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

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Message-ID: <20240413115324.53303a68@canb.auug.org.au>
Date: Sat, 13 Apr 2024 11:53:24 +1000
From: Stephen Rothwell <sfr@...b.auug.org.au>
To: 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, 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>, Michael
 Ellerman <mpe@...erman.id.au>
Subject: Re: [PATCH 1/3] x86/cpu: Actually turn off mitigations by default
 for SPECULATION_MITIGATIONS=n

Hi Sean,

I noticed this commit in linux-next.

On Tue,  9 Apr 2024 10:51:05 -0700 Sean Christopherson <seanjc@...gle.com> wrote:
>
> Initialize cpu_mitigations to CPU_MITIGATIONS_OFF if the kernel is built
> with CONFIG_SPECULATION_MITIGATIONS=n, as the help text quite clearly
> states that disabling SPECULATION_MITIGATIONS is supposed to turn off all
> mitigations by default.
> 
>   │ If you say N, all mitigations will be disabled. You really
>   │ should know what you are doing to say so.
> 
> As is, the kernel still defaults to CPU_MITIGATIONS_AUTO, which results in
> some mitigations being enabled in spite of SPECULATION_MITIGATIONS=n.
> 
> Fixes: f43b9876e857 ("x86/retbleed: Add fine grained Kconfig knobs")
> Cc: stable@...r.kernel.org
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>  kernel/cpu.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> 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)
>  {
> -- 
> 2.44.0.478.gd926399ef9-goog
> 

I noticed because it turned off all mitigations for my PowerPC qemu
boot tests - probably because CONFIG_SPECULATION_MITIGATIONS only
exists in arch/x86/Kconfig ... thus for other architectures that have
cpu mitigations, this will always default them to off, right?

-- 
Cheers,
Stephen Rothwell

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