[vcpu hotplug] the cpu is offline after migration successfully

CNV: registry-proxy.engineering.redhat.com/rh-osbs/iib:977162
OCP 4.19.0-ec.4

100%

1. Create a VM by following yaml config.

apiVersion: kubevirt.io/v1
kind: VirtualMachine
metadata:
  annotations:
    kubemacpool.io/transaction-timestamp: "2025-05-30T15:05:57.204944275Z"
    kubevirt.io/latest-observed-api-version: v1
    kubevirt.io/storage-observed-api-version: v1
    vm.kubevirt.io/validations: |
      [
        {
          "name": "minimal-required-memory",
          "path": "jsonpath::.spec.domain.memory.guest",
          "rule": "integer",
          "message": "This VM requires more memory.",
          "min": 1610612736
        }
      ]
  creationTimestamp: "2025-05-30T09:44:26Z"
  finalizers:
  - kubevirt.io/virtualMachineControllerFinalize
  generation: 9
  labels:
    app: rhel97
    kubevirt.io/dynamic-credentials-support: "true"
    vm.kubevirt.io/template: rhel9-server-small
    vm.kubevirt.io/template.namespace: openshift
    vm.kubevirt.io/template.revision: "1"
    vm.kubevirt.io/template.version: v0.34.0
  name: rhel97
  namespace: default
  resourceVersion: "53900460"
  uid: 82b148ed-198b-4436-acfd-214bc9067822
spec:
  dataVolumeTemplates:
  - apiVersion: cdi.kubevirt.io/v1beta1
    kind: DataVolume
    metadata:
      creationTimestamp: null
      name: rhel97
    spec:
      source:
        http:
          url: http://<internal_server>/libvirt-CI-resources/RHEL-9.7-x86_64-latest-ovmf.qcow2
      storage:
        accessModes:
        - ReadWriteMany
        resources:
          requests:
            storage: 16Gi
        storageClassName: ocs-storagecluster-cephfs
        volumeMode: Filesystem
  runStrategy: RerunOnFailure
  template:
    metadata:
      annotations:
        vm.kubevirt.io/flavor: small
        vm.kubevirt.io/os: rhel9
        vm.kubevirt.io/workload: server
      creationTimestamp: null
      labels:
        kubevirt.io/domain: rhel97
        kubevirt.io/size: small
        network.kubevirt.io/headlessService: headless
    spec:
      architecture: amd64
      domain:
        cpu:
          cores: 1
          sockets: 1
          threads: 1
        devices:
          disks:
          - disk:
              bus: virtio
            name: rootdisk
          - disk:
              bus: virtio
            name: cloudinitdisk
          interfaces:
          - macAddress: 02:0d:38:00:00:00
            masquerade: {}
            model: virtio
            name: default
          networkInterfaceMultiqueue: true
          rng: {}
        features:
          acpi: {}
          smm:
            enabled: true
        firmware:
          bootloader:
            efi: {}
        machine:
          type: pc-q35-rhel9.6.0
        memory:
          guest: 2Gi
        resources: {}
      networks:
      - name: default
        pod: {}
      terminationGracePeriodSeconds: 180
      volumes:
      - dataVolume:
          name: rhel97
        name: rootdisk
      - cloudInitNoCloud:
          userData: |-
            #cloud-config
            user: cloud-user
            password: jaom-1g5t-ohlm
            chpasswd: { expire: False }
        name: cloudinitdisk 2.
2. Change the socket to 2 to trigger vcpu hotplug.
3. Login the VM after successfully migration and check the cpuinfo. 
The new cpu is offline.

[root@rhel97 ~]# lscpu  
Architecture:             x86_64
  CPU op-mode(s):         32-bit, 64-bit
  Address sizes:          46 bits physical, 57 bits virtual
  Byte Order:             Little Endian
CPU(s):                   2
  On-line CPU(s) list:    0
  Off-line CPU(s) list:   1
Vendor ID:                GenuineIntel
  BIOS Vendor ID:         Red Hat
  Model name:             Intel Xeon Processor (Icelake)
    BIOS Model name:      RHEL-9.6.0 PC (Q35 + ICH9, 2009)
    CPU family:           6
    Model:                134
    Thread(s) per core:   1
    Core(s) per socket:   1
    Socket(s):            1
    Stepping:             0
    BogoMIPS:             4190.15
    Flags:                fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge m
                          ca cmov pat pse36 clflush dts mmx fxsr sse sse2 ss sys
                          call nx pdpe1gb rdtscp lm constant_tsc pebs bts rep_go
                          od nopl xtopology cpuid tsc_known_freq pni pclmulqdq d
                          tes64 vmx ssse3 fma cx16 pcid sse4_1 sse4_2 x2apic mov
                          be popcnt tsc_deadline_timer aes xsave avx f16c rdrand
                           hypervisor lahf_lm abm 3dnowprefetch cpuid_fault ssbd
                           ibrs ibpb stibp ibrs_enhanced tpr_shadow flexpriority
                           ept vpid ept_ad fsgsbase tsc_adjust bmi1 avx2 smep bm
                          i2 erms invpcid avx512f avx512dq rdseed adx smap avx51
                          2ifma clflushopt clwb avx512cd sha_ni avx512bw avx512v
                          l xsaveopt xsavec xgetbv1 xsaves wbnoinvd arat vnmi av
                          x512vbmi umip pku ospke avx512_vbmi2 gfni vaes vpclmul
                          qdq avx512_vnni avx512_bitalg avx512_vpopcntdq la57 rd
                          pid fsrm md_clear flush_l1d arch_capabilities
Virtualization features:  
  Virtualization:         VT-x
  Hypervisor vendor:      KVM
  Virtualization type:    full
Caches (sum of all):      
  L1d:                    32 KiB (1 instance)
  L1i:                    32 KiB (1 instance)
  L2:                     4 MiB (1 instance)
  L3:                     16 MiB (1 instance)
NUMA:                     
  NUMA node(s):           1
  NUMA node0 CPU(s):      0
Vulnerabilities:          
  Gather data sampling:   Not affected
  Itlb multihit:          Not affected
  L1tf:                   Not affected
  Mds:                    Not affected
  Meltdown:               Not affected
  Mmio stale data:        Mitigation; Clear CPU buffers; SMT Host state unknown
  Reg file data sampling: Not affected
  Retbleed:               Not affected
  Spec rstack overflow:   Not affected
  Spec store bypass:      Mitigation; Speculative Store Bypass disabled via prct
                          l
  Spectre v1:             Mitigation; usercopy/swapgs barriers and __user pointe
                          r sanitization
  Spectre v2:             Mitigation; Enhanced / Automatic IBRS; IBPB conditiona
                          l; RSB filling; PBRSB-eIBRS Not affected; BHI SW loop,
                           KVM SW loop
  Srbds:                  Not affected
  Tsx async abort:        Mitigation; TSX disabled

4. Check the dmesg log.

[   70.601402] clocksource: timekeeping watchdog on CPU0: Marking clocksource 'tsc' as unstable because the skew is too large:
[   70.601462] clocksource:                       'kvm-clock' wd_nsec: 543445712 wd_now: 117af3fcc2 wd_last: 115a8fa9f2 mask: ffffffffffffffff
[   70.601467] clocksource:                       'tsc' cs_nsec: 479651937 cs_now: 24b63d5eb6 cs_last: 247a57b034 mask: ffffffffffffffff
[   70.601470] clocksource:                       'kvm-clock' (not 'tsc') is current clocksource.
[   70.601473] tsc: Marking TSC unstable due to clocksource watchdog
[   70.718258] ACPI: CPU1 has been hot-added
[   70.728190] SMP alternatives: switching to SMP code
[   70.733376] smpboot: Booting Node 0 Processor 1 APIC 0x1
[   70.734082] kvm_intel: Inconsistent VMCS config on CPU 1
[   70.734130] kvm: enabling virtualization on CPU1 failed
[   70.735933] smpboot: CPU 1 is now offline


5. Soft reboot the VM, the 2 cpu will be online. Then do vcpu hotplug again by changing the sockets to 3.
6. Check the cpu info after migration. The new cpu can be hotpluged successfully.

[   57.549015] ACPI: CPU2 has been hot-added
[   57.565879] smpboot: Booting Node 0 Processor 2 APIC 0x2
[   57.566014] TSC ADJUST compensate: CPU2 observed 130448479191 warp. Adjust: 130448479191
[   57.566014] TSC ADJUST compensate: CPU2 observed 12 warp. Adjust: 130448479203
[   57.567354] TSC synchronization [CPU#0 -> CPU#2]:
[   57.567354] Measured 4 cycles TSC warp between CPUs, turning off TSC clock.
[   57.626457] clocksource: timekeeping watchdog on CPU0: Marking clocksource 'tsc' as unstable because the skew is too large:
[   57.626469] clocksource:                       'kvm-clock' wd_nsec: 519620295 wd_now: 3ec4536741 wd_last: 3ea55aa07a mask: ffffffffffffffff
[   57.626473] clocksource:                       'tsc' cs_nsec: 467819394 cs_now: 1e6b2800fb cs_last: 1e30bcac51 mask: ffffffffffffffff
[   57.626480] clocksource:                       'kvm-clock' (not 'tsc') is current clocksource.
[   57.626483] tsc: Marking TSC unstable due to clocksource watchdog
[   57.627411] Will online and init hotplugged CPU: 2

Do vcpu hotplug on a new created VM, the new cpu is offline on the VM.
Soft reboot the VM can fix the cpu offline issue. And the following vcpu hotplug can be success after soft reboot the VM.

The vcpu hotplug on a new created VM should make the new cpu online.