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Yes—QEMU supports virtual CPU hot-plug, but only when the machine type, CPU topology, firmware, and guest operating system are prepared for it. You normally boot with fewer vCPUs than a declared maxcpus value, then add an available CPU slot through QMP or a management layer such as libvirt. Hot-unplug is more constrained: device_del starts an asynchronous request, and the guest must offline the processor before QEMU can finish removing it.
What QEMU CPU hot-plug actually changes
CPU hot-plug adds or removes virtual processors presented to a running guest. QEMU and KVM schedule the resulting vCPU threads on the host’s existing physical CPUs. It does not hot-add physical cores, change host CPU capacity, or replace CPU affinity and NUMA planning.
This is different from:
- Host CPU hotplug, which changes the host kernel’s physical-CPU inventory.
- CPU pinning, which controls where vCPU threads run.
- Guest-side online/offline operations, which change whether an already-present CPU is used but do not create a new QEMU CPU device.
- Changing a VM’s vCPU count for its next boot.
Adding vCPUs can improve throughput only when the host has capacity and the workload can use additional parallelism. Oversubscription, poor NUMA placement, synchronization, or guest-scheduler effects can make performance unchanged or worse.
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Hot-plug is a pre-planned capacity feature. The VM must start with an initial CPU count below a maximum and with a topology that contains all possible vCPUs. QEMU’s -smp syntax is documented in the QEMU system manual.
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-smp cpus=2,maxcpus=8,sockets=1,cores=4,threads=2Here, two CPUs start present and eight slots are reserved. The product of the topology hierarchy must equal maxcpus, and the initial count cannot exceed it. A simpler test launch is:
qemu-system-x86_64
-enable-kvm
-machine pc
-smp cpus=1,maxcpus=4,sockets=1,cores=4,threads=1
-m 2G
-qmp unix:/tmp/qmp.sock,server=on,wait=offOther requirements are equally important:
- A machine type with CPU hot-plug support.
- A CPU model and topology valid for that machine type.
- Firmware and ACPI support for CPU notifications.
- A guest kernel and policy that support processor hot-add and, if needed, hot-remove.
- Enough host capacity, with suitable KVM support.
- A migration plan that keeps machine type, CPU model, topology, and QEMU compatibility consistent.
If maxcpus equals the initial count, there are no spare slots to add later. Reserving a maximum is not an unrestricted promise that every arbitrary topology can be inserted.
Direct QMP workflow
QMP is the lowest-level supported interface. The safest procedure is to ask the running QEMU process which CPU devices are available instead of constructing a device definition from memory. QEMU’s CPU hotplug guide and QMP reference describe these commands.
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- Connect to the QMP UNIX socket and negotiate capabilities.
- Run
query-hotpluggable-cpus. - Choose an unused result. Possible CPUs generally have no
qom-path; present CPUs do. - Copy the returned CPU type and topology properties into
device_add. - Wait for the guest to discover and online the processor.
- Use
query-cpus-fastto inspect QEMU’s current CPU devices and verify the guest separately.
{ "execute": "qmp_capabilities" }
{ "execute": "query-hotpluggable-cpus" }
{ "execute": "device_add",
"arguments": {
"id": "cpu-hotplug-1",
"driver": "MODEL_FROM_QUERY",
"socket-id": 0,
"core-id": 1,
"thread-id": 0
}
}
{ "execute": "query-cpus-fast" }The driver name and properties above are deliberately placeholders. For one documented x86 example, QEMU returns an IvyBridge-IBRS-x86_64-cpu type and socket/core/thread values, which are then passed to device_add. Your machine type, CPU model, and QEMU release can return different values. A successful device_add reply means QEMU accepted the request; it does not prove that the guest has onlined the CPU.
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Production software normally sends JSON over QMP rather than relying on an interactive qmp-shell. Track replies and asynchronous events, and avoid reusing a device ID.
Verify the guest, not just QEMU
On Linux, check both the CPUs that exist and those currently online:
lscpu
nproc
cat /sys/devices/system/cpu/online
cat /sys/devices/system/cpu/present
cat /sys/devices/system/cpu/cpu2/onlineA newly discovered CPU may be automatically onlined, depending on the kernel and distribution. If the online file exists and policy permits it, an administrator can request that it be onlined:
echo 1 | sudo tee /sys/devices/system/cpu/cpu2/onlineThis is a guest operating-system operation, not a QEMU command. CPU numbering, file availability, and automatic-online policy vary. Windows and other operating systems have their own processor-hotplug rules, editions, and possible reboot requirements; validate the exact guest version rather than assuming universal support.
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Hot-unplug is a negotiated, asynchronous operation
To request removal through QMP:
{ "execute": "device_del",
"arguments": { "id": "cpu-hotplug-1" }
}QEMU sends a CPU hotplug notification through the ACPI interface. The guest must receive it, identify the processor, take it offline, and signal that removal can proceed. The ACPI notification and status mechanism is described in QEMU’s ACPI CPU hotplug specification.
Therefore, device_del is a request, not a synchronous deletion. Poll QMP or the management API, inspect guest CPU state, and handle refusal or delay. Removal may fail for the boot CPU, a CPU needed by an active workload or interrupt path, an indivisible topology unit, or a guest subsystem that has not released the processor.
Using libvirt and virsh
For managed VMs, libvirt is usually preferable to hand-written QMP because it maintains domain XML and lifecycle state. Inspect the current and maximum counts first:
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virsh help setvcpusTo request a live count change:
virsh setvcpus DOMAIN COUNT --liveOn versions that expose it and for domains whose hypervisor configuration supports it, request hotpluggable vCPUs explicitly:
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virsh setvcpus DOMAIN COUNT --live --hotpluggableCommon scope flags have different meanings:
--live: change the running guest.--config: change the persistent definition used on a future boot.--current: apply to the current definition according to libvirt’s rules.--maximum --config: change the persistent maximum rather than only the currently active count.
Consult the installed command help: accepted flags and behavior depend on libvirt and QEMU versions. Libvirt maps a requested count onto eligible hotplug entities; it cannot create capacity that was not reserved in the domain’s topology. XML may represent individual vCPU enabled and hotpluggable state, but its exact form is version- and configuration-dependent. The libvirt setvcpus reference documents the core operation.
QMP, libvirt, or a higher-level platform?
| Method | Best use | Strength | Main risk |
|---|---|---|---|
| QMP | Custom orchestration and debugging | Exact device and topology control | Easy to violate slot or topology rules; asynchronous state is your responsibility |
| libvirt | Ordinary production administration | Persistent XML and lifecycle integration | Topology details may be abstracted; behavior varies by versions |
| Higher-level platforms | Fleet automation | Policy, scheduling, and audit | The platform may restrict or hide QEMU’s capability |
Architecture, topology, and migration
x86 pc machines commonly use ACPI CPU hotplug with socket, core, and thread placement. Do not copy that command to another architecture. AArch64 virt has machine- and GIC-dependent CPU limits and topology rules; its documentation also warns that -cpu max can hurt migration compatibility when feature sets differ between QEMU versions. See the ARM virt documentation and CPU model guidance. s390x, pSeries, and other machines have their own layouts and interfaces.
Versioned machine types matter. A VM that works locally can fail to migrate if source and destination QEMU versions, host CPU features, machine aliases, or final vCPU topology differ. Treat the maximum topology and CPU model as part of the migration contract. Test migration after both hot-add and hot-unplug, not merely at the initial boot state.
Troubleshooting
| Symptom | Likely cause | What to check |
|---|---|---|
| No hotplug slots | No spare maxcpus capacity, full topology, unsupported machine type, or management-layer configuration |
Compare initial CPUs with maxcpus, inspect the complete QEMU command line and machine type, and recreate the VM if capacity was never reserved. |
device_add rejects CPU/topology |
Wrong model, occupied slot, or invalid socket/core/thread combination | Run query-hotpluggable-cpus and copy the exact type and properties from an unused result. |
| QEMU accepts add; guest count is unchanged | Missing guest ACPI support, CPU discovered but offline, or guest policy prevents online | Check lscpu, present, online, guest logs, and the individual CPU’s online file. |
| Unplug never completes | Guest did not process ACPI, CPU is not removable, or caller assumed synchronous behavior | Check guest kernel logs and offline state; poll QMP/libvirt until completion or an explicit failure. |
| Migration fails afterward | Incompatible QEMU versions, machine types, CPU features, or topology | Use a stable CPU model, avoid untested -cpu max combinations, and test the post-hotplug state on the destination. |
| Performance does not scale | Host oversubscription, NUMA mismatch, emulator contention, or workload serialization | Review placement and scheduling; hotplug changes available capacity, not guaranteed application throughput. |
When a reboot is the better choice
Use a shutdown-and-reconfigure resize when the guest’s hotplug support is unreliable, topology must remain NUMA-symmetric, licensing is tied to a fixed CPU count, latency sensitivity makes topology changes undesirable, or migration compatibility cannot be guaranteed. Other alternatives include booting with the maximum and controlling utilization, scaling stateless services across more VMs or containers, and using pinning or NUMA tuning when the real requirement is isolation rather than changing capacity. Memory hotplug addresses a different resource and is not a substitute for vCPU hotplug.
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Operational checklist
- Choose and pin a supported machine type and CPU model.
- Reserve a complete, valid maximum topology with
maxcpus. - Confirm firmware and guest processor-hotplug support.
- Test QMP discovery with
query-hotpluggable-cpus. - Add a slot and verify both QEMU state and guest online state.
- Test unplug, including delayed or refused completion.
- Test live migration after topology changes.
- Automate polling, guest-state checks, and rollback rather than treating replies as proof of completion.
Frequently Asked Questions
Does QEMU CPU hot-plug add physical CPU capacity?
No. It adds virtual CPUs to the guest; QEMU still schedules their threads on the host’s existing processors.
Can I hot-add CPUs if I forgot maxcpus at boot?
Usually not. Without spare topology slots reserved at startup, you generally must reconfigure and reboot the VM.
Is device_del immediate for a vCPU?
No. It requests removal through ACPI, and the guest must offline the CPU before QEMU can complete the operation.
The Bottom Line
QEMU vCPU hot-plug is reliable when treated as planned topology capacity: reserve maxcpus, query the running instance for valid slots, verify the guest independently, and handle unplug and migration asynchronously.
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