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

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Message-ID: <87mspp9bm6.fsf@mail.lhotse>
Date: Sat, 20 Apr 2024 09:27:45 +1000
From: Michael Ellerman <mpe@...erman.id.au>
To: Josh Poimboeuf <jpoimboe@...nel.org>, Sean Christopherson
 <seanjc@...gle.com>
Cc: 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>,
 linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org, Stephen Rothwell
 <sfr@...b.auug.org.au>, Geert Uytterhoeven <geert@...ux-m68k.org>
Subject: Re: [PATCH 1/2] cpu: Re-enable CPU mitigations by default for !X86
 architectures

Josh Poimboeuf <jpoimboe@...nel.org> writes:
> On Tue, Apr 16, 2024 at 05:15:06PM -0700, Sean Christopherson wrote:
>> 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>
>
> It seems confusing to have two config options which have very similar
> names and similar purposes (with subtle differences depending on the
> arch).

I agree.

But can we please get Sean's fix into mainline before rc5.

cheers