does this bug <https://bugs.launchpad.net/ubuntu/+source/libvirt/+bug/2106812> related your report? On Thu, Nov 20, 2025 at 9:23 PM Hai Pham Thanh <haipt43@fpt.com> wrote:

Hi community, We would like to report an issue we are encountering with nested virtualization on Windows 11 instances running on OpenStack. On our OpenStack platform, a Windows 11 VM becomes unable to boot after enabling Virtual Machine Platform/Hyper-V inside the VM and performing a hard reboot (VM does not show a BSOD but fails to boot into the OS and gets stuck in a boot loop in Tianocore logo). This issue does not occur on Windows 10 under the same conditions. System:

- Compute node CPU: Intel Xeon Gold 6538Y+ (Emerald Rapids) with nested virtualization is enabled. - OpenStack 2024.2 - libvirt 8.0.0 - QEMU API 8.0.0 - QEMU hypervisor 6.2.0

Initially, our Nova CPU configuration was:

- cpu_mode=host-model

- cpu_model_extra_flags=+vmx,-hypervisor,-xsaves

According to virsh dumpxml, host-model maps to Icelake-Server. We tested several cpu_mode=custom configurations and observed the following:

- cpu_models=Icelake-Server-noTSX → error (boot loop) - cpu_models=Icelake-Server → error (boot loop) - cpu_models=Broadwell-noTSX-IBRS → works - cpu_models=Cascadelake-Server-noTSX → works

The working models correspond to the Preferred CPU models recommended by QEMU:

https://qemu-project.gitlab.io/qemu/system/qemu-cpu-models.html#preferred-cp... We would like to ask:

1. Has anyone encountered this issue with Windows 11 + nested virtualization on Icelake/Emerald Rapids hosts? 2. Are there known root causes explaining why newer CPU models fail while preferred (older) models work? 3. Is there a recommended fix other than switching to a preferred older CPU model?

Best regards, Hai Pham

-- Thu.