KVM: arm64: Cap KVM_CAP_NR_VCPUS by kvm_arm_default_max_vcpus()

Generally, it doesn't make sense to return the recommended maximum number
of vCPUs which exceeds the maximum possible number of vCPUs.

Note: ARM64 is special as the value returned by KVM_CAP_MAX_VCPUS differs
depending on whether it is a system-wide ioctl or a per-VM one. Previously,
KVM_CAP_NR_VCPUS didn't have this difference and it seems preferable to
keep the status quo. Cap KVM_CAP_NR_VCPUS by kvm_arm_default_max_vcpus()
which is what gets returned by system-wide KVM_CAP_MAX_VCPUS.

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Message-Id: <20211116163443.88707-2-vkuznets@redhat.com>
Acked-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/arm64/kvm/arm.c

@@ -223,7 +223,14 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
 		r = 1;
 		break;
 	case KVM_CAP_NR_VCPUS:
-		r = num_online_cpus();
+		/*
+		 * ARM64 treats KVM_CAP_NR_CPUS differently from all other
+		 * architectures, as it does not always bound it to
+		 * KVM_CAP_MAX_VCPUS. It should not matter much because
+		 * this is just an advisory value.
+		 */
+		r = min_t(unsigned int, num_online_cpus(),
+			  kvm_arm_default_max_vcpus());
 		break;
 	case KVM_CAP_MAX_VCPUS:
 	case KVM_CAP_MAX_VCPU_ID: