Re: objtool warning for next-20221118

[Juergen Gross <jgross@xxxxxxxx>]

On 24.11.22 17:39, Josh Poimboeuf wrote:
> On Thu, Nov 24, 2022 at 08:47:47AM +0100, Juergen Gross wrote:
> > > > +++ b/arch/x86/xen/smp_pv.c
> > > > @@ -385,17 +385,9 @@ static void xen_pv_play_dead(void) /* used only
> > > > with HOTPLUG_CPU */
> > > >    {
> > > >        play_dead_common();
> > > >        HYPERVISOR_vcpu_op(VCPUOP_down, xen_vcpu_nr(smp_processor_id()), NULL);
> > > > -    cpu_bringup();
> > > > -    /*
> > > > -     * commit 4b0c0f294 (tick: Cleanup NOHZ per cpu data on cpu down)
> > > > -     * clears certain data that the cpu_idle loop (which called us
> > > > -     * and that we return from) expects. The only way to get that
> > > > -     * data back is to call:
> > > > -     */
> > > > -    tick_nohz_idle_enter();
> > > > -    tick_nohz_idle_stop_tick_protected();
> > > > -    cpuhp_online_idle(CPUHP_AP_ONLINE_IDLE);
> > > > +    /* FIXME: converge cpu_bringup_and_idle() and start_secondary() */
> > > > +    cpu_bringup_and_idle();
> > > I think this will leak stack memory. Multiple cpu offline/online cycles of
> > > the same cpu will finally exhaust the idle stack.
> Doh!  Of course...
>
> I was actually thinking ahead, to where eventually xen_pv_play_dead()
> can call start_cpu0(), which can be changed to automatically reset the
> stack pointer like this:
>
> SYM_CODE_START(start_cpu0)
>         ANNOTATE_NOENDBR
>         UNWIND_HINT_EMPTY
>         movq    PER_CPU_VAR(pcpu_hot + X86_top_of_stack), %rax
>         leaq    -PTREGS_SIZE(%rax), %rsp
>         jmp     .Ljump_to_C_code
> SYM_CODE_END(start_cpu0)
>
> but that would only be possible be after more cleanups which converge
> cpu_bringup_and_idle() with start_secondary().
>
> > The attached patch seems to work fine.
> The patch looks good to me.
>
> It doesn't solve Paul's original issue where arch_cpu_idle_dead() needs
> to be __noreturn.  But that should probably be a separate patch anyway.
Okay, I'll split this off.

> > The __noreturn annotation seems to trigger an objtool warning, though, in
> > spite of the added BUG() at the end of xen_pv_play_dead():
> >
> > arch/x86/xen/smp_pv.o: warning: objtool: xen_pv_play_dead() falls through to
> > next function xen_pv_cpu_die()
> You'll need to tell objtool that xen_cpu_bringup_again() is noreturn by
> adding "xen_cpu_bringup_again" to global_noreturns[] in
> tools/objtool/check.c.
Ah, okay. Will do that.

> (Yes it's a pain, I'll be working an improved solution to the noreturn
> thing...)
Should be fairly easy, no?

"Just" extend the __noreturn macro to put the function into a ".text.noreturn"
section, which can be handled in a special way by objtool. This would need
an __init_noreturn macro, of course, for a ".init.text.noreturn" section.


Juergen

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