Re: [PATCH 1/6] x86/bugs: Move the X86_FEATURE_USE_IBPB check into callers

[Yosry Ahmed]

February 25, 2025 at 11:47 AM, "Sean Christopherson" <seanjc@xxxxxxxxxx> 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!