KVM Guests: Manipulating Libvirt XML for Guest Creation

KVM Guests: Manipulating Libvirt XML for Guest Creation

Today we are going to discuss using the underlying XML which libvirt uses to define a VM to either create VMs or to make simple changes.  This method has the advantage of more control over what you are creating, although it is more manual.  Now first off, when you use virt-install, virt-manager, or another tool which uses libvirt under the covers an XML file is created for every VM that you define, libvirt stores these in  /etc/libvirt/qemu/ and names them vmname.xml.

View VMs XML Configuration

# virsh dumpxml vmname

Example XML

<domain type='kvm'>
<name>vmname</name>
<uuid>8ca8ebe2-1eaa-1de6-b110-9823b99297ba</uuid>
<memory>2097152</memory>
<currentMemory>2097152</currentMemory>
<vcpu>1</vcpu>
<os>
<type arch='x86_64' machine='pc-0.12'>hvm</type>
<boot dev='hd'/>
</os>
<features>
<acpi/>
<apic/>
<pae/>
</features>
<clock offset='localtime'/>
<on_poweroff>destroy</on_poweroff>
<on_reboot>restart</on_reboot>
<on_crash>restart</on_crash>
<devices>
<emulator>/usr/bin/kvm</emulator>
<disk type='file' device='disk'>
<driver name='qemu' type='raw'/>
<source file='/kvm/images/disk/vmname_boot.img'/>
<target dev='vda' bus='virtio'/>
<address type='pci' domain='0x0000' bus='0x00' slot='0x04' function='0x0'/>
</disk>
<disk type='file' device='cdrom'>
<driver name='qemu' type='raw'/>
<source file='/kvm/images/iso/cdimage.iso'/>
<target dev='hdc' bus='ide'/>
<readonly/>
<address type='drive' controller='0' bus='1' unit='0'/>
</disk>
<controller type='ide' index='0'>
<address type='pci' domain='0x0000' bus='0x00' slot='0x01' function='0x1'/>
</controller>
<interface type='bridge'>
<mac address='52:54:00:07:ab:b0'/>
<source bridge='br0'/>
<model type='virtio'/>
<address type='pci' domain='0x0000' bus='0x00' slot='0x03' function='0x0'/>
</interface>
<serial type='pty'>
<target port='0'/>
</serial>
<console type='pty'>
<target type='serial' port='0'/>
</console>
<input type='tablet' bus='usb'/>
<input type='mouse' bus='ps2'/>
<graphics type='vnc' port='-1' autoport='yes' keymap='en-us'/>
<video>
<model type='vga' vram='9216' heads='1'/>
<address type='pci' domain='0x0000' bus='0x00' slot='0x02' function='0x0'/>
</video>
<memballoon model='virtio'>
<address type='pci' domain='0x0000' bus='0x00' slot='0x05' function='0x0'/>
</memballoon>
</devices>
</domain>

Above you will see an example XML which we will talk about.  Additionally I will have some different configuration examples below after we have discussed the different areas of the file. Line 2 contains the VMs Name and can be changed to reflect the name of the new VM. Line 3 is the UUID of the VM, this needs to be unique, so you can either generate and input one manually, or you can choose the super duper easy method and delete this entire line and allow libvirt to plug it in for you. Lines 4 & 5 are the Memory Values these must match each other, although I suspect if you are using a balloon driver you could use that here. Line 6 allows you to specify the number of virtual CPUs the guest will have. Lines 22-27 specifies the virtual disk which the VM will use.  Line 24 contains the path of the image, which you will want to reflect your path. Lines 28-34 specifies the virtual cdrom device.  Line 30 contains the path of the image, I like to have a cdrom if I need it but don’t necessarily want an image mounted.  If you don’t want an image, remove line 30 completely. Line 38-43 specifies the virtual network card, obviously with networking duplicate MACs is a bad thing, so you can either specify a unique MAC or you can as with the UUID remove line 39 in its entirety. Line 44-49 specifies the virtual serial device.  You should not need to adjust this. Line 52 specifies the method with which you can connect to the console of the guest, which is most likely going to be VNC.  However one thing to watch here is for listen=0.0.0.0 this will allow all machines to connect to your guests via VNC without a password (so if the console was not locked this would be a problem).  If listen is not defined then it defaults to listen=127.0.0.1 which is what you actually want. Line 53-56 specifies the virtual video card, the only thing I have had to adjust here is for Windows XP guests I use the cirrus type on line 54, XP doesn’t have a driver for vga. Below you will find a series of “templates” meaning that I have removed the automatically generated lines so all you would need to do is copy these and then adjust the names and paths of the devices as I outlined above.  The only thing to keep in mind is the Video, if you are running Windows XP you will want to switch the video type to cirrus.  It didn’t make sense to do that for you.  The only other bit that we have not covered is the actual creation of VMs.  This is the ridiculously easy part. Create a VM

# virsh define vmname.xml

Start a VM

# virsh start vmname

Create and Start a VM

# virsh create vmname.xml

Shutdown a VM

# virsh shutdown vmname

Poweroff a VM (Pull the Plug)

I know this sounds really scary and counter-intuitive, however the destroy parameter actually refers to the fact that you are destroying the contents of the memory.  They also don’t want you to get used to using this instead of a more orderly shutdown.

# virsh destroy vmname

Delete a VMs Configuration (Does not delete disk images)

# virsh undefine vmname

Edit a Defined VMs Configuration

# virsh edit vmname

Libvirt XML Template (Single Disk with VirtIO, Empty CDROM)

<domain type='kvm'>
<name>vmname</name>
<memory>2097152</memory>
<currentMemory>2097152</currentMemory>
<vcpu>1</vcpu>
<os>
<type arch='x86_64' machine='pc-0.12'>hvm</type>
<boot dev='hd'/>
</os>
<features>
<acpi/>
<apic/>
<pae/>
</features>
<clock offset='localtime'/>
<on_poweroff>destroy</on_poweroff>
<on_reboot>restart</on_reboot>
<on_crash>restart</on_crash>
<devices>
<emulator>/usr/bin/kvm</emulator>
<disk type='file' device='disk'>
<driver name='qemu' type='raw'/>
<source file='/kvm/images/disk/vmname_boot.img'/>
<target dev='vda' bus='virtio'/>
<address type='pci' domain='0x0000' bus='0x00' slot='0x04' function='0x0'/>
</disk>
<disk type='file' device='cdrom'>
<driver name='qemu' type='raw'/>
<source file='/kvm/images/iso/cdimage.iso'/>
<target dev='hdc' bus='ide'/>
<readonly/>
<address type='drive' controller='0' bus='1' unit='0'/>
</disk>
<controller type='ide' index='0'>
<address type='pci' domain='0x0000' bus='0x00' slot='0x01' function='0x1'/>
</controller>
<interface type='bridge'>
<source bridge='br0'/>
<model type='virtio'/>
<address type='pci' domain='0x0000' bus='0x00' slot='0x03' function='0x0'/>
</interface>
<serial type='pty'>
<target port='0'/>
</serial>
<console type='pty'>
<target type='serial' port='0'/>
</console>
<input type='tablet' bus='usb'/>
<input type='mouse' bus='ps2'/>
<graphics type='vnc' port='-1' autoport='yes' keymap='en-us'/>
<video>
<model type='vga' vram='9216' heads='1'/>
<address type='pci' domain='0x0000' bus='0x00' slot='0x02' function='0x0'/>
</video>
<memballoon model='virtio'>
<address type='pci' domain='0x0000' bus='0x00' slot='0x05' function='0x0'/>
</memballoon>
</devices>
</domain>

Libvirt XML Template (Single Disk without VirtIO, Empty CDROM)

<domain type='kvm'>
<name>vmname</name>
<memory>2097152</memory>
<currentMemory>2097152</currentMemory>
<vcpu>1</vcpu>
<os>
<type arch='x86_64' machine='pc-0.12'>hvm</type>
<boot dev='hd'/>
</os>
<features>
<acpi/>
<apic/>
<pae/>
</features>
<clock offset='localtime'/>
<on_poweroff>destroy</on_poweroff>
<on_reboot>restart</on_reboot>
<on_crash>restart</on_crash>
<devices>
<emulator>/usr/bin/kvm</emulator>
<disk type='file' device='disk'>
<driver name='qemu' type='raw'/>
<source file='/kvm/images/disk/vmname_boot.img'/>
<target dev='hda' bus='ide'/>
<address type='drive' controller='0' bus='0' unit='0'/>
</disk>
<disk type='file' device='cdrom'>
<driver name='qemu' type='raw'/>
<target dev='hdc' bus='ide'/>
<readonly/>
<address type='drive' controller='0' bus='1' unit='0'/>
</disk>
<controller type='ide' index='0'>
<address type='pci' domain='0x0000' bus='0x00' slot='0x01' function='0x1'/>
</controller>
<interface type='bridge'>
<source bridge='br0'/>
<model type='e1000'/>
<address type='pci' domain='0x0000' bus='0x00' slot='0x03' function='0x0'/>
</interface>
<serial type='pty'>
<target port='0'/>
</serial>
<console type='pty'>
<target type='serial' port='0'/>
</console>
<input type='tablet' bus='usb'/>
<input type='mouse' bus='ps2'/>
<graphics type='vnc' port='-1' autoport='yes' keymap='en-us'/>
<video>
<model type='vga' vram='9216' heads='1'/>
<address type='pci' domain='0x0000' bus='0x00' slot='0x02' function='0x0'/>
</video>
<memballoon model='virtio'>
<address type='pci' domain='0x0000' bus='0x00' slot='0x05' function='0x0'/>
</memballoon>
</devices>
</domain>

Libvirt XML Template (Multiple Disks with VirtIO, Empty CDROM)

<domain type='kvm'>
<name>vmname</name>
<memory>2097152</memory>
<currentMemory>2097152</currentMemory>
<vcpu>1</vcpu>
<os>
<type arch='x86_64' machine='pc-0.12'>hvm</type>
<boot dev='hd'/>
</os>
<features>
<acpi/>
<apic/>
<pae/>
</features>
<clock offset='localtime'/>
<on_poweroff>destroy</on_poweroff>
<on_reboot>restart</on_reboot>
<on_crash>restart</on_crash>
<devices>
<emulator>/usr/bin/kvm</emulator>
<disk type='file' device='disk'>
<driver name='qemu' type='raw'/>
<source file='/kvm/images/disk/vmname_boot.img'/>
<target dev='vda' bus='virtio'/>
<address type='pci' domain='0x0000' bus='0x00' slot='0x04' function='0x0'/>
</disk>
<disk type='file' device='disk'>
<driver name='qemu' type='raw'/>
<source file='/kvm/images/disk/vmname_data_1.img'/>
<target dev='vdb' bus='virtio'/>
<address type='pci' domain='0x0000' bus='0x00' slot='0x06' function='0x0'/>
</disk>
<disk type='file' device='cdrom'>
<driver name='qemu' type='raw'/>
<target dev='hdc' bus='ide'/>
<readonly/>
<address type='drive' controller='0' bus='1' unit='0'/>
</disk>
<controller type='ide' index='0'>
<address type='pci' domain='0x0000' bus='0x00' slot='0x01' function='0x1'/>
</controller>
<interface type='bridge'>
<source bridge='br0'/>
<model type='virtio'/>
<address type='pci' domain='0x0000' bus='0x00' slot='0x03' function='0x0'/>
</interface>
<serial type='pty'>
<target port='0'/>
</serial>
<console type='pty'>
<target type='serial' port='0'/>
</console>
<input type='tablet' bus='usb'/>
<input type='mouse' bus='ps2'/>
<graphics type='vnc' port='-1' autoport='yes' keymap='en-us'/>
<video>
<model type='vga' vram='9216' heads='1'/>
<address type='pci' domain='0x0000' bus='0x00' slot='0x02' function='0x0'/>
</video>
<memballoon model='virtio'>
<address type='pci' domain='0x0000' bus='0x00' slot='0x05' function='0x0'/>
</memballoon>
</devices>
</domain>

Libvirt XML Template (Multiple Disks without VirtIO, Empty CDROM)

<domain type='kvm'>
<name>vmname</name>
<memory>2097152</memory>
<currentMemory>2097152</currentMemory>
<vcpu>1</vcpu>
<os>
<type arch='x86_64' machine='pc-0.12'>hvm</type>
<boot dev='hd'/>
</os>
<features>
<acpi/>
<apic/>
<pae/>
</features>
<clock offset='localtime'/>
<on_poweroff>destroy</on_poweroff>
<on_reboot>restart</on_reboot>
<on_crash>restart</on_crash>
<devices>
<emulator>/usr/bin/kvm</emulator>
<disk type='file' device='disk'>
<driver name='qemu' type='raw'/>
<source file='/kvm/images/disk/vmname_boot.img'/>
<target dev='hda' bus='ide'/>
<address type='drive' controller='0' bus='0' unit='0'/>
</disk>
<disk type='file' device='disk'>
<driver name='qemu' type='raw'/>
<source file='/kvm/images/disk/vmname_data_1.img'/>
<target dev='hdb' bus='ide'/>
<address type='drive' controller='0' bus='0' unit='1'/>
</disk>
<disk type='file' device='cdrom'>
<driver name='qemu' type='raw'/>
<target dev='hdc' bus='ide'/>
<readonly/>
<address type='drive' controller='0' bus='1' unit='0'/>
</disk>
<controller type='ide' index='0'>
<address type='pci' domain='0x0000' bus='0x00' slot='0x01' function='0x1'/>
</controller>
<interface type='bridge'>
<source bridge='br0'/>
<model type='e1000'/>
<address type='pci' domain='0x0000' bus='0x00' slot='0x03' function='0x0'/>
</interface>
<serial type='pty'>
<target port='0'/>
</serial>
<console type='pty'>
<target type='serial' port='0'/>
</console>
<input type='tablet' bus='usb'/>
<input type='mouse' bus='ps2'/>
<graphics type='vnc' port='-1' autoport='yes' keymap='en-us'/>
<video>
<model type='vga' vram='9216' heads='1'/>
<address type='pci' domain='0x0000' bus='0x00' slot='0x02' function='0x0'/>
</video>
<memballoon model='virtio'>
<address type='pci' domain='0x0000' bus='0x00' slot='0x05' function='0x0'/>
</memballoon>
</devices>
</domain>