x86/smpboot: Mark native_play_dead() as __noreturn

native_play_dead() ends by calling the non-returning function
hlt_play_dead() and therefore also never returns.

The !CONFIG_HOTPLUG_CPU stub version of native_play_dead()
unconditionally calls BUG() and does not return either.

Add the __noreturn attribute to both function definitions and their
declaration to document this behavior and to potentially improve
compiler optimizations.

Remove the obsolete comment, and add native_play_dead() to the objtool's
list of __noreturn functions.

Signed-off-by: Thorsten Blum <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Josh Poimboeuf <[e-mail]>

arch/x86/include/asm/smp.h

···
 int native_kick_ap(unsigned int cpu, struct task_struct *tidle);
 int native_cpu_disable(void);
 void __noreturn hlt_play_dead(void);
-void native_play_dead(void);
+void __noreturn native_play_dead(void);
 void play_dead_common(void);
 void wbinvd_on_cpu(int cpu);
 void wbinvd_on_all_cpus(void);

arch/x86/kernel/smpboot.c

···
 		native_halt();
 }
 
-/*
- * native_play_dead() is essentially a __noreturn function, but it can't
- * be marked as such as the compiler may complain about it.
- */
-void native_play_dead(void)
+void __noreturn native_play_dead(void)
 {
 	if (cpu_feature_enabled(X86_FEATURE_KERNEL_IBRS))
 		__update_spec_ctrl(0);
···
 	return -ENOSYS;
 }
 
-void native_play_dead(void)
+void __noreturn native_play_dead(void)
 {
 	BUG();
 }

tools/objtool/noreturns.h

···
 NORETURN(make_task_dead)
 NORETURN(mpt_halt_firmware)
 NORETURN(mwait_play_dead)
+NORETURN(native_play_dead)
 NORETURN(nmi_panic_self_stop)
 NORETURN(panic)
 NORETURN(vpanic)