linux-kernel - Re: [RFC PATCH 2/2] x86/xen: Make the secondary CPU idle tasks reliable

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Date:   Mon, 16 Mar 2020 15:35:14 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Miroslav Benes <mbenes@...e.cz>
Cc:     Jürgen Groß <jgross@...e.com>,
        boris.ostrovsky@...cle.com, sstabellini@...nel.org,
        tglx@...utronix.de, mingo@...hat.com, bp@...en8.de, hpa@...or.com,
        x86@...nel.org, xen-devel@...ts.xenproject.org,
        linux-kernel@...r.kernel.org, live-patching@...r.kernel.org,
        jslaby@...e.cz, Peter Zijlstra <peterz@...radead.org>
Subject: Re: [RFC PATCH 2/2] x86/xen: Make the secondary CPU idle tasks
 reliable

On Mon, Mar 16, 2020 at 04:51:12PM +0100, Miroslav Benes wrote:
> > diff --git a/arch/x86/xen/smp_pv.c b/arch/x86/xen/smp_pv.c
> > index 6b88cdcbef8f..39afd88309cb 100644
> > --- a/arch/x86/xen/smp_pv.c
> > +++ b/arch/x86/xen/smp_pv.c
> > @@ -92,6 +92,7 @@ asmlinkage __visible void cpu_bringup_and_idle(void)
> >  {
> >         cpu_bringup();
> >         boot_init_stack_canary();
> > +       asm volatile (UNWIND_HINT(ORC_REG_UNDEFINED, 0, ORC_TYPE_CALL, 1));
> >         cpu_startup_entry(CPUHP_AP_ONLINE_IDLE);
> >  }
> > 
> > and that seems to work. I need to properly verify and test, but the 
> > explanation is that as opposed to the above, cpu_startup_entry() is on the 
> > idle task's stack and the hint is then taken into account. The unwound 
> > stack seems to be complete, so it could indeed be the fix.
> 
> Not the correct one though. Objtool rightfully complains with
> 
> arch/x86/xen/smp_pv.o: warning: objtool: cpu_bringup_and_idle()+0x6a: undefined stack state
> 
> and all the other hacks I tried ended up in the same dead alley. It seems 
> to me the correct fix is that all orc entries for cpu_bringup_and_idle() 
> should have "end" property set to 1, since it is the first function on the 
> stack. I don't know how to achieve that without the assembly hack in the 
> patch I sent. If I am not missing something, of course.
> 
> Josh, any idea?

Yeah, I think mucking with the unwind hints in C code is going to be
precarious.  You could maybe have something like

	asm("
	  UNWIND_HINT_EMPTY\n
	  mov $CPUHP_AP_ONLINE_IDLE, %rdi\n
	  call cpu_startup_entry\n
	)"
	unreachable();

but that's pretty ugly (and it might not work anyway).

I suppose we could add a new facility to mark an entire C function as an
"end" point.

But I think it would be cleanest to just do something like your patch
and have the entry code be asm which then calls cpu_bringup_and_idle().
That would make it consistent with all other entry code, which all lives
in asm.

-- 
Josh