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

Sean Christopherson
2025-02-25T19:49:26Z

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@linux.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.