KVM: arm64: vgic: Don't hold config_lock while unregistering redistributors

We recently moved the teardown of the vgic part of a vcpu inside
a critical section guarded by the config_lock. This teardown phase
involves calling into kvm_io_bus_unregister_dev(), which takes the
kvm->srcu lock.

However, this violates the established order where kvm->srcu is
taken on a memory fault (such as an MMIO access), possibly
followed by taking the config_lock if the GIC emulation requires
mutual exclusion from the other vcpus.

It therefore results in a bad lockdep splat, as reported by Zenghui.

Fix this by moving the call to kvm_io_bus_unregister_dev() outside
of the config_lock critical section. At this stage, there shouln't
be any need to hold the config_lock.

As an additional bonus, document the ordering between kvm->slots_lock,
kvm->srcu and kvm->arch.config_lock so that I cannot pretend I didn't
know about those anymore.

Fixes: 9eb18136 ("KVM: arm64: vgic: Hold config_lock while tearing down a CPU interface")
Reported-by: Zenghui Yu <yuzenghui@huawei.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Zenghui Yu <yuzenghui@huawei.com>
Tested-by: Zenghui Yu <yuzenghui@huawei.com>
Link: https://lore.kernel.org/r/20240819125045.3474845-1-maz@kernel.orgSigned-off-by: Oliver Upton <oliver.upton@linux.dev>

arch/arm64/kvm/vgic/vgic-init.c

@@ -417,10 +417,8 @@ static void __kvm_vgic_vcpu_destroy(struct kvm_vcpu *vcpu)
 	kfree(vgic_cpu->private_irqs);
 	vgic_cpu->private_irqs = NULL;
 
-	if (vcpu->kvm->arch.vgic.vgic_model == KVM_DEV_TYPE_ARM_VGIC_V3) {
-		vgic_unregister_redist_iodev(vcpu);
+	if (vcpu->kvm->arch.vgic.vgic_model == KVM_DEV_TYPE_ARM_VGIC_V3)
 		vgic_cpu->rd_iodev.base_addr = VGIC_ADDR_UNDEF;
-	}
 }
 
 void kvm_vgic_vcpu_destroy(struct kvm_vcpu *vcpu)
@@ -448,6 +446,11 @@ void kvm_vgic_destroy(struct kvm *kvm)
 	kvm_vgic_dist_destroy(kvm);
 
 	mutex_unlock(&kvm->arch.config_lock);
+
+	if (kvm->arch.vgic.vgic_model == KVM_DEV_TYPE_ARM_VGIC_V3)
+		kvm_for_each_vcpu(i, vcpu, kvm)
+			vgic_unregister_redist_iodev(vcpu);
+
 	mutex_unlock(&kvm->slots_lock);
 }
 

arch/arm64/kvm/vgic/vgic.c

@@ -36,6 +36,11 @@ struct vgic_global kvm_vgic_global_state __ro_after_init = {
  * we have to disable IRQs before taking this lock and everything lower
  * than it.
  *
+ * The config_lock has additional ordering requirements:
+ * kvm->slots_lock
+ *   kvm->srcu
+ *     kvm->arch.config_lock
+ *
  * If you need to take multiple locks, always take the upper lock first,
  * then the lower ones, e.g. first take the its_lock, then the irq_lock.
  * If you are already holding a lock and need to take a higher one, you