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Date: Mon, 15 Apr 2024 07:31:23 -0700
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Subject: Re: [PATCH 1/3] x86/cpu: Actually turn off mitigations by default for SPECULATION_MITIGATIONS=n
From: Sean Christopherson <seanjc@google.com>
To: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Michael Ellerman <mpe@ellerman.id.au>, Stephen Rothwell <sfr@canb.auug.org.au>, 
	Jonathan Corbet <corbet@lwn.net>, Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>, 
	Borislav Petkov <bp@alien8.de>, Dave Hansen <dave.hansen@linux.intel.com>, x86@kernel.org, 
	Peter Zijlstra <peterz@infradead.org>, Josh Poimboeuf <jpoimboe@kernel.org>, linux-doc@vger.kernel.org, 
	linux-kernel@vger.kernel.org, Pawan Gupta <pawan.kumar.gupta@linux.intel.com>, 
	Daniel Sneddon <daniel.sneddon@linux.intel.com>, linuxppc-dev@lists.ozlabs.org, 
	linux-arch@vger.kernel.org, Catalin Marinas <catalin.marinas@arm.com>, 
	Will Deacon <will@kernel.org>, Heiko Carstens <hca@linux.ibm.com>, 
	Linux-Renesas <linux-renesas-soc@vger.kernel.org>
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On Mon, Apr 15, 2024, Geert Uytterhoeven wrote:
> Hi Michael,
>=20
> On Sat, Apr 13, 2024 at 11:38=E2=80=AFAM Michael Ellerman <mpe@ellerman.i=
d.au> wrote:
> > Michael Ellerman <mpe@ellerman.id.au> writes:
> > > Stephen Rothwell <sfr@canb.auug.org.au> writes:
> > ...
> > >> On Tue,  9 Apr 2024 10:51:05 -0700 Sean Christopherson <seanjc@googl=
e.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 =3D
> > >>> -   CPU_MITIGATIONS_AUTO;
> > >>> +   IS_ENABLED(CONFIG_SPECULATION_MITIGATIONS) ? CPU_MITIGATIONS_AU=
TO :
> > >>> +                                                CPU_MITIGATIONS_OF=
F;
> > >>>
> > >>>  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
>=20
> Thanks, that works for me (on arm64), so
> Tested-by: Geert Uytterhoeven <geert+renesas@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
=20
+config SPECULATION_MITIGATIONS
+       def_bool !X86
+
 config SOC_BUS
        bool
        select GLOB