On Mon, 16 Mar 2020, Josh Poimboeuf wrote:
> 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.
I think it would be an overkill for what I perceive as one-off scenario.
Maybe if there are more use cases in the future, but I doubt it.
> 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.
Ack.
Thanks
Miroslav
