[PATCH v8 8/9] KVM: arm/arm64: Avoid work when userspace iqchips are not used

[Christoffer Dall]

We currently check if the VM has a userspace irqchip on every exit from
the VCPU, and if so, we do some work to ensure correct timer behavior.
This is unfortunate, as we could avoid doing any work entirely, if we
didn't have to support irqchip in userspace.

Realizing the userspace irqchip on ARM is mostly a developer or hobby
feature, and is unlikely to be used in servers or other scenarios where
performance is a priority, we can use a refcounted static key to only
check the irqchip configuration when we have at least one VM that uses
an irqchip in userspace.

Reviewed-by: Eric Auger <eric.auger at redhat.com>
Signed-off-by: Christoffer Dall <christoffer.dall at linaro.org>
---
 virt/kvm/arm/arch_timer.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/virt/kvm/arm/arch_timer.c b/virt/kvm/arm/arch_timer.c
index f8d09665ddce..73d262c4712b 100644
--- a/virt/kvm/arm/arch_timer.c
+++ b/virt/kvm/arm/arch_timer.c
@@ -51,6 +51,8 @@ static void kvm_timer_update_irq(struct kvm_vcpu *vcpu, bool new_level,
 				 struct arch_timer_context *timer_ctx);
 static bool kvm_timer_should_fire(struct arch_timer_context *timer_ctx);
 
+static DEFINE_STATIC_KEY_FALSE(userspace_irqchip_in_use);
+
 u64 kvm_phys_timer_read(void)
 {
 	return timecounter->cc->read(timecounter->cc);
@@ -562,7 +564,8 @@ static void unmask_vtimer_irq_user(struct kvm_vcpu *vcpu)
 
 void kvm_timer_sync_hwstate(struct kvm_vcpu *vcpu)
 {
-	unmask_vtimer_irq_user(vcpu);
+	if (static_branch_unlikely(&userspace_irqchip_in_use))
+		unmask_vtimer_irq_user(vcpu);
 }
 
 int kvm_timer_vcpu_reset(struct kvm_vcpu *vcpu)
@@ -767,6 +770,8 @@ void kvm_timer_vcpu_terminate(struct kvm_vcpu *vcpu)
 	soft_timer_cancel(&timer->bg_timer, &timer->expired);
 	soft_timer_cancel(&timer->phys_timer, NULL);
 	kvm_vgic_unmap_phys_irq(vcpu, vtimer->irq.irq);
+	if (timer->enabled && !irqchip_in_kernel(vcpu->kvm))
+		static_branch_dec(&userspace_irqchip_in_use);
 }
 
 static bool timer_irqs_are_valid(struct kvm_vcpu *vcpu)
@@ -819,8 +824,10 @@ int kvm_timer_enable(struct kvm_vcpu *vcpu)
 		return 0;
 
 	/* Without a VGIC we do not map virtual IRQs to physical IRQs */
-	if (!irqchip_in_kernel(vcpu->kvm))
+	if (!irqchip_in_kernel(vcpu->kvm)) {
+		static_branch_inc(&userspace_irqchip_in_use);
 		goto no_vgic;
+	}
 
 	if (!vgic_initialized(vcpu->kvm))
 		return -ENODEV;
-- 
2.14.2