Re: [PATCH 2/2] KVM: x86: check_nested_events if there is an injectable NMI

[Sean Christopherson]

On Tue, Apr 14, 2020 at 04:11:07PM -0400, Cathy Avery wrote:
> With NMI intercept moved to check_nested_events there is a race
> condition where vcpu->arch.nmi_pending is set late causing

How is nmi_pending set late?  The KVM_{G,S}ET_VCPU_EVENTS paths can't set
it because the current KVM_RUN thread holds the mutex, and the only other
call to process_nmi() is in the request path of vcpu_enter_guest, which has
already executed.

> the execution of check_nested_events to not setup correctly
> for nested.exit_required. A second call to check_nested_events
> allows the injectable nmi to be detected in time in order to
> require immediate exit from L2 to L1.
> 
> Signed-off-by: Cathy Avery <cavery@xxxxxxxxxx>
> ---
>  arch/x86/kvm/x86.c | 15 +++++++++++----
>  1 file changed, 11 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 027dfd278a97..ecfafcd93536 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -7734,10 +7734,17 @@ static int inject_pending_event(struct kvm_vcpu *vcpu)
>  		vcpu->arch.smi_pending = false;
>  		++vcpu->arch.smi_count;
>  		enter_smm(vcpu);
> -	} else if (vcpu->arch.nmi_pending && kvm_x86_ops.nmi_allowed(vcpu)) {
> -		--vcpu->arch.nmi_pending;
> -		vcpu->arch.nmi_injected = true;
> -		kvm_x86_ops.set_nmi(vcpu);
> +	} else if (vcpu->arch.nmi_pending) {
> +		if (is_guest_mode(vcpu) && kvm_x86_ops.check_nested_events) {
> +			r = kvm_x86_ops.check_nested_events(vcpu);
> +			if (r != 0)
> +				return r;
> +		}
> +		if (kvm_x86_ops.nmi_allowed(vcpu)) {
> +			--vcpu->arch.nmi_pending;
> +			vcpu->arch.nmi_injected = true;
> +			kvm_x86_ops.set_nmi(vcpu);
> +		}
>  	} else if (kvm_cpu_has_injectable_intr(vcpu)) {
>  		/*
>  		 * Because interrupts can be injected asynchronously, we are
> -- 
> 2.20.1
>