KVM: arm64: Make KVM_CAP_MAX_VCPUS compatible with the selected GIC version

KVM_CAP_MAX_VCPUS always return the maximum possible number of
VCPUs, irrespective of the selected interrupt controller. This
is pretty misleading for userspace that selects a GICv2 on a GICv3
system that supports v2 compat: It always gets a maximum of 512
VCPUs, even if the effective limit is 8. The 9th VCPU will fail
to be created, which is unexpected as far as userspace is concerned.

Fortunately, we already have the right information stashed in the
kvm structure, and we can return it as requested.
Reported-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Tested-by: Alexandru Elisei <alexandru.elisei@arm.com>
Reviewed-by: Alexandru Elisei <alexandru.elisei@arm.com>
Link: https://lore.kernel.org/r/20200427141507.284985-1-maz@kernel.org

arch/arm64/kvm/arm.c

@@ -95,6 +95,11 @@ int kvm_vm_ioctl_enable_cap(struct kvm *kvm,
 	return r;
 }
 
+static int kvm_arm_default_max_vcpus(void)
+{
+	return vgic_present ? kvm_vgic_get_max_vcpus() : KVM_MAX_VCPUS;
+}
+
 /**
  * kvm_arch_init_vm - initializes a VM data structure
  * @kvm:	pointer to the KVM struct
@@ -128,8 +133,7 @@ int kvm_arch_init_vm(struct kvm *kvm, unsigned long type)
 	kvm->arch.vmid.vmid_gen = 0;
 
 	/* The maximum number of VCPUs is limited by the host's GIC model */
-	kvm->arch.max_vcpus = vgic_present ?
-				kvm_vgic_get_max_vcpus() : KVM_MAX_VCPUS;
+	kvm->arch.max_vcpus = kvm_arm_default_max_vcpus();
 
 	return ret;
 out_free_stage2_pgd:
@@ -204,10 +208,11 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
 		r = num_online_cpus();
 		break;
 	case KVM_CAP_MAX_VCPUS:
-		r = KVM_MAX_VCPUS;
-		break;
 	case KVM_CAP_MAX_VCPU_ID:
-		r = KVM_MAX_VCPU_ID;
+		if (kvm)
+			r = kvm->arch.max_vcpus;
+		else
+			r = kvm_arm_default_max_vcpus();
 		break;
 	case KVM_CAP_MSI_DEVID:
 		if (!kvm)