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

[Greg Kurz <groug@kaod.org>]

On Wed, 1 Jun 2022 09:27:31 +0200
Thomas Huth <thuth@redhat.com> 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.
> 

I didn't even know there was a "max" alias :-)

This was introduced by commit:

commit 3fc6c082e3aad85addf25d36740030982963c0c8
Author: Fabrice Bellard <fabrice@bellard.org>
Date:   Sat Jul 2 20:59:34 2005 +0000

    preliminary patch to support more PowerPC CPUs (Jocelyn Mayer)

This was already a 7400 model at the time. Obviously we've never
maintained that and I hardly see how it is useful... As Thomas
noted, "max" has implicit semantics that depend on the host.
This isn't migration friendly and I'm pretty sure libvirt
doesn't use it (Daniel ?)

We already have the concept of default CPU for the spapr
machine types, that is usually popped up to the newer
CPU model that is going to be supported in production.
This goes with a bump of the machine type version as
well for the sake of migration. This seems a lot more
reliable than the "max" thingy IMHO.

Unless there's a very important use case I'm missing,
I'd rather kill the thing instead of trying to resurrect
it.

Cheers,

--
Greg

>   Thomas
>