linux-kernel - Re: [PATCH 4/6] x86/bugs: Use a static branch to guard IBPB on vCPU load

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Message-ID: <Z74exImxJpQI9iyA@google.com>
Date: Tue, 25 Feb 2025 11:49:24 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Yosry Ahmed <yosry.ahmed@...ux.dev>
Cc: x86@...nel.org, Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, 
	Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>, 
	"H. Peter Anvin" <hpa@...or.com>, Peter Zijlstra <peterz@...radead.org>, 
	Josh Poimboeuf <jpoimboe@...nel.org>, Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>, 
	Andy Lutomirski <luto@...nel.org>, Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/6] x86/bugs: Use a static branch to guard IBPB on vCPU load

On Wed, Feb 19, 2025, Yosry Ahmed wrote:
> Instead of using X86_FEATURE_USE_IBPB to guard the IBPB execution in the
> vCPU load path, introduce a static branch, similar to switch_mm_*_ibpb.
> 
> This makes it obvious in spectre_v2_user_select_mitigation() what
> exactly is being toggled, instead of the unclear X86_FEATURE_USE_IBPB
> (which will be shortly removed). It also provides more fine-grained
> control, making it simpler to change/add paths that control the IBPB in
> the vCPU load path without affecting other IBPBs.
> 
> Signed-off-by: Yosry Ahmed <yosry.ahmed@...ux.dev>
> ---
>  arch/x86/include/asm/nospec-branch.h | 2 ++
>  arch/x86/kernel/cpu/bugs.c           | 5 +++++
>  arch/x86/kvm/svm/svm.c               | 2 +-
>  arch/x86/kvm/vmx/vmx.c               | 2 +-
>  4 files changed, 9 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
> index 7cbb76a2434b9..a22836c5fb338 100644
> --- a/arch/x86/include/asm/nospec-branch.h
> +++ b/arch/x86/include/asm/nospec-branch.h
> @@ -552,6 +552,8 @@ DECLARE_STATIC_KEY_FALSE(switch_to_cond_stibp);
>  DECLARE_STATIC_KEY_FALSE(switch_mm_cond_ibpb);
>  DECLARE_STATIC_KEY_FALSE(switch_mm_always_ibpb);
>  
> +DECLARE_STATIC_KEY_FALSE(vcpu_load_ibpb);

How about ibpb_on_vcpu_load?  To make it easy for readers to understand exactly
what the knob controls.