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

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Message-ID: <59ea1984b2893be8a3a72855b022d16c67b857e9@linux.dev>
Date: Tue, 25 Feb 2025 21:27:06 +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 4/6] x86/bugs: Use a static branch to guard IBPB on vCPU
 load

February 25, 2025 at 11:49 AM, "Sean Christopherson" <seanjc@...gle.com> wrote:
>
> 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.

I was trying to remain consistent with the existing static branches' names, but I am fine with ibpb_on_vcpu_load if others don't object.