[PATCH] KVM: arm/arm64: initialize new VGIC even without host GIC

[Andre Przywara <andre.przywara@arm.com>]

The new VGIC code is always in the VCPU entry/exit path, even when the
actual GIC hardware initialization found the VGIC unusable (due to
non-aligned base addresses or a missing maintenance interrupt, for
instance).
Since in this case the VGIC structures aren't initialized properly, the
host kernel crashes on a NULL pointer dereference.
Initialize each VCPU's ap_list even with the VGIC unavailable, so the
VGIC code now just iterates an empty list in that case and no longer
crashes the kernel.
Also take care of the arch timer (which becomes unusable as well without
a VGIC) by using a static key to prevent arch timer code to run in the
hot path.
Much of the code was inspired by Marc Zyngier.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reported-by: Stefan Agner <stefan@agner.ch>
---
Hi Stefan, Drew,

can you test this on your systems which showed the issue?

Cheers,
Andre.

 arch/arm/kvm/arm.c            | 14 ++++++++++----
 include/kvm/arm_arch_timer.h  |  2 ++
 virt/kvm/arm/arch_timer.c     | 11 +++++++++++
 virt/kvm/arm/vgic/vgic-init.c |  9 ++++-----
 4 files changed, 27 insertions(+), 9 deletions(-)

diff --git a/arch/arm/kvm/arm.c b/arch/arm/kvm/arm.c
index f1bde7c..cef35ce 100644
--- a/arch/arm/kvm/arm.c
+++ b/arch/arm/kvm/arm.c
@@ -294,7 +294,8 @@ int kvm_arch_vcpu_init(struct kvm_vcpu *vcpu)
 	bitmap_zero(vcpu->arch.features, KVM_VCPU_MAX_FEATURES);
 
 	/* Set up the timer */
-	kvm_timer_vcpu_init(vcpu);
+	if (vgic_present)
+		kvm_timer_vcpu_init(vcpu);
 
 	kvm_arm_reset_debug_ptr(vcpu);
 
@@ -1208,6 +1209,7 @@ static int init_subsystems(void)
 		break;
 	case -ENODEV:
 	case -ENXIO:
+		kvm_err("No useable vgic detected\n");
 		vgic_present = false;
 		err = 0;
 		break;
@@ -1218,9 +1220,13 @@ static int init_subsystems(void)
 	/*
 	 * Init HYP architected timer support
 	 */
-	err = kvm_timer_hyp_init();
-	if (err)
-		goto out;
+	if (vgic_present) {
+		err = kvm_timer_hyp_init();
+		if (err)
+			goto out;
+	} else {
+		static_branch_enable(&kvm_arch_timer_disabled);
+	}
 
 	kvm_perf_init();
 	kvm_coproc_table_init();
diff --git a/include/kvm/arm_arch_timer.h b/include/kvm/arm_arch_timer.h
index dda39d8..64f361f 100644
--- a/include/kvm/arm_arch_timer.h
+++ b/include/kvm/arm_arch_timer.h
@@ -23,6 +23,8 @@
 #include <linux/hrtimer.h>
 #include <linux/workqueue.h>
 
+extern struct static_key_false kvm_arch_timer_disabled;
+
 struct arch_timer_kvm {
 	/* Virtual offset */
 	cycle_t			cntvoff;
diff --git a/virt/kvm/arm/arch_timer.c b/virt/kvm/arm/arch_timer.c
index 4fde8c7..71fb04c 100644
--- a/virt/kvm/arm/arch_timer.c
+++ b/virt/kvm/arm/arch_timer.c
@@ -34,6 +34,8 @@ static struct timecounter *timecounter;
 static struct workqueue_struct *wqueue;
 static unsigned int host_vtimer_irq;
 
+DEFINE_STATIC_KEY_FALSE(kvm_arch_timer_disabled);
+
 void kvm_timer_vcpu_put(struct kvm_vcpu *vcpu)
 {
 	vcpu->arch.timer_cpu.active_cleared_last = false;
@@ -41,6 +43,9 @@ void kvm_timer_vcpu_put(struct kvm_vcpu *vcpu)
 
 static cycle_t kvm_phys_timer_read(void)
 {
+	if (static_branch_unlikely(&kvm_arch_timer_disabled))
+		return 0;
+
 	return timecounter->cc->read(timecounter->cc);
 }
 
@@ -104,6 +109,9 @@ static u64 kvm_timer_compute_delta(struct kvm_vcpu *vcpu)
 {
 	cycle_t cval, now;
 
+	if (static_branch_unlikely(&kvm_arch_timer_disabled))
+		return 0;
+
 	cval = vcpu->arch.timer_cpu.cntv_cval;
 	now = kvm_phys_timer_read() - vcpu->kvm->arch.timer.cntvoff;
 
@@ -148,6 +156,9 @@ static bool kvm_timer_irq_can_fire(struct kvm_vcpu *vcpu)
 {
 	struct arch_timer_cpu *timer = &vcpu->arch.timer_cpu;
 
+	if (static_branch_unlikely(&kvm_arch_timer_disabled))
+		return false;
+
 	return !(timer->cntv_ctl & ARCH_TIMER_CTRL_IT_MASK) &&
 		(timer->cntv_ctl & ARCH_TIMER_CTRL_ENABLE);
 }
diff --git a/virt/kvm/arm/vgic/vgic-init.c b/virt/kvm/arm/vgic/vgic-init.c
index 2c7f0d5..b92133c 100644
--- a/virt/kvm/arm/vgic/vgic-init.c
+++ b/virt/kvm/arm/vgic/vgic-init.c
@@ -56,6 +56,10 @@ void kvm_vgic_early_init(struct kvm *kvm)
 
 void kvm_vgic_vcpu_early_init(struct kvm_vcpu *vcpu)
 {
+	struct vgic_cpu *vgic_cpu = &vcpu->arch.vgic_cpu;
+
+	INIT_LIST_HEAD(&vgic_cpu->ap_list_head);
+	spin_lock_init(&vgic_cpu->ap_list_lock);
 }
 
 /* CREATION */
@@ -392,11 +396,6 @@ int kvm_vgic_hyp_init(void)
 	if (!gic_kvm_info)
 		return -ENODEV;
 
-	if (!gic_kvm_info->maint_irq) {
-		kvm_err("No vgic maintenance irq\n");
-		return -ENXIO;
-	}
-
 	switch (gic_kvm_info->type) {
 	case GIC_V2:
 		ret = vgic_v2_probe(gic_kvm_info);
-- 
2.9.0