13.3. Managing devices attached to virtual machines using the CLI

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To modify the functionality of your virtual machine (VM), you can manage the devices attached to your VM using the command-line interface (CLI).

You can use the CLI to:

13.3.1. Attaching devices to virtual machines

You can add a specific functionality to your virtual machines (VMs) by attaching a new virtual device.

The following procedure creates and attaches virtual devices to your virtual machines (VMs) using the command-line interface (CLI). Some devices can also be attached to VMs using the RHEL web console.

For example, you can increase the storage capacity of a VM by attaching a new virtual disk device to it. This is also referred to as memory hot plug.

Avertissement

Removing a memory device from a VM, also known as memory hot unplug, is not supported in RHEL 9, and Red Hat highly discourages its use.

Conditions préalables

Obtain the required options for the device you intend to attach to a VM. To see the available options for a specific device, use the virt-xml --device=? command. For example:
```
# virt-xml --network=?
--network options:
[...]
address.unit
boot_order
clearxml
driver_name
[...]
```

Procédure

To attach a device to a VM, use the virt-xml --add-device command, including the definition of the device and the required options:
- For example, the following command creates a 20GB newdisk qcow2 disk image in the /var/lib/libvirt/images/ directory, and attaches it as a virtual disk to the running testguest VM on the next start-up of the VM:
```
# virt-xml testguest --add-device --disk /var/lib/libvirt/images/newdisk.qcow2,format=qcow2,size=20
Domain 'testguest' defined successfully.
Changes will take effect after the domain is fully powered off.
```
- The following attaches a USB flash drive, attached as device 004 on bus 002 on the host, to the testguest2 VM while the VM is running:
```
# virt-xml testguest2 --add-device --update --hostdev 002.004
Device hotplug successful.
Domain 'testguest2' defined successfully.
```
  The bus-device combination for defining the USB can be obtained using the lsusb command.

Vérification

To verify the device has been added, do any of the following:

Use the virsh dumpxml command and see if the device’s XML definition has been added to the <devices> section in the VM’s XML configuration.
For example, the following output shows the configuration of the testguest VM and confirms that the 002.004 USB flash disk device has been added.
```
# virsh dumpxml testguest
[...]
<hostdev mode='subsystem' type='usb' managed='yes'>
  <source>
    <vendor id='0x4146'/>
    <product id='0x902e'/>
    <address bus='2' device='4'/>
  </source>
  <alias name='hostdev0'/>
  <address type='usb' bus='0' port='3'/>
</hostdev>
[...]
```
Run the VM and test if the device is present and works properly.

Ressources supplémentaires

The man virt-xml command

13.3.2. Modifying devices attached to virtual machines

You can change the functionality of your virtual machines (VMs) by editing a configuration of the attached virtual devices. For example, if you want to optimize the performance of your VMs, you can change their virtual CPU models to better match the CPUs of the hosts.

The following procedure provides general instructions for modifying virtual devices using the command-line interface (CLI). Some devices attached to your VM, such as disks and NICs, can also be modified using the RHEL 9 web console.

Conditions préalables

Obtain the required options for the device you intend to attach to a VM. To see the available options for a specific device, use the virt-xml --device=? command. For example:

# virt-xml --network=?
--network options:
[...]
address.unit
boot_order
clearxml
driver_name
[...]

Optional: Back up the XML configuration of your VM by using virsh dumpxml vm-name and sending the output to a file. For example, the following backs up the configuration of your Motoko VM as the motoko.xml file:

# virsh dumpxml Motoko > motoko.xml
# cat motoko.xml
<domain type='kvm' xmlns:qemu='http://libvirt.org/schemas/domain/qemu/1.0'>
  <name>Motoko</name>
  <uuid>ede29304-fe0c-4ca4-abcd-d246481acd18</uuid>
  [...]
</domain>

Procédure

Use the virt-xml --edit command, including the definition of the device and the required options:
For example, the following clears the <cpu> configuration of the shut-off testguest VM and sets it to host-model:
```
# virt-xml testguest --edit --cpu host-model,clearxml=yes
Domain 'testguest' defined successfully.
```

Vérification

To verify the device has been modified, do any of the following:

Run the VM and test if the device is present and reflects the modifications.
Use the virsh dumpxml command and see if the device’s XML definition has been modified in the VM’s XML configuration.
For example, the following output shows the configuration of the testguest VM and confirms that the CPU mode has been configured as host-model.
```
# virsh dumpxml testguest
[...]
<cpu mode='host-model' check='partial'>
  <model fallback='allow'/>
</cpu>
[...]
```

Résolution de problèmes

If modifying a device causes your VM to become unbootable, use the virsh define utility to restore the XML configuration by reloading the XML configuration file you backed up previously.
```
# virsh define testguest.xml
```

Note

For small changes to the XML configuration of your VM, you can use the virsh edit command - for example virsh edit testguest. However, do not use this method for more extensive changes, as it is more likely to break the configuration in ways that could prevent the VM from booting.

Ressources supplémentaires

The man virt-xml command

13.3.3. Removing devices from virtual machines

You can change the functionality of your virtual machines (VMs) by removing a virtual device. For example, you can remove a virtual disk device from one of your VMs if it is no longer needed.

The following procedure demonstrates how to remove virtual devices from your virtual machines (VMs) using the command-line interface (CLI). Some devices, such as disks or NICs, can also be removed from VMs using the RHEL 9 web console.

Conditions préalables

Optional: Back up the XML configuration of your VM by using virsh dumpxml vm-name and sending the output to a file. For example, the following backs up the configuration of your Motoko VM as the motoko.xml file:

# virsh dumpxml Motoko > motoko.xml
# cat motoko.xml
<domain type='kvm' xmlns:qemu='http://libvirt.org/schemas/domain/qemu/1.0'>
  <name>Motoko</name>
  <uuid>ede29304-fe0c-4ca4-abcd-d246481acd18</uuid>
  [...]
</domain>

Procédure

Use the virt-xml --remove-device command, including a definition of the device. For example:

The following removes the storage device marked as vdb from the running testguest VM after it shuts down:

# virt-xml testguest --remove-device --disk target=vdb
Domain 'testguest' defined successfully.
Changes will take effect after the domain is fully powered off.

The following immediately removes a USB flash drive device from the running testguest2 VM:

# virt-xml testguest2 --remove-device --update --hostdev type=usb
Device hotunplug successful.
Domain 'testguest2' defined successfully.

Résolution de problèmes

If removing a device causes your VM to become unbootable, use the virsh define utility to restore the XML configuration by reloading the XML configuration file you backed up previously.
```
# virsh define testguest.xml
```

Ressources supplémentaires

The man virt-xml command

13.3. Managing devices attached to virtual machines using the CLI

13.3.1. Attaching devices to virtual machines

13.3.2. Modifying devices attached to virtual machines

13.3.3. Removing devices from virtual machines

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