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

Sean Christopherson
2025-02-27T00:46:08Z

On Wed, Feb 26, 2025, Yosry Ahmed wrote:
> On Tue, Feb 25, 2025 at 02:40:24PM -0800, Sean Christopherson wrote:
> > On Tue, Feb 25, 2025, Yosry Ahmed wrote:
> > > February 25, 2025 at 11:49 AM, "Sean Christopherson" <seanjc@google.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@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.
> > >
> > > 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.
>
> To be honest looking at this again I think I prefer consistency, so if
> you don't mind and others don't chime in I'd rather keep it as-is.

Works for me.

Actually, looking at the names again, wouldn't "switch_vcpu_ibpb" be better?
KVM doesn't do IBPB on every vCPU load or even every VMCS load, only when a
different vCPU is being loaded.

> Alternatively I can rename all the static branches (e.g.
> ibpb_always_on_switch_mm) :P

LOL, also works for me.