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>

diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
index 2f03cbf..e4727dc 100644 (file)
--- a/arch/arm64/kvm/arm.c
+++ b/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: