linux-kernel - Re: [PATCH v4 06/11] x86/vmscape: Move mitigation selection to a switch()

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Message-ID: <c8d197cb-bd8d-42b0-a32b-8d8f77c96567@suse.com>
Date: Fri, 21 Nov 2025 16:27:05 +0200
From: Nikolay Borisov <nik.borisov@...e.com>
To: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>, x86@...nel.org,
 David Kaplan <david.kaplan@....com>, "H. Peter Anvin" <hpa@...or.com>,
 Josh Poimboeuf <jpoimboe@...nel.org>, Sean Christopherson
 <seanjc@...gle.com>, Paolo Bonzini <pbonzini@...hat.com>,
 Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>
Cc: linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
 Asit Mallick <asit.k.mallick@...el.com>, Tao Zhang <tao1.zhang@...el.com>
Subject: Re: [PATCH v4 06/11] x86/vmscape: Move mitigation selection to a
 switch()

On 11/20/25 08:19, Pawan Gupta wrote:
> This ensures that all mitigation modes are explicitly handled, while
> keeping the mitigation selection for each mode together. This also prepares
> for adding BHB-clearing mitigation mode for VMSCAPE.
> 
> Signed-off-by: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
> ---
>   arch/x86/kernel/cpu/bugs.c | 22 ++++++++++++++++++----
>   1 file changed, 18 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
> index 1e9b11198db0fe2483bd17b1327bcfd44a2c1dbf..233594ede19bf971c999f4d3cc0f6f213002c16c 100644
> --- a/arch/x86/kernel/cpu/bugs.c
> +++ b/arch/x86/kernel/cpu/bugs.c
> @@ -3231,17 +3231,31 @@ early_param("vmscape", vmscape_parse_cmdline);
>   
>   static void __init vmscape_select_mitigation(void)
>   {
> -	if (!boot_cpu_has_bug(X86_BUG_VMSCAPE) ||
> -	    !boot_cpu_has(X86_FEATURE_IBPB)) {
> +	if (!boot_cpu_has_bug(X86_BUG_VMSCAPE)) {
>   		vmscape_mitigation = VMSCAPE_MITIGATION_NONE;
>   		return;
>   	}
>   
> -	if (vmscape_mitigation == VMSCAPE_MITIGATION_AUTO) {
> -		if (should_mitigate_vuln(X86_BUG_VMSCAPE))
> +	if ((vmscape_mitigation == VMSCAPE_MITIGATION_AUTO) &&
> +	    !should_mitigate_vuln(X86_BUG_VMSCAPE))
> +		vmscape_mitigation = VMSCAPE_MITIGATION_NONE;
> +
> +	switch (vmscape_mitigation) {
> +	case VMSCAPE_MITIGATION_NONE:
> +		break;
> +
> +	case VMSCAPE_MITIGATION_IBPB_ON_VMEXIT:
> +	case VMSCAPE_MITIGATION_IBPB_EXIT_TO_USER:
> +		if (!boot_cpu_has(X86_FEATURE_IBPB))
> +			vmscape_mitigation = VMSCAPE_MITIGATION_NONE;
> +		break;
> +
> +	case VMSCAPE_MITIGATION_AUTO:
> +		if (boot_cpu_has(X86_FEATURE_IBPB))
>   			vmscape_mitigation = VMSCAPE_MITIGATION_IBPB_EXIT_TO_USER;

IMO this patch is a net-negative because as per my reply to patch 9 you 
have effectively a dead branch:

The clear BHB_CLEAR_USER one, however it turns out you have yet another 
one: VMSCAPE_MITIGATION_IBPB_ON_VMEXIT as it's only ever set in 
vmscape_update_mitigation() which executes after '_select()' as well and 
additionally you duplicate the FEATURE_IBPB check.

So I think either dropping it or removing the superfluous branches is in 
order.

>   		else
>   			vmscape_mitigation = VMSCAPE_MITIGATION_NONE;
> +		break;
>   	}
>   }
>   
>