Re: [PATCH 07/15] KVM: Use dedicated flag to track if KVM is handling an NMI from guest

[Sean Christopherson <seanjc@xxxxxxxxxx>]

On Fri, Aug 27, 2021, Peter Zijlstra wrote:
> On Thu, Aug 26, 2021 at 05:57:10PM -0700, Sean Christopherson wrote:
> > diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
> > index 5cedc0e8a5d5..4c5ba4128b38 100644
> > --- a/arch/x86/kvm/x86.h
> > +++ b/arch/x86/kvm/x86.h
> > @@ -395,9 +395,10 @@ static inline void kvm_unregister_perf_callbacks(void)
> >  
> >  DECLARE_PER_CPU(struct kvm_vcpu *, current_vcpu);
> >  
> > -static inline void kvm_before_interrupt(struct kvm_vcpu *vcpu)
> > +static inline void kvm_before_interrupt(struct kvm_vcpu *vcpu, bool is_nmi)
> >  {
> >     __this_cpu_write(current_vcpu, vcpu);
> > +   WRITE_ONCE(vcpu->arch.handling_nmi_from_guest, is_nmi);
> >  
> >     kvm_register_perf_callbacks();
> >  }
> > @@ -406,6 +407,7 @@ static inline void kvm_after_interrupt(struct kvm_vcpu 
> > *vcpu)
> >  {
> >     kvm_unregister_perf_callbacks();
> >  
> > +   WRITE_ONCE(vcpu->arch.handling_nmi_from_guest, false);
> >     __this_cpu_write(current_vcpu, NULL);
> >  }
> 
> Does this rely on kvm_{,un}register_perf_callback() being a function
> call and thus implying a sequence point to order the stores? 

No, I'm just terrible at remembering which macros provide what ordering 
guarantees,
i.e. I was thinking WRITE_ONCE provided guarantees against compiler reordering.