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Date:   Wed, 23 Nov 2022 09:55:30 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Josh Poimboeuf <jpoimboe@kernel.org>
Cc:     "Paul E. McKenney" <paulmck@kernel.org>, sfr@canb.auug.org.au,
        linux-kernel@vger.kernel.org, jgross@suse.com,
        sstabellini@kernel.org, boris.ostrovsky@oracle.com,
        xen-devel@lists.xenproject.org,
        Andrew Cooper <Andrew.Cooper3@citrix.com>
Subject: Re: objtool warning for next-20221118
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On Tue, Nov 22, 2022 at 05:23:50PM -0800, Josh Poimboeuf 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.
> 
> o_O
> 
> How the @#$% is that a valid way of doing it?  Why not just do it the
> normal way?

Well, if you return from arch_cpu_idle_dead() you're back in the idle
loop -- exactly where you would be if you were to bootstrap the whole
CPU -- provided you have it remember the whole state (easier with a
vCPU).

But maybe I'm missing something, lets add Xen folks on.