Re: [RFC/PATCH] powerpc: Use rtas query-cpu-stopped-state in smp spinup

[Nathan Lynch <ntl@pobox.com>]

Michael Ellerman wrote:
> Currently we use a cpumask called of_spin_map to keep track of which threads
> have been spun up. We basically guess that OF has spun up all even numbered
> threads, and so all the odd numbered threads need to be brought up.
> 
> That's a bit of a dicey assumption at best, and is totally incorrect for
> kexec.
>
> Luckily we have an rtas call which can tell us whether a cpu is up
> or not, so let's use it?

1. query-cpu-stopped-state isn't available on all RTAS systems (I
   believe it's required only on systems that support cpu offline).

2. I've tried it before.  Hope I'm remembering this correctly, but I
   think my experience was that query-cpu-stopped-state reported
   nonsense for cpus that were started by OF.  This was on Power5,
   btw.

3. This isn't how query-cpu-stopped-state was intended to be used.
   It's meant to be used by the OS to determine when a thread has
   stopped itself using the stop-self method.  Which might partially
   explain (2).

I'm fine with getting rid of the assumptions about thread numbering,
but I don't think this is gonna work, sorry.