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

[David Gibson]

On Fri, Jul 08, 2016 at 12:11:12PM +0530, Bharata B Rao wrote:
> 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 <address@hidden>
> > > ---
> > >  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.

Hrm, that's true.  But when you describe it like that it sounds like a
really non-obvious and fragile dependency between different components.

> > 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.

This is all seeming terribly awkward.  Can we try investigating a
different approach:

    1. Rename cpu_index to cpu_id, but it's still used in the same
       places it's used.

    2. Remove assumptions that cpu_id values are contiguous or
       dense
    
    3. Machine type decides whether it wants to populate the cpu_id
       values explicitly, or leave it to generic code to calculate
       them as cpu_index is calculated now.

    4. Ideally, generic code enforces that the machine type populates
       either all or none of the cpu_id values.

Does that seem workable?

-- 
David Gibson                    | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au  | minimalist, thank you.  NOT _the_ _other_
                                | _way_ _around_!
http://www.ozlabs.org/~dgibson

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