Re: [Qemu-devel] [RFC PATCH v2 3/5] spapr: Set stable_cpu_id for threads of CPU cores

[Bharata B Rao <bharata@linux.vnet.ibm.com>]

On Fri, Jul 08, 2016 at 03:24:13PM +1000, David Gibson wrote:
> On Thu, Jul 07, 2016 at 08:20:23PM +0530, Bharata B Rao wrote:
> > Conditonally set stable_cpu_id for CPU threads that are created as part
> > of spapr CPU cores. The use of stable_cpu_id is enabled for pseries-2.7
> > onwards.
> > 
> > Signed-off-by: Bharata B Rao <bharata@linux.vnet.ibm.com>
> > ---
> >  hw/ppc/spapr_cpu_core.c | 7 +++++++
> >  1 file changed, 7 insertions(+)
> > 
> > diff --git a/hw/ppc/spapr_cpu_core.c b/hw/ppc/spapr_cpu_core.c
> > index b104778..0ec3513 100644
> > --- a/hw/ppc/spapr_cpu_core.c
> > +++ b/hw/ppc/spapr_cpu_core.c
> > @@ -293,8 +293,15 @@ static void spapr_cpu_core_realize(DeviceState *dev, Error **errp)
> >      for (i = 0; i < cc->nr_threads; i++) {
> >          char id[32];
> >          obj = sc->threads + i * size;
> > +        CPUState *cs;
> >  
> >          object_initialize(obj, size, typename);
> > +        cs = CPU(obj);
> > +
> > +        /* Use core_id (which is actually cpu_dt_id) as stable CPU id */
> > +        if (cs->has_stable_cpu_id) {
> > +            cs->stable_cpu_id = cc->core_id + i;
> > +        }
> 
> Testing cs->has_stable_cpu_id here in machine type specific code seems
> really weird.  It's the machine type that knows whether it has a
> stable ID to give to the CPU or not, rather than the other way around.
> 
> Since we haven't yet had a release with cpu cores, I think the right
> thing is for cpu_core to unconditionally set the stable ID (and set
> has_stable_id to true).

Right, we can set cpu_stable_id unconditionally here since this code path
(core realize) will be taken only for pseries-2.7 onwards. has_stable_id
will get set as part of the property we defined in SPAPR_COMPAT_2_7.

> The backup path that does thread-based cpu
> init, can set has_stable_id to false (if that's not the default).

Default is off, but turning it on for 2.7 will be inherited by 2.6
and others below. Hence I have code to explicitly disable this prop
for 2.6 and below via SPAPR_COMPAT_2_6.

Regards,
Bharata.