);
+void cpu_hotplug_disable_offlining(void);
 void cpu_hotplug_disable(void);
 void cpu_hotplug_enable(void);
 void clear_tasks_mm_cpumask(int cpu);
···
 static inline void cpus_read_unlock(void) { }
 static inline int  cpus_read_trylock(void) { return true; }
 static inline void lockdep_assert_cpus_held(void) { }
+static inline void cpu_hotplug_disable_offlining(void) { }
 static inline void cpu_hotplug_disable(void) { }
 static inline void cpu_hotplug_enable(void) { }
 static inline int remove_cpu(unsigned int cpu) { return -EPERM; }

kernel/cpu.c

···
 
 DEFINE_STATIC_PERCPU_RWSEM(cpu_hotplug_lock);
 
+static bool cpu_hotplug_offline_disabled __ro_after_init;
+
 void cpus_read_lock(void)
 {
 	percpu_down_read(&cpu_hotplug_lock);
···
 static void lockdep_release_cpus_lock(void)
 {
 	rwsem_release(&cpu_hotplug_lock.dep_map, _THIS_IP_);
+}
+
+/* Declare CPU offlining not supported */
+void cpu_hotplug_disable_offlining(void)
+{
+	cpu_maps_update_begin();
+	cpu_hotplug_offline_disabled = true;
+	cpu_maps_update_done();
 }
 
 /*
···
 	 * If the platform does not support hotplug, report it explicitly to
 	 * differentiate it from a transient offlining failure.
 	 */
-	if (cc_platform_has(CC_ATTR_HOTPLUG_DISABLED))
+	if (cc_platform_has(CC_ATTR_HOTPLUG_DISABLED) ||
+	    cpu_hotplug_offline_disabled)
 		return -EOPNOTSUPP;
 	if (cpu_hotplug_disabled)
 		return -EBUSY;