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

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Message-ID: <76526510ba3a81f812d16aabb3b45e2dead2fa35@linux.dev>
Date: Tue, 25 Feb 2025 21:28:56 +0000
From: "Yosry Ahmed" <yosry.ahmed@...ux.dev>
To: "Sean Christopherson" <seanjc@...gle.com>
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

February 25, 2025 at 11:47 AM, "Sean Christopherson" <seanjc@...gle.com> wrote:
> 
> 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.

SGTM, will do that in the next version. Thanks!