linux-kernel - Re: [RFC PATCH 04/56] x86/bugs: Reset spectre

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Message-ID: <20251029115719.GEaQIBH1j4vVEQLas9@fat_crate.local>
Date: Wed, 29 Oct 2025 12:57:19 +0100
From: Borislav Petkov <bp@...en8.de>
To: David Kaplan <david.kaplan@....com>
Cc: Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Josh Poimboeuf <jpoimboe@...nel.org>,
	Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
	Ingo Molnar <mingo@...hat.com>,
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
	"H . Peter Anvin" <hpa@...or.com>, Alexander Graf <graf@...zon.com>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 04/56] x86/bugs: Reset spectre_v1 mitigations

On Mon, Oct 13, 2025 at 09:33:52AM -0500, David Kaplan wrote:
> diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
> index 6a526ae1fe99..9d5c6a3e50e1 100644
> --- a/arch/x86/kernel/cpu/bugs.c
> +++ b/arch/x86/kernel/cpu/bugs.c
> @@ -46,6 +46,8 @@
>   *				   may want to change based on other choices
>   *				   made.  This function is optional.
>   *   <vuln>_apply_mitigation() -- Enable the selected mitigation.
> + *   <vuln>_reset_mitigation() -- Undo's the apply_mitigation step, this is used

"Undoes" no?

> + *				  with runtime mitigation patching.
>   *
>   * The compile-time mitigation in all cases should be AUTO.  An explicit
>   * command-line option can override AUTO.  If no such option is
> @@ -1247,6 +1249,15 @@ static void __init spectre_v1_apply_mitigation(void)
>  	pr_info("%s\n", spectre_v1_strings[spectre_v1_mitigation]);
>  }
>  
> +#ifdef CONFIG_DYNAMIC_MITIGATIONS
> +static void spectre_v1_reset_mitigation(void)
> +{
> +	setup_clear_cpu_cap(X86_FEATURE_FENCE_SWAPGS_USER);
> +	setup_clear_cpu_cap(X86_FEATURE_FENCE_SWAPGS_KERNEL);
> +	spectre_v1_mitigation = SPECTRE_V1_MITIGATION_AUTO;
> +}
> +#endif

Right, let's see in the end what is easier: we have the cross-dependencies
between mitigations so having separate "undo" functions might require them to
run in reverse order to the "apply" functions. I can imagine a single "undo"
function would be easier because you have everything in one place.

We'll see.

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette