linux-kernel - Re: [PATCH 1/6] x86/bugs: Move the X86_FEATURE_USE

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Message-ID: <Z74eaeYm_EgHbmNn@google.com>
Date: Tue, 25 Feb 2025 11:47:53 -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 1/6] x86/bugs: Move the X86_FEATURE_USE_IBPB check into callers

On Wed, Feb 19, 2025, Yosry Ahmed wrote:
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 6c56d5235f0f3..729a8ee24037b 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -1478,7 +1478,8 @@ void vmx_vcpu_load_vmcs(struct kvm_vcpu *vcpu, int cpu,
>  		 * may switch the active VMCS multiple times).
>  		 */
>  		if (!buddy || WARN_ON_ONCE(buddy->vmcs != prev))
> -			indirect_branch_prediction_barrier();
> +			if (cpu_feature_enabled(X86_FEATURE_USE_IBPB))

Combine this into a single if-statement, to make it readable and because as-is
the outer if would need curly braces.

And since this check will stay around in the form of a static_branch, I vote to
check it first so that the checks on "buddy" are elided if vcpu_load_ibpb is disabled.
That'll mean the WARN_ON_ONCE() won't fire if we have a bug and someone is running
with mitigations disabled, but I'm a-ok with that.