linux-kernel - Re: [PATCH v6 6/9] x86/vmscape: Use static

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Message-ID: <20251211105050.GB3707891@noisy.programming.kicks-ass.net>
Date: Thu, 11 Dec 2025 11:50:50 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
Cc: x86@...nel.org, David Kaplan <david.kaplan@....com>,
	Nikolay Borisov <nik.borisov@...e.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>,
	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 v6 6/9] x86/vmscape: Use static_call() for predictor flush

On Mon, Dec 01, 2025 at 10:20:14PM -0800, Pawan Gupta wrote:
> Adding more mitigation options at exit-to-userspace for VMSCAPE would
> usually require a series of checks to decide which mitigation to use. In
> this case, the mitigation is done by calling a function, which is decided
> at boot. So, adding more feature flags and multiple checks can be avoided
> by using static_call() to the mitigating function.
> 
> Replace the flag-based mitigation selector with a static_call(). This also
> frees the existing X86_FEATURE_IBPB_EXIT_TO_USER.
> 
> Suggested-by: Dave Hansen <dave.hansen@...ux.intel.com>
> Signed-off-by: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
> ---
>  arch/x86/Kconfig                     | 1 +
>  arch/x86/include/asm/cpufeatures.h   | 2 +-
>  arch/x86/include/asm/entry-common.h  | 7 +++----
>  arch/x86/include/asm/nospec-branch.h | 3 +++
>  arch/x86/kernel/cpu/bugs.c           | 5 ++++-
>  arch/x86/kvm/x86.c                   | 2 +-
>  6 files changed, 13 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index fa3b616af03a2d50eaf5f922bc8cd4e08a284045..066f62f15e67e85fda0f3fd66acabad9a9794ff8 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -2706,6 +2706,7 @@ config MITIGATION_TSA
>  config MITIGATION_VMSCAPE
>  	bool "Mitigate VMSCAPE"
>  	depends on KVM
> +	select HAVE_STATIC_CALL

That can't be right.