Re: [PATCH v2 35/43] KVM: SVM: Signal AVIC doorbell iff vCPU is in guest mode

[Maxim Levitsky]

On Fri, 2021-10-08 at 19:12 -0700, Sean Christopherson wrote:
> Signal the AVIC doorbell iff the vCPU is running in the guest.  If the vCPU
> is not IN_GUEST_MODE, it's guaranteed to pick up any pending IRQs on the
> next VMRUN, which unconditionally processes the vIRR.
> 
> Add comments to document the logic.
> 
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> ---
>  arch/x86/kvm/svm/avic.c | 14 ++++++++++++--
>  1 file changed, 12 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kvm/svm/avic.c b/arch/x86/kvm/svm/avic.c
> index 208c5c71e827..cbf02e7e20d0 100644
> --- a/arch/x86/kvm/svm/avic.c
> +++ b/arch/x86/kvm/svm/avic.c
> @@ -674,7 +674,12 @@ int svm_deliver_avic_intr(struct kvm_vcpu *vcpu, int vec)
>  	kvm_lapic_set_irr(vec, vcpu->arch.apic);
>  	smp_mb__after_atomic();
>  
> -	if (avic_vcpu_is_running(vcpu)) {
> +	/*
> +	 * Signal the doorbell to tell hardware to inject the IRQ if the vCPU
> +	 * is in the guest.  If the vCPU is not in the guest, hardware will
> +	 * automatically process AVIC interrupts at VMRUN.
> +	 */
> +	if (vcpu->mode == IN_GUEST_MODE) {
>  		int cpu = READ_ONCE(vcpu->cpu);
>  
>  		/*
> @@ -687,8 +692,13 @@ int svm_deliver_avic_intr(struct kvm_vcpu *vcpu, int vec)
>  		if (cpu != get_cpu())
>  			wrmsrl(SVM_AVIC_DOORBELL, kvm_cpu_get_apicid(cpu));
>  		put_cpu();
> -	} else
> +	} else {
> +		/*
> +		 * Wake the vCPU if it was blocking.  KVM will then detect the
> +		 * pending IRQ when checking if the vCPU has a wake event.
> +		 */
>  		kvm_vcpu_wake_up(vcpu);
> +	}
>  
>  	return 0;
>  }

It makes sense indeed to avoid ringing the doorbell when the vCPU is not in the guest mode.

I do wonder if we want to call kvm_vcpu_wake_up always otherwise, as the vCPU might
be just outside of the guest mode and not scheduled out. I don't know how expensive
is kvm_vcpu_wake_up in this case.

Before this patch, the avic_vcpu_is_running would only be false when the vCPU is scheduled out
(e.g when vcpu_put was done on it)

Best regards,
	Maxim Levitsky