KVM: arm/arm64: Avoid work when userspace iqchips are not used

We currently check if the VM has a userspace irqchip in several places
along the critical path, and if so, we do some work which is only
required for having an irqchip in userspace.  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: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>

arch/arm/include/asm/kvm_host.h

@@ -48,6 +48,8 @@
 	KVM_ARCH_REQ_FLAGS(0, KVM_REQUEST_WAIT | KVM_REQUEST_NO_WAKEUP)
 #define KVM_REQ_IRQ_PENDING	KVM_ARCH_REQ(1)
 
+DECLARE_STATIC_KEY_FALSE(userspace_irqchip_in_use);
+
 u32 *kvm_vcpu_reg(struct kvm_vcpu *vcpu, u8 reg_num, u32 mode);
 int __attribute_const__ kvm_target_cpu(void);
 int kvm_reset_vcpu(struct kvm_vcpu *vcpu);

arch/arm64/include/asm/kvm_host.h

@@ -47,6 +47,8 @@
 	KVM_ARCH_REQ_FLAGS(0, KVM_REQUEST_WAIT | KVM_REQUEST_NO_WAKEUP)
 #define KVM_REQ_IRQ_PENDING	KVM_ARCH_REQ(1)
 
+DECLARE_STATIC_KEY_FALSE(userspace_irqchip_in_use);
+
 int __attribute_const__ kvm_target_cpu(void);
 int kvm_reset_vcpu(struct kvm_vcpu *vcpu);
 int kvm_arch_dev_ioctl_check_extension(struct kvm *kvm, long ext);

virt/kvm/arm/arch_timer.c

@@ -103,7 +103,8 @@ static irqreturn_t kvm_arch_timer_handler(int irq, void *dev_id)
 	if (kvm_timer_irq_can_fire(vtimer))
 		kvm_timer_update_irq(vcpu, true, vtimer);
 
-	if (unlikely(!irqchip_in_kernel(vcpu->kvm)))
+	if (static_branch_unlikely(&userspace_irqchip_in_use) &&
+	    unlikely(!irqchip_in_kernel(vcpu->kvm)))
 		kvm_vtimer_update_mask_user(vcpu);
 
 	return IRQ_HANDLED;
@@ -284,7 +285,8 @@ static void kvm_timer_update_irq(struct kvm_vcpu *vcpu, bool new_level,
 	trace_kvm_timer_update_irq(vcpu->vcpu_id, timer_ctx->irq.irq,
 				   timer_ctx->irq.level);
 
-	if (likely(irqchip_in_kernel(vcpu->kvm))) {
+	if (!static_branch_unlikely(&userspace_irqchip_in_use) &&
+	    likely(irqchip_in_kernel(vcpu->kvm))) {
 		ret = kvm_vgic_inject_irq(vcpu->kvm, vcpu->vcpu_id,
 					  timer_ctx->irq.irq,
 					  timer_ctx->irq.level,

virt/kvm/arm/arm.c

@@ -74,6 +74,8 @@ static void kvm_arm_set_running_vcpu(struct kvm_vcpu *vcpu)
 	__this_cpu_write(kvm_arm_running_vcpu, vcpu);
 }
 
+DEFINE_STATIC_KEY_FALSE(userspace_irqchip_in_use);
+
 /**
  * kvm_arm_get_running_vcpu - get the vcpu running on the current CPU.
  * Must be called from non-preemptible context
@@ -302,6 +304,8 @@ void kvm_arch_vcpu_free(struct kvm_vcpu *vcpu)
 
 void kvm_arch_vcpu_destroy(struct kvm_vcpu *vcpu)
 {
+	if (vcpu->arch.has_run_once && unlikely(!irqchip_in_kernel(vcpu->kvm)))
+		static_branch_dec(&userspace_irqchip_in_use);
 	kvm_arch_vcpu_free(vcpu);
 }
 
@@ -522,14 +526,22 @@ static int kvm_vcpu_first_run_init(struct kvm_vcpu *vcpu)
 
 	vcpu->arch.has_run_once = true;
 
-	/*
-	 * Map the VGIC hardware resources before running a vcpu the first
-	 * time on this VM.
-	 */
-	if (unlikely(irqchip_in_kernel(kvm) && !vgic_ready(kvm))) {
-		ret = kvm_vgic_map_resources(kvm);
-		if (ret)
-			return ret;
+	if (likely(irqchip_in_kernel(kvm))) {
+		/*
+		 * Map the VGIC hardware resources before running a vcpu the
+		 * first time on this VM.
+		 */
+		if (unlikely(!vgic_ready(kvm))) {
+			ret = kvm_vgic_map_resources(kvm);
+			if (ret)
+				return ret;
+		}
+	} else {
+		/*
+		 * Tell the rest of the code that there are userspace irqchip
+		 * VMs in the wild.
+		 */
+		static_branch_inc(&userspace_irqchip_in_use);
 	}
 
 	ret = kvm_timer_enable(vcpu);
@@ -664,18 +676,29 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu, struct kvm_run *run)
 		kvm_vgic_flush_hwstate(vcpu);
 
 		/*
-		 * If we have a singal pending, or need to notify a userspace
-		 * irqchip about timer or PMU level changes, then we exit (and
-		 * update the timer level state in kvm_timer_update_run
-		 * below).
+		 * Exit if we have a signal pending so that we can deliver the
+		 * signal to user space.
 		 */
-		if (signal_pending(current) ||
-		    kvm_timer_should_notify_user(vcpu) ||
-		    kvm_pmu_should_notify_user(vcpu)) {
+		if (signal_pending(current)) {
 			ret = -EINTR;
 			run->exit_reason = KVM_EXIT_INTR;
 		}
 
+		/*
+		 * If we're using a userspace irqchip, then check if we need
+		 * to tell a userspace irqchip about timer or PMU level
+		 * changes and if so, exit to userspace (the actual level
+		 * state gets updated in kvm_timer_update_run and
+		 * kvm_pmu_update_run below).
+		 */
+		if (static_branch_unlikely(&userspace_irqchip_in_use)) {
+			if (kvm_timer_should_notify_user(vcpu) ||
+			    kvm_pmu_should_notify_user(vcpu)) {
+				ret = -EINTR;
+				run->exit_reason = KVM_EXIT_INTR;
+			}
+		}
+
 		/*
 		 * Ensure we set mode to IN_GUEST_MODE after we disable
 		 * interrupts and before the final VCPU requests check.
@@ -688,7 +711,8 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu, struct kvm_run *run)
 		    kvm_request_pending(vcpu)) {
 			vcpu->mode = OUTSIDE_GUEST_MODE;
 			kvm_pmu_sync_hwstate(vcpu);
-			kvm_timer_sync_hwstate(vcpu);
+			if (static_branch_unlikely(&userspace_irqchip_in_use))
+				kvm_timer_sync_hwstate(vcpu);
 			kvm_vgic_sync_hwstate(vcpu);
 			local_irq_enable();
 			preempt_enable();
@@ -732,7 +756,8 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu, struct kvm_run *run)
 		 * we don't want vtimer interrupts to race with syncing the
 		 * timer virtual interrupt state.
 		 */
-		kvm_timer_sync_hwstate(vcpu);
+		if (static_branch_unlikely(&userspace_irqchip_in_use))
+			kvm_timer_sync_hwstate(vcpu);
 
 		/*
 		 * We may have taken a host interrupt in HYP mode (ie