Re: [PATCH] target/ppc/cpu-models: Update max alias to power10

[Murilo Opsfelder Araújo]

Hi, Cédric.

On 6/1/22 04:44, Cédric Le Goater wrote:
> On 6/1/22 09:27, Thomas Huth wrote:
> > On 31/05/2022 19.27, Murilo Opsfelder Araujo wrote:
> > > Update max alias to power10 so users can take advantage of a more
> > > recent CPU model when '-cpu max' is provided.
> > >
> > > Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1038
> > > Cc: Daniel P. Berrangé <berrange@redhat.com>
> > > Cc: Thomas Huth <thuth@redhat.com>
> > > Cc: Cédric Le Goater <clg@kaod.org>
> > > Cc: Daniel Henrique Barboza <danielhb413@gmail.com>
> > > Cc: Fabiano Rosas <farosas@linux.ibm.com>
> > > Signed-off-by: Murilo Opsfelder Araujo <muriloo@linux.ibm.com>
> > > ---
> > >   target/ppc/cpu-models.c | 3 ++-
> > >   1 file changed, 2 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/target/ppc/cpu-models.c b/target/ppc/cpu-models.c
> > > index 976be5e0d1..c15fcb43a1 100644
> > > --- a/target/ppc/cpu-models.c
> > > +++ b/target/ppc/cpu-models.c
> > > @@ -879,7 +879,6 @@ PowerPCCPUAlias ppc_cpu_aliases[] = {
> > >       { "755", "755_v2.8" },
> > >       { "goldfinger", "755_v2.8" },
> > >       { "7400", "7400_v2.9" },
> > > -    { "max", "7400_v2.9" },
> > >       { "g4",  "7400_v2.9" },
> > >       { "7410", "7410_v1.4" },
> > >       { "nitro", "7410_v1.4" },
> > > @@ -910,6 +909,8 @@ PowerPCCPUAlias ppc_cpu_aliases[] = {
> > >       { "power8nvl", "power8nvl_v1.0" },
> > >       { "power9", "power9_v2.0" },
> > >       { "power10", "power10_v2.0" },
> > > +    /* Update the 'max' alias to the latest CPU model */
> > > +    { "max", "power10_v2.0" },
> > >   #endif
> >
> > I'm not sure whether "max" should really be fixed alias in this list here? What if a user 
> > runs with KVM on a POWER8 host - then "max" won't work this way, will it?
> >
> > And in the long run, it would also be good if this would work with other machines like 
> > the "g3beige", too (which don't support the new 64-bit POWER CPUs), so you 
> > should at least mention in the commit description that this is only a temporary hack for 
> > the pseries machine, I think.
>
> Yes. You are right, Thomas.
>
> s390 and x86 have a nice way to address "max".

If I understood the code correctly, they implement "-cpu max" based on
a CPU model with additional CPU features enabled.  The resulting
emulated CPU is not necessarily a CPU model that exists as a hardware.
So, the "-cpu max" never gets any CPU feature dropped.  Features are
only added in.

I'm not keen on this idea of having a CPU model that doesn't even
exist as a hardware.

--
Murilo