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After recent Fedora 27 update, my KVM VM guest ended up in the kernel panic during boot process.

The system was updated to libvirt-3.7.0-4.fc27.x86_64 (and many other packages during this update) and rebooted. Before the VM guest running Kubuntu 16.04 had no problem. Previous kernel on the hypervisor doesn't help.

The host CPU is AMD Athlon(tm) X4 750.

What's the cause of this sudden issue?

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The culprit is really the system update (most probably the libvirt).

Before the VM guest was running with cpu mode=host-model more than one year. However now it was necessary to edit VM guest XML deffinition file - virsh edit <domain>. The cpu mode entry must have been changed to cpu mode=host-passthrough:

<cpu mode='host-passthrough' check='partial'/>

After this change, the VM guest with Kubuntu booted up without any problem.

According to libvirt documentation, the host-model can cause CPU detection problems to VM guest OS. The host-passthrough avoids this issues as provides exactly the same type of CPU (and it's features). However makes difficulties with migration to different VM host. The other VM host must be exactly of the same type of CPU.

Interesting thing is that another VM guest with CentOS was still able to boot without any problem...

  • 1
    +1 to the question and answer. I also had the same problem with cpu mode recently. – Rui F Ribeiro Aug 1 '18 at 12:01

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