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

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Message-ID: <Z75G2L6N1vR3DslT@google.com>
Date: Tue, 25 Feb 2025 14:40:24 -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 Tue, Feb 25, 2025, Yosry Ahmed wrote:
> 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.

I assumed as much :-)  I'm ok with vcpu_load_ibpb if that's what others prefer.