[PATCH 1/3] x86/cpu: Actually turn off mitigations by default for SPECULATION_MITIGATIONS=n

[Will Deacon]

On Mon, Apr 15, 2024 at 07:31:23AM -0700, Sean Christopherson wrote:
> On Mon, Apr 15, 2024, Geert Uytterhoeven wrote:
> > On Sat, Apr 13, 2024 at 11:38 AM Michael Ellerman <mpe at ellerman.id.au> wrote:
> > > Michael Ellerman <mpe at ellerman.id.au> writes:
> > > > Stephen Rothwell <sfr at canb.auug.org.au> writes:
> > > ...
> > > >> On Tue,  9 Apr 2024 10:51:05 -0700 Sean Christopherson <seanjc at google.com> wrote:
> > > ...
> > > >>> diff --git a/kernel/cpu.c b/kernel/cpu.c
> > > >>> index 8f6affd051f7..07ad53b7f119 100644
> > > >>> --- a/kernel/cpu.c
> > > >>> +++ b/kernel/cpu.c
> > > >>> @@ -3207,7 +3207,8 @@ enum cpu_mitigations {
> > > >>>  };
> > > >>>
> > > >>>  static enum cpu_mitigations cpu_mitigations __ro_after_init =
> > > >>> -   CPU_MITIGATIONS_AUTO;
> > > >>> +   IS_ENABLED(CONFIG_SPECULATION_MITIGATIONS) ? CPU_MITIGATIONS_AUTO :
> > > >>> +                                                CPU_MITIGATIONS_OFF;
> > > >>>
> > > >>>  static int __init mitigations_parse_cmdline(char *arg)
> > > >>>  {
> > >
> > > I think a minimal workaround/fix would be:
> > >
> > > diff --git a/drivers/base/Kconfig b/drivers/base/Kconfig
> > > index 2b8fd6bb7da0..290be2f9e909 100644
> > > --- a/drivers/base/Kconfig
> > > +++ b/drivers/base/Kconfig
> > > @@ -191,6 +191,10 @@ config GENERIC_CPU_AUTOPROBE
> > >  config GENERIC_CPU_VULNERABILITIES
> > >         bool
> > >
> > > +config SPECULATION_MITIGATIONS
> > > +       def_bool y
> > > +       depends on !X86
> > > +
> > >  config SOC_BUS
> > >         bool
> > >         select GLOB
> > 
> > Thanks, that works for me (on arm64), so
> > Tested-by: Geert Uytterhoeven <geert+renesas at glider.be>
> 
> Oof.  I completely missed that "cpu_mitigations" wasn't x86-only.  I can't think
> of better solution than an on-by-default generic Kconfig, though can't that it
> more simply be:
> 
> diff --git a/drivers/base/Kconfig b/drivers/base/Kconfig
> index 2b8fd6bb7da0..5930cb56ee29 100644
> --- a/drivers/base/Kconfig
> +++ b/drivers/base/Kconfig
> @@ -191,6 +191,9 @@ config GENERIC_CPU_AUTOPROBE
>  config GENERIC_CPU_VULNERABILITIES
>         bool
>  
> +config SPECULATION_MITIGATIONS
> +       def_bool !X86
> +
>  config SOC_BUS
>         bool
>         select GLOB

I can't see this in -next yet. Do you plan to post it as a proper patch
to collect acks etc?

Cheers,

Will