Re: [Qemu-ppc] [PATCHv4 0/5] Clean up compatibility mode handling

[David Gibson]

On Thu, Jun 01, 2017 at 03:09:15PM +0200, Greg Kurz wrote:
> On Thu, 1 Jun 2017 13:59:14 +0200
> Cédric Le Goater <address@hidden> wrote:
> 
> > On 06/01/2017 08:52 AM, David Gibson wrote:
> > > On Wed, May 31, 2017 at 10:58:57AM +0200, Greg Kurz wrote:  
> > >> On Wed, 31 May 2017 12:57:48 +1000
> > >> David Gibson <address@hidden> wrote:  
> > >>> [...]  
> > >>>> All old non-pseries machine types already complain when started with
> > >>>> a POWER7 or newer CPU. Providing the extra error message looks weird:
> > >>>>
> > >>>> qemu-system-ppc64 -machine ppce500 \
> > >>>>                   -cpu POWER7,compat=power6
> > >>>> qemu-system-ppc64: CPU 'compat' property is deprecated and has no 
> > >>>> effect;
> > >>>>  use max-cpu-compat machine property instead
> > >>>> MMU model 983043 not supported by this machine.
> > >>>>
> > >>>> but I guess it's better than crashing. :)    
> > >>>
> > >>> Well, sure POWER7 doesn't make sense for an e500 machine for other
> > >>> reasons.  But POWER7 or POWER8 _would_ make sense for powernv, where
> > >>> compat= doesn't.
> > >>>  
> > >>
> > >> The powernv machine type doesn't even support CPU features at all:
> > >>
> > >>     chip_typename = g_strdup_printf(TYPE_PNV_CHIP "-%s", 
> > >> machine->cpu_model);
> > >>     if (!object_class_by_name(chip_typename)) {
> > >>         error_report("invalid CPU model '%s' for %s machine",
> > >>                      machine->cpu_model, 
> > >> MACHINE_GET_CLASS(machine)->name);
> > >>         exit(1);
> > >>     }  
> > > 
> > > Ah, well, that's another bug, but not one that's in scope for this
> > > series.  
> > 
> > PowerNV is still work in progress. I would not worry about it too much.
> > 
> 
> Of course and this isn't the purpose of the discussion actually. We were
> talking about CPU features being relevant or not depending on the machine
> type.
> 
> But I'm not even sure that CPU features are useful at all for ppc, not to
> say very confusing (otherwise this series wouldn't be needed for example).
> 
> Speaking of PowerNV, just as an example, I guess the fix would be to
> forbid machine->cpu_model if it contains features. And probably the same
> for all other machine types, except pseries for backward compatibility
> reasons.

I don't think that's correct in principle.  I can imagine CPU
properties it might make sense to really set on the cpu, regardless of
machine type.  A quick look says we don't have any such at the moment,
but I don't think it's something we should prevent as a matter of policy.

-- 
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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