When configuring a QEMU-KVM virtual machine through libvirt, simply setting <boot dev='cdrom'/> in the XML configuration might not be sufficient. The boot order is actually determined by multiple factors:

• The os section boot device specification
• The bootable flag on storage devices
• The boot order attribute in device definitions

Here's how to properly configure your VM to boot from CDROM:

<domain type='kvm'>
  <name>example-vm</name>
  <os>
    <type arch='x86_64' machine='pc-i440fx-2.1'>hvm</type>
    <boot dev='cdrom'/>
  </os>
  <devices>
    <disk type='file' device='disk'>
      <driver name='qemu' type='qcow2'/>
      <source file='/var/lib/libvirt/images/example.qcow2'/>
      <target dev='vda' bus='virtio'/>
      <boot order='2'/>
    </disk>
    <disk type='file' device='cdrom'>
      <driver name='qemu' type='raw'/>
      <source file='/path/to/install.iso'/>
      <target dev='hda' bus='ide'/>
      <readonly/>
      <boot order='1'/>
    </disk>
  </devices>
</domain>

Three critical changes are needed:

1. Explicit boot order numbering (1 for CDROM, 2 for HDD)
2. Proper device addressing (IDE bus for CDROM works best for boot)
3. Configuration reload after changes

After editing the XML file, you must:

# Define the VM (if new)
virsh define /etc/libvirt/qemu/example.xml

# For existing VMs:
virsh destroy example-vm
virsh undefine example-vm
virsh define /etc/libvirt/qemu/example.xml
virsh start example-vm

Check if your configuration was applied correctly:

virsh dumpxml example-vm | grep -A5 "boot order"

This should show the CDROM device with boot order="1" and the HDD with boot order="2".

• Ensure the ISO is properly mounted by checking with virsh domblklist example-vm
• Verify the CDROM device is using IDE bus (not SCSI or virtio)
• Check QEMU command line with ps aux | grep qemu to see boot parameters

When working with QEMU-KVM virtual machines, you might encounter situations where the VM stubbornly boots from the hard drive despite explicit configuration to use the CDROM. The XML configuration appears correct with <boot dev='cdrom'/> set, yet the VM ignores this setting.



The most reliable method involves multiple verification steps:

1. Full Domain Restart Required
Simple VM reboot isn't sufficient - you need to completely destroy and recreate the domain:


# virsh destroy vm_name
# virsh undefine vm_name
# virsh define /etc/libvirt/qemu/vm_name.xml
# virsh start vm_name


2. Verify CDROM Device Configuration
Your CDROM device must be properly defined in the XML. Here's a correctly configured example:


<disk type='file' device='cdrom'>
  <driver name='qemu' type='raw'/>
  <source file='/path/to/your.iso'/>
  <target dev='hdc' bus='ide'/>
  <readonly/>
  <address type='drive' controller='0' bus='1' unit='0'/>
</disk>




Method 1: Using virsh edit
Instead of manually editing XML files, use:


# virsh edit vm_name


Make your changes and save - libvirt will automatically reload the configuration.

Method 2: Temporary Boot Override
For one-time boot from CDROM without XML changes:


# virsh start vm_name --cdrom /path/to/your.iso





Ensure the ISO path is accessible by the libvirt-qemu user
Verify no other boot devices have higher priority
Check for syntax errors in XML with virt-xml-validate
Some older libvirt versions require additional boot parameters




For complete control, you can specify the exact boot order:


<os>
  <type arch='x86_64' machine='pc-i440fx-2.1'>hvm</type>
  <boot dev='hd'/>
  <boot dev='cdrom'/>
  <bootmenu enable='yes'/>
</os>


This ensures CDROM is tried before HDD during boot.