linux-kernel - Re: [PATCH 1/3] x86/cpu: Actually turn off mitigations by default for SPECULATION

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Message-ID: <20240419140321.GF3148@willie-the-truck>
Date: Fri, 19 Apr 2024 15:03:22 +0100
From: Will Deacon <will@...nel.org>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Geert Uytterhoeven <geert@...ux-m68k.org>,
	Michael Ellerman <mpe@...erman.id.au>,
	Stephen Rothwell <sfr@...b.auug.org.au>,
	Jonathan Corbet <corbet@....net>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
	Peter Zijlstra <peterz@...radead.org>,
	Josh Poimboeuf <jpoimboe@...nel.org>, linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
	Daniel Sneddon <daniel.sneddon@...ux.intel.com>,
	linuxppc-dev@...ts.ozlabs.org, linux-arch@...r.kernel.org,
	Catalin Marinas <catalin.marinas@....com>,
	Heiko Carstens <hca@...ux.ibm.com>,
	Linux-Renesas <linux-renesas-soc@...r.kernel.org>
Subject: Re: [PATCH 1/3] x86/cpu: Actually turn off mitigations by default
 for SPECULATION_MITIGATIONS=n

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@...erman.id.au> wrote:
> > > Michael Ellerman <mpe@...erman.id.au> writes:
> > > > Stephen Rothwell <sfr@...b.auug.org.au> writes:
> > > ...
> > > >> On Tue,  9 Apr 2024 10:51:05 -0700 Sean Christopherson <seanjc@...gle.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@...der.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