linux-kernel - Re: [PATCH 1/2] cpu: Re-enable CPU mitigations by default for !X86 architectures

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Message-ID: <87jzkvbid7.fsf@mail.lhotse>
Date: Thu, 18 Apr 2024 10:54:28 +1000
From: Michael Ellerman <mpe@...erman.id.au>
To: Sean Christopherson <seanjc@...gle.com>, 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, Greg Kroah-Hartman
 <gregkh@...uxfoundation.org>, Peter Zijlstra <peterz@...radead.org>
Cc: linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org, Stephen
 Rothwell <sfr@...b.auug.org.au>, Geert Uytterhoeven
 <geert@...ux-m68k.org>, Sean Christopherson <seanjc@...gle.com>
Subject: Re: [PATCH 1/2] cpu: Re-enable CPU mitigations by default for !X86
 architectures

Sean Christopherson <seanjc@...gle.com> writes:
> Add a generic Kconfig, CPU_MITIGATIONS, to control whether or not CPU
> mitigations are enabled by default, and force it on for all architectures
> except x86.  A recent commit to turn mitigations off by default if
> SPECULATION_MITIGATIONS=n kinda sorta missed that "cpu_mitigations" is
> completely generic, where as SPECULATION_MITIGATIONS is x86 specific.
>
> Alternatively, SPECULATION_MITIGATIONS could simply be defined in common
> code, but that creates weirdness for x86 because SPECULATION_MITIGATIONS
> ends up being defined twice, and the default behavior would likely depend
> on the arbitrary include order (if the two definitions diverged).
>
> Ideally, CPU_MITIGATIONS would be unconditionally on by default for all
> architectures, and manually turned off, but there is no way to unselect a
> Kconfig.
>
> Reported-by: Stephen Rothwell <sfr@...b.auug.org.au>
> Reported-by: Michael Ellerman <mpe@...erman.id.au>
> Reported-by: Geert Uytterhoeven <geert@...ux-m68k.org>
> Closes: https://lkml.kernel.org/r/20240413115324.53303a68%40canb.auug.org.au
> Fixes: f337a6a21e2f ("x86/cpu: Actually turn off mitigations by default for SPECULATION_MITIGATIONS=n")
> Cc: stable@...r.kernel.org
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---

Thanks for fixing it up.

Tested-by: Michael Ellerman <mpe@...erman.id.au> (powerpc)

cheers