Re: objtool warning for next-20221118

[Paul E. McKenney]

On Tue, Nov 22, 2022 at 09:35:17AM +0100, Peter Zijlstra wrote:
> On Mon, Nov 21, 2022 at 09:16:05PM -0800, Josh Poimboeuf wrote:
> 
> > It's complaining about an unreachable instruction after a call to
> > arch_cpu_idle_dead().  In this case objtool detects the fact
> > arch_cpu_idle_dead() doesn't return due to its call to the
> > non-CONFIG_SMP version of play_dead().  But GCC has no way of detecting
> > that because the caller is in another translation unit.
> > 
> > As far as I can tell, that function should never return.  Though it
> > seems to have some dubious semantics (see xen_pv_play_dead() for
> > example, which *does* seem to return?).  I'm thinking it would be an
> > improvement to enforce that noreturn behavior across all arches and
> > platforms, sprinkling __noreturn and BUG() on arch_cpu_idle_dead() and
> > maybe some of it callees, where needed.
> > 
> > Peter, what do you think?  I could attempt a patch.
> 
> I'm thinking the Xen case makes all this really rather difficult :/
> 
> While normally a CPU is brought up through a trampoline, Xen seems to
> have implemented it by simply returning from play_dead(), and afaict
> that is actually a valid way to go about doing it.
> 
> Perhaps the best way would be to stick a REACHABLE annotation in
> arch_cpu_idle_dead() or something?

When I apply this on -next, I still get the objtool complaint.
Is there something else I should also be doing?

							Thanx, Paul

> ---
> diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
> index c21b7347a26d..0354be027eb0 100644
> --- a/arch/x86/kernel/process.c
> +++ b/arch/x86/kernel/process.c
> @@ -712,6 +712,7 @@ void arch_cpu_idle_enter(void)
>  void arch_cpu_idle_dead(void)
>  {
>  	play_dead();
> +	asm(ASM_REACHABLE);
>  }
>  
>  /*