linux-kernel - Re: [RFC PATCH 14/56] x86/bugs: Reset GDS mitigations

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Message-ID: <20251024024054.6gum6dmztyrd6u46@desk>
Date: Thu, 23 Oct 2025 19:40:54 -0700
From: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
To: David Kaplan <david.kaplan@....com>
Cc: Thomas Gleixner <tglx@...utronix.de>, Borislav Petkov <bp@...en8.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Josh Poimboeuf <jpoimboe@...nel.org>,
	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 14/56] x86/bugs: Reset GDS mitigations

On Mon, Oct 13, 2025 at 09:34:02AM -0500, David Kaplan wrote:
> Add function to reset GDS mitigations back to their boot-time defaults.
> 
> Signed-off-by: David Kaplan <david.kaplan@....com>
> ---
>  arch/x86/kernel/cpu/bugs.c | 13 +++++++++++++
>  1 file changed, 13 insertions(+)
> 
> diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
> index 112553058ccc..e765ac0b9240 100644
> --- a/arch/x86/kernel/cpu/bugs.c
> +++ b/arch/x86/kernel/cpu/bugs.c
> @@ -1182,6 +1182,18 @@ static void __init gds_apply_mitigation(void)
>  	pr_info("%s\n", gds_strings[gds_mitigation]);
>  }
>  
> +#ifdef CONFIG_DYNAMIC_MITIGATIONS
> +static void gds_reset_mitigation(void)
> +{
> +	/* To cause the MSR bit to be cleared. */
> +	gds_mitigation = GDS_MITIGATION_OFF;
> +	if (x86_arch_cap_msr & ARCH_CAP_GDS_CTRL)
> +		update_gds_msr();

This also needs to check for GDS_MITIGATION_FULL_LOCKED, otherwise in the
locked case it would trigger this warning in update_gds_msr():

	WARN_ON_ONCE(mcu_ctrl != mcu_ctrl_after);