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

[Michael Ellerman]

Sean Christopherson <seanjc at google.com> writes:
> 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
> +

Yeah that works too.

cheers