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Date:   Tue, 22 Nov 2022 17:48:12 -0800
From:   Josh Poimboeuf <jpoimboe@kernel.org>
To:     "Paul E. McKenney" <paulmck@kernel.org>
Cc:     Peter Zijlstra <peterz@infradead.org>, sfr@canb.auug.org.au,
        linux-kernel@vger.kernel.org
Subject: Re: objtool warning for next-20221118
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On Tue, Nov 22, 2022 at 04:22:58PM -0800, Paul E. McKenney wrote:
> 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?

Silly GCC is folding the inline asm.  This works (but still doesn't seem
like the right approach):

diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
index 26e8f57c75ad..128e7d78fedf 100644
--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c
@@ -702,7 +702,7 @@ static void (*x86_idle)(void);
 #ifndef CONFIG_SMP
 static inline void play_dead(void)
 {
-	BUG();
+	_BUG_FLAGS(ASM_UD2, 0, ASM_REACHABLE);
 }
 #endif