305-300 Practice Question: Troubleshooting a KVM guest that fails to start with the error 'unsupported…
You are troubleshooting a KVM guest that fails to start with the error 'unsupported configuration: CPU mode 'host-passthrough' requires KVM'. Which of the following is the most likely cause?
⚠ Common exam trap
The trap here is assuming the error relates to CPU hardware capabilities, when it is actually about the hypervisor type not being KVM.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The guest is configured to use a CPU mode that is only supported by the KVM hypervisor, but the hypervisor is not KVM (e.g., QEMU TCG).
The error 'unsupported configuration: CPU mode 'host-passthrough' requires KVM' occurs when the guest is configured with <cpu mode='host-passthrough'/> but libvirt is not using the KVM hypervisor. This can happen if KVM is not enabled on the host, or if the domain XML explicitly sets a different hypervisor type. host-passthrough exposes the host CPU model and features directly to the guest, which is only possible with hardware virtualization.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
The host CPU is too old and does not support the host-passthrough mode.
Why it's wrong here
The host-passthrough mode does not depend on the CPU's age or specific features; it simply passes through the host CPU model. The limitation is not about CPU capabilities but about the hypervisor. Even an older CPU can use host-passthrough if KVM is available. Therefore, this is not the likely cause of the error.
- ✗
The guest's XML configuration has a typo in the CPU mode attribute.
Why it's wrong here
A typo in the CPU mode attribute would likely result in an XML validation error or an 'unknown CPU mode' message. The error specifically mentions 'host-passthrough' and 'requires KVM', which indicates that the mode is recognized but not supported by the current hypervisor. A typo would not produce this specific error.
- ✓
The guest is configured to use a CPU mode that is only supported by the KVM hypervisor, but the hypervisor is not KVM (e.g., QEMU TCG).
Why this is correct
The host-passthrough CPU mode exposes the host CPU features directly to the guest and requires hardware virtualization (KVM). If libvirt is using a different hypervisor, such as QEMU's TCG (software emulation), this mode is not supported. The error clearly states that host-passthrough requires KVM, so the most likely cause is that the guest is running under a non-KVM hypervisor, possibly due to KVM not being enabled or the domain XML specifying a different hypervisor.
- ✗
The host CPU does not support hardware virtualization extensions.
Why it's wrong here
If the host CPU did not support hardware virtualization, the error would typically be about KVM not being available or /dev/kvm missing. The specific error about host-passthrough requiring KVM indicates that the guest is configured to use host-passthrough CPU mode, but the hypervisor is not KVM. This is not about the CPU's virtualization capabilities but about the hypervisor in use.
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Written and reviewed by Johnson Ajibi, MSc IT Security
Security Systems Architect · founder of Courseiva
Last reviewed September 2026 · checked against the official LPI exam blueprint
This 305-300 practice question is part of Courseiva's free LPI certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 305-300 exam.