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

[Daniel Henrique Barboza]

On 6/1/22 07:03, Greg Kurz wrote:
> On Wed, 1 Jun 2022 11:25:43 +0200
> Thomas Huth <thuth@redhat.com> wrote:
>
> > On 01/06/2022 10.38, Greg Kurz wrote:
> > > 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.
> > ...
> > > 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.
> >
> > It's about making ppc similar to other architectures, which
> > have "-cpu max" as well, see:
> >
> >    https://gitlab.com/qemu-project/qemu/-/issues/1038
> >
> > It would be nice to get something similar on ppc.
>
> Problem is that on ppc, given the variety of models and boards,
> the concept of "max" is quite fuzzy... i.e. a lot of cases to
> consider for a benefit that is unclear to me. Hence my questioning.
> If the idea is just to match what other targets do without a specific
> use case in mind, this looks quite useless to me.

I mean, yes, the use case is that users/tooling are using -cpu max with x86
and arm. We'd rather not increase the gap between them and ppc64 because we
ended up removing -cpu max.

Even if the concept might not be applicable to every machine we have we can 
alias
-cpu max to the default machine CPU.

> > By the way, the warnings that you currently get when running with
> > TCG are quite ugly, too:
> >
> > $ ./qemu-system-ppc64
> > qemu-system-ppc64: warning: TCG doesn't support requested feature,
> > cap-cfpc=workaround
> > qemu-system-ppc64: warning: TCG doesn't support requested feature,
> > cap-sbbc=workaround
> > qemu-system-ppc64: warning: TCG doesn't support requested feature,
> > cap-ibs=workaround
> > qemu-system-ppc64: warning: TCG doesn't support requested feature,
> > cap-ccf-assist=on
> >
> > Maybe these could get fixed with a proper "max" CPU in TCG
> > mode, too?
>
> I don't think so. These warnings are the consequence of pseries
> being the default machine for ppc64, and the default pseries
> machine decides on the default CPU model and default values for
> features (in this case, these are mitigations for spectre/meltdown).
> TCG doesn't support them but we certainly don't want to add more
> divergence between TCG and KVM.

I sent a patch last year trying to suppress the warning:

https://lists.gnu.org/archive/html/qemu-devel/2021-01/msg05029.html

I proposed to suppress these warnings when the user didn't specifically
set those caps to true (which TCG doesn't support). David thought that
this was also a bad idea and we reached an impasse. Back then seemed like
I was the only one severely aggravated by these messages so I gave up :)


Thanks,


Daniel

> Cheers,
>
> --
> Greg
>
> >    Thomas