x86/smp: Fix mwait_play_dead() and acpi_processor_ffh_play_dead() noreturn behavior

Fix some related issues (done in a single patch to avoid introducing
intermediate bisect warnings):

1) The SMP version of mwait_play_dead() doesn't return, but its
!SMP counterpart does. Make its calling behavior consistent by
resolving the !SMP version to a BUG(). It should never be called
anyway, this just enforces that at runtime and enables its callers
to be marked as __noreturn.

2) While the SMP definition of mwait_play_dead() is annotated as
__noreturn, the declaration isn't. Nor is it listed in
tools/objtool/noreturns.h. Fix that.

3) Similar to #1, the SMP version of acpi_processor_ffh_play_dead()
doesn't return but its !SMP counterpart does. Make the !SMP
version a BUG(). It should never be called.

4) acpi_processor_ffh_play_dead() doesn't return, but is lacking any
__noreturn annotations. Fix that.

This fixes the following objtool warnings:

vmlinux.o: warning: objtool: acpi_processor_ffh_play_dead+0x67: mwait_play_dead() is missing a __noreturn annotation
vmlinux.o: warning: objtool: acpi_idle_play_dead+0x3c: acpi_processor_ffh_play_dead() is missing a __noreturn annotation

Fixes: a7dd183f0b38 ("x86/smp: Allow calling mwait_play_dead with an arbitrary hint")
Fixes: 541ddf31e300 ("ACPI/processor_idle: Add FFH state handling")
Reported-by: Paul E. McKenney <[e-mail]>
Signed-off-by: Josh Poimboeuf <[e-mail]>
Signed-off-by: Ingo Molnar <[e-mail]>
Tested-by: Paul E. McKenney <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

arch/x86/include/asm/smp.h

···
 int wbinvd_on_all_cpus(void);
 
 void smp_kick_mwait_play_dead(void);
-void mwait_play_dead(unsigned int eax_hint);
+void __noreturn mwait_play_dead(unsigned int eax_hint);
 
 void native_smp_send_reschedule(int cpu);
 void native_send_call_func_ipi(const struct cpumask *mask);
···
 	return (struct cpumask *)cpumask_of(0);
 }
 
-static inline void mwait_play_dead(unsigned int eax_hint) { }
+static inline void __noreturn mwait_play_dead(unsigned int eax_hint) { BUG(); }
 #endif /* CONFIG_SMP */
 
 #ifdef CONFIG_DEBUG_NMI_SELFTEST

arch/x86/kernel/acpi/cstate.c

···
 }
 EXPORT_SYMBOL_GPL(acpi_processor_ffh_cstate_probe);
 
-void acpi_processor_ffh_play_dead(struct acpi_processor_cx *cx)
+void __noreturn acpi_processor_ffh_play_dead(struct acpi_processor_cx *cx)
 {
 	unsigned int cpu = smp_processor_id();
 	struct cstate_entry *percpu_entry;

include/acpi/processor.h

···
 				    struct acpi_processor_cx *cx,
 				    struct acpi_power_register *reg);
 void acpi_processor_ffh_cstate_enter(struct acpi_processor_cx *cstate);
-void acpi_processor_ffh_play_dead(struct acpi_processor_cx *cx);
+void __noreturn acpi_processor_ffh_play_dead(struct acpi_processor_cx *cx);
 #else
 static inline void acpi_processor_power_init_bm_check(struct
 						      acpi_processor_flags
···
 {
 	return;
 }
-static inline void acpi_processor_ffh_play_dead(struct acpi_processor_cx *cx)
+static inline void __noreturn acpi_processor_ffh_play_dead(struct acpi_processor_cx *cx)
 {
-	return;
+	BUG();
 }
 #endif
 

tools/objtool/noreturns.h

···
 NORETURN(__ubsan_handle_builtin_unreachable)
 NORETURN(__x64_sys_exit)
 NORETURN(__x64_sys_exit_group)
+NORETURN(acpi_processor_ffh_play_dead)
 NORETURN(arch_cpu_idle_dead)
 NORETURN(bch2_trans_in_restart_error)
 NORETURN(bch2_trans_restart_error)
···
 NORETURN(machine_real_restart)
 NORETURN(make_task_dead)
 NORETURN(mpt_halt_firmware)
+NORETURN(mwait_play_dead)
 NORETURN(nmi_panic_self_stop)
 NORETURN(panic)
 NORETURN(panic_smp_self_stop)