linux-kernel - Re: [RFC PATCH 15/56] x86/bugs: Reset BHI mitigations

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Message-ID: <20251024024918.5ytpa7vrcny6tngt@desk>
Date: Thu, 23 Oct 2025 19:49:18 -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 15/56] x86/bugs: Reset BHI mitigations

On Mon, Oct 13, 2025 at 09:34:03AM -0500, David Kaplan wrote:
> Add function to reset BHI mitigations back to their boot-time defaults.
> 
> Signed-off-by: David Kaplan <david.kaplan@....com>
> ---
>  arch/x86/kernel/cpu/bugs.c | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
> 
> diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
> index e765ac0b9240..67561e5c2154 100644
> --- a/arch/x86/kernel/cpu/bugs.c
> +++ b/arch/x86/kernel/cpu/bugs.c
> @@ -2360,6 +2360,17 @@ static void __init bhi_apply_mitigation(void)
>  	setup_force_cpu_cap(X86_FEATURE_CLEAR_BHB_VMEXIT);
>  }
>  
> +#ifdef CONFIG_DYNAMIC_MITIGATIONS
> +static void bhi_reset_mitigation(void)
> +{
> +	/* RRSBA already cleared in spectre_v2_reset_mitigation() */
> +	setup_clear_cpu_cap(X86_FEATURE_CLEAR_BHB_VMEXIT);
> +	setup_clear_cpu_cap(X86_FEATURE_CLEAR_BHB_LOOP);

Also needs to reset SPEC_CTRL_BHI_DIS_S in x86_spec_ctrl_base.

An alternative is to add spec_ctrl_reset_mitigation() that resets
x86_spec_ctrl_base for SPEC_CTRL_MITIGATIONS_MASK. To be consistent with
reset functions of other mitigations, probably also reset the MSR.

> +	bhi_mitigation = IS_ENABLED(CONFIG_MITIGATION_SPECTRE_BHI) ?
> +		BHI_MITIGATION_AUTO : BHI_MITIGATION_OFF;
> +}