I recently experienced some KVM migration problems while moving virtual machines from my development laptop to my server. The laptop has a Core i7 processor but the server is running a Core i5 and, as you might expect, they have different features available. In this article I’ll explain what I did to fix my KVM migration problems.

KVM Migration Problems – Unsupported Configuration

To perform the migration I was following the steps I’ve documented here and everything was going well until I came to start the moved machine. I was presented with the error message:

virsh start main
error: Failed to start domain main
error: unsupported configuration: guest and host CPU are not compatible: Host CPU does not provide required features: x2apic

X2APIC, it turns out, is an interrupt controller but that’s not terribly important as the processor I’m moving too apparently doesn’t have one – erm, it has an interrupt controller obviously just not this one. When I looked in the main.xml configuration file (found in /etc/libvirt/qemu) I was surprised by how little configuration there was for the CPU:

<cpu mode='custom' match='exact'>
    <model fallback='allow'>SandyBridge</model>
</cpu>

Now run the following command to determine what the capabilites of the target machine are – there’s quite a bit of output, CPU information is near the top:

virsh capabilities

<cpu>
    <arch>x86_64</arch>
    <model>Westmere</model>
    <vendor>Intel</vendor>
    <topology sockets='1' cores='4' threads='1'/>
    <feature name='rdtscp'/>
    <feature name='avx'/>
    <feature name='osxsave'/>
    <feature name='xsave'/>
    <feature name='tsc-deadline'/>
    <feature name='pcid'/>
    <feature name='pdcm'/>
    <feature name='xtpr'/>
    <feature name='tm2'/>
    <feature name='est'/>
    <feature name='vmx'/>
    <feature name='ds_cpl'/>
    <feature name='monitor'/>
    <feature name='dtes64'/>
    <feature name='pclmuldq'/>
    <feature name='pbe'/>
    <feature name='tm'/>
    <feature name='ht'/>
    <feature name='ss'/>
    <feature name='acpi'/>
    <feature name='ds'/>
    <feature name='vme'/>
</cpu>

Now that you know what model of processor is in the target host machine edit the configuration file:

virsh edit main

so that the CPU configuration looks like this (yours will probably be different):

<cpu mode='custom' match='exact'>
    <model fallback='allow'>Westmere</model>
</cpu>

And restart your virtual machine…

Note: it’s also possible to make this change using Virtual Machine Manager. Open the virtual machine settings and under Processor > Configuration you’ll see a check box that will all you to copy the host CPU configuration.

KVM Migration Problems – Unsupported Machine Type

After fixing the CPU issue described above I then ran into this new KVM migration problem:

error: Failed to start domain main
error: internal error: process exited while connecting to monitor: qemu-system-x86_64: -machine pc-i440fx-utopic,accel=kvm,usb=off: Unsupported machine type
Use -machine help to list supported machines!

I initially installed this virtual machine on a host running Ubuntu 14.10 Utopic. It hadn’t occurred to me that the virtual machine would care what the host was running beyond the version of KVM / Qemu it was on. A scan though the configuration found this:

<os>
    <type arch='x86_64' machine='pc-i440fx-utopic'>hvm</type>
    <boot dev='hd'/>
</os>

As above I used “virsh edit main” to modify the configuration and changed “utopic” to “trusty” which is what the server is running. This was enough to get the virtual machine started.