KVM: arm64: vgic: Rely on RCU protection in vgic_get_lpi()

Stop acquiring the lpi_list_lock in favor of RCU for protecting
the read-side critical section in vgic_get_lpi(). In order for this to
be safe, we also need to be careful not to take a reference on an irq
with a refcount of 0, as it is about to be freed.

Reviewed-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20240221054253.3848076-9-oliver.upton@linux.dev
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>

Author: oupton

arch/arm64/kvm/vgic/vgic.c

@@ -63,15 +63,14 @@ static struct vgic_irq *vgic_get_lpi(struct kvm *kvm, u32 intid)
 {
 	struct vgic_dist *dist = &kvm->arch.vgic;
 	struct vgic_irq *irq = NULL;
-	unsigned long flags;
 
-	raw_spin_lock_irqsave(&dist->lpi_list_lock, flags);
+	rcu_read_lock();
 
 	irq = xa_load(&dist->lpi_xa, intid);
-	if (irq)
-		vgic_get_irq_kref(irq);
+	if (!vgic_try_get_irq_kref(irq))
+		irq = NULL;
 
-	raw_spin_unlock_irqrestore(&dist->lpi_list_lock, flags);
+	rcu_read_unlock();
 
 	return irq;
 }

arch/arm64/kvm/vgic/vgic.h

@@ -220,12 +220,20 @@ void vgic_v2_vmcr_sync(struct kvm_vcpu *vcpu);
 void vgic_v2_save_state(struct kvm_vcpu *vcpu);
 void vgic_v2_restore_state(struct kvm_vcpu *vcpu);
 
-static inline void vgic_get_irq_kref(struct vgic_irq *irq)
+static inline bool vgic_try_get_irq_kref(struct vgic_irq *irq)
 {
+	if (!irq)
+		return false;
+
 	if (irq->intid < VGIC_MIN_LPI)
-		return;
+		return true;
 
-	kref_get(&irq->refcount);
+	return kref_get_unless_zero(&irq->refcount);
+}
+
+static inline void vgic_get_irq_kref(struct vgic_irq *irq)
+{
+	WARN_ON_ONCE(!vgic_try_get_irq_kref(irq));
 }
 
 void vgic_v3_fold_lr_state(struct kvm_vcpu *vcpu);