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
