[PATCH 2/9] KVM: arm/arm64: vgic: Avoid flushing vgic state when there's no pending IRQ

[Christoffer Dall]

From: Shih-Wei Li <shihwei at cs.columbia.edu>

We do not need to flush vgic states in each world switch unless
there is pending IRQ queued to the vgic's ap list. We can thus reduce
the overhead by not grabbing the spinlock and not making the extra
function call to vgic_flush_lr_state.

Signed-off-by: Shih-Wei Li <shihwei at cs.columbia.edu>
Signed-off-by: Christoffer Dall <cdall at linaro.org>
---
 virt/kvm/arm/vgic/vgic.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/virt/kvm/arm/vgic/vgic.c b/virt/kvm/arm/vgic/vgic.c
index 2ac0def..1436c2e 100644
--- a/virt/kvm/arm/vgic/vgic.c
+++ b/virt/kvm/arm/vgic/vgic.c
@@ -637,12 +637,17 @@ static void vgic_flush_lr_state(struct kvm_vcpu *vcpu)
 /* Sync back the hardware VGIC state into our emulation after a guest's run. */
 void kvm_vgic_sync_hwstate(struct kvm_vcpu *vcpu)
 {
+	struct vgic_cpu *vgic_cpu = &vcpu->arch.vgic_cpu;
+
 	if (unlikely(!vgic_initialized(vcpu->kvm)))
 		return;
 
 	vgic_process_maintenance_interrupt(vcpu);
 	vgic_fold_lr_state(vcpu);
 	vgic_prune_ap_list(vcpu);
+
+	/* Make sure we can fast-path in flush_hwstate */
+	vgic_cpu->used_lrs = 0;
 }
 
 /* Flush our emulation state into the GIC hardware before entering the guest. */
@@ -651,6 +656,9 @@ void kvm_vgic_flush_hwstate(struct kvm_vcpu *vcpu)
 	if (unlikely(!vgic_initialized(vcpu->kvm)))
 		return;
 
+	if (list_empty(&vcpu->arch.vgic_cpu.ap_list_head))
+		return;
+
 	spin_lock(&vcpu->arch.vgic_cpu.ap_list_lock);
 	vgic_flush_lr_state(vcpu);
 	spin_unlock(&vcpu->arch.vgic_cpu.ap_list_lock);
-- 
2.9.0