Live Migration Failure Due to CPU Model Mismatch

Problem

Instances fail to live migrate between hypervisors, with errors Unacceptable CPU info: CPU doesn't have compatibility indicating that no suitable host is found, even though resources are available.

Environment

• Private Cloud Director Virtualisation - v2025.4 and Higher
• Self-Hosted Private Cloud Director Virtualisation - v2025.4 and Higher

Cause

The issue was caused by a CPU model mismatch between the source and destination hypervisors. The source hypervisor was running a different CPU model compared to other compute nodes. Nova’s CPU compatibility checks during live migration failed because the CPU feature set did not match across the involved hosts.

When Nova detects incompatible CPU models, it will reject the migration with a "No valid host found" error, even if other resources (CPU, RAM, Disk) are sufficient.

Diagnostics

1. Verify the supported CPU Model from the underlying hypervisor:

$ virsh cpu-models x86_64

...

SandyBridge

SandyBridge-IBRS

IvyBridge

IvyBridge-IBRS

Haswell-noTSX

Haswell-noTSX-IBRS

Haswell

Haswell-IBRS

2. Check the current cpu_models on the source hypervisor from the file /opt/pf9/etc/nova/conf.d/nova_override.conf , and check whether it is same on other hypervisor hosts:

$ cat /opt/pf9/etc/nova/conf.d/nova_override.conf

[libvirt]

cpu_mode = custom

cpu_models = SandyBridge-IBRS   ## Example CPU Model

3. Migration attempts from any source hypervisor to a destination hypervisor with a different CPU model configuration failed, displaying the following error in /var/log/pf9/ostackhost.log on the destination host:

nova.exception.InvalidCPUInfo: Unacceptable CPU info: CPU doesn't have compatibility.

Resolution

1. Ensure the source and destination hypervisor nodes have the supported CPU model /opt/pf9/etc/nova/conf.d/nova_override.conf. And this CPU model should be same as configured on other hypervisors.
2. Restart ostackhost service on hypervisor where the cpu_model is updated:

$ sudo systemctl restart pf9-ostackhost

Validation

1. Re-attempt migration of VMs hosted on the source hypervisor to another hypervisor — Check if the migration is successful.
2. Check the /var/log/pf9/ostackhost.log logs to ensure no CPU mismatch errors were reported.