Sample files for libvirt define behaviour

output.log

initial definition
Domain 'vagrant-libvirt_default' defined from test.xml

display vcpus
  <vcpu placement='static'>2</vcpu>

update with nvram XML tag
Domain 'vagrant-libvirt_default' defined from test_with_nvram.xml

check if apply worked
  <vcpu placement='static'>4</vcpu>

update with both nvram and loader XML tags
Domain 'vagrant-libvirt_default' defined from test_with_nvram_and_loader.xml

check if apply worked (should also revert the vcpu)
    <nvram>/usr/share/edk2-ovmf/OVMF_VARS.fd</nvram>
  <vcpu placement='static'>2</vcpu>

test.bash

#!/bin/bash

echo "initial definition"
virsh -c qemu:///system define test.xml

echo "display vcpus"
virsh -c qemu:///system dumpxml vagrant-libvirt_default | grep vcpu
echo

echo "update with nvram XML tag"
virsh -c qemu:///system define test_with_nvram.xml

echo "check if apply worked"
virsh -c qemu:///system dumpxml vagrant-libvirt_default | grep nvram
virsh -c qemu:///system dumpxml vagrant-libvirt_default | grep vcpu
echo

echo "update with both nvram and loader XML tags"
virsh -c qemu:///system define test_with_nvram_and_loader.xml

echo "check if apply worked (should also revert the vcpu)"
virsh -c qemu:///system dumpxml vagrant-libvirt_default | grep nvram
virsh -c qemu:///system dumpxml vagrant-libvirt_default | grep vcpu

test.xml

<domain type='kvm'>
  <name>vagrant-libvirt_default</name>
  <uuid>b12bdd34-9c8d-4bd5-b2c7-ea1e2359bd64</uuid>
  <description>Source: /home/test/git/github.com/vagrant-libvirt/vagrant-libvirt/Vagrantfile</description>
  <memory unit='KiB'>2097152</memory>
  <currentMemory unit='KiB'>2097152</currentMemory>
  <vcpu placement='static'>2</vcpu>
  <os>
    <type arch='x86_64' machine='pc-i440fx-7.0'>hvm</type>
    <boot dev='hd'/>
  </os>
  <features>
    <acpi/>
    <apic/>
    <pae/>
  </features>
  <cpu mode='host-model' check='partial'/>
  <clock offset='utc'/>
  <on_poweroff>destroy</on_poweroff>
  <on_reboot>restart</on_reboot>
  <on_crash>destroy</on_crash>
  <devices>
    <emulator>/usr/bin/qemu-system-x86_64</emulator>
    <disk type='file' device='disk'>
      <driver name='qemu' type='qcow2'/>
      <source file='/var/lib/libvirt/images/vagrant-libvirt_default.img'/>
      <target dev='vda' bus='virtio'/>
      <alias name='ua-box-volume-0'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x03' function='0x0'/>
    </disk>
    <controller type='usb' index='0' model='piix3-uhci'>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x01' function='0x2'/>
    </controller>
    <controller type='pci' index='0' model='pci-root'/>
    <interface type='network'>
      <mac address='52:54:00:a9:7e:c5'/>
      <source network='vagrant-libvirt'/>
      <model type='virtio'/>
      <alias name='ua-net-0'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x05' function='0x0'/>
    </interface>
    <serial type='pty'>
      <target type='isa-serial' port='0'>
        <model name='isa-serial'/>
      </target>
    </serial>
    <console type='pty'>
      <target type='serial' port='0'/>
    </console>
    <input type='mouse' bus='ps2'/>
    <input type='keyboard' bus='ps2'/>
    <graphics type='vnc' port='-1' autoport='yes' listen='127.0.0.1' keymap='en-us'>
      <listen type='address' address='127.0.0.1'/>
    </graphics>
    <audio id='1' type='none'/>
    <video>
      <model type='cirrus' vram='256' heads='1' primary='yes'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x02' function='0x0'/>
    </video>
    <memballoon model='virtio'>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x04' function='0x0'/>
    </memballoon>
  </devices>
</domain>

test_with_nvram.xml

<domain type='kvm'>
  <name>vagrant-libvirt_default</name>
  <uuid>b12bdd34-9c8d-4bd5-b2c7-ea1e2359bd64</uuid>
  <description>Source: /home/test/git/github.com/vagrant-libvirt/vagrant-libvirt/Vagrantfile</description>
  <memory unit='KiB'>2097152</memory>
  <currentMemory unit='KiB'>2097152</currentMemory>
  <vcpu placement='static'>4</vcpu>
  <os>
    <type arch='x86_64' machine='pc-i440fx-7.0'>hvm</type>
    <boot dev='hd'/>
    <nvram>/usr/share/edk2-ovmf/OVMF_VARS.fd</nvram>
  </os>
  <features>
    <acpi/>
    <apic/>
    <pae/>
  </features>
  <cpu mode='host-model' check='partial'/>
  <clock offset='utc'/>
  <on_poweroff>destroy</on_poweroff>
  <on_reboot>restart</on_reboot>
  <on_crash>destroy</on_crash>
  <devices>
    <emulator>/usr/bin/qemu-system-x86_64</emulator>
    <disk type='file' device='disk'>
      <driver name='qemu' type='qcow2'/>
      <source file='/var/lib/libvirt/images/vagrant-libvirt_default.img'/>
      <target dev='vda' bus='virtio'/>
      <alias name='ua-box-volume-0'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x03' function='0x0'/>
    </disk>
    <controller type='usb' index='0' model='piix3-uhci'>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x01' function='0x2'/>
    </controller>
    <controller type='pci' index='0' model='pci-root'/>
    <interface type='network'>
      <mac address='52:54:00:a9:7e:c5'/>
      <source network='vagrant-libvirt'/>
      <model type='virtio'/>
      <alias name='ua-net-0'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x05' function='0x0'/>
    </interface>
    <serial type='pty'>
      <target type='isa-serial' port='0'>
        <model name='isa-serial'/>
      </target>
    </serial>
    <console type='pty'>
      <target type='serial' port='0'/>
    </console>
    <input type='mouse' bus='ps2'/>
    <input type='keyboard' bus='ps2'/>
    <graphics type='vnc' port='-1' autoport='yes' listen='127.0.0.1' keymap='en-us'>
      <listen type='address' address='127.0.0.1'/>
    </graphics>
    <audio id='1' type='none'/>
    <video>
      <model type='cirrus' vram='256' heads='1' primary='yes'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x02' function='0x0'/>
    </video>
    <memballoon model='virtio'>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x04' function='0x0'/>
    </memballoon>
  </devices>
</domain>

test_with_nvram_and_loader.xml

<domain type='kvm'>
  <name>vagrant-libvirt_default</name>
  <uuid>b12bdd34-9c8d-4bd5-b2c7-ea1e2359bd64</uuid>
  <description>Source: /home/test/git/github.com/vagrant-libvirt/vagrant-libvirt/Vagrantfile</description>
  <memory unit='KiB'>2097152</memory>
  <currentMemory unit='KiB'>2097152</currentMemory>
  <vcpu placement='static'>2</vcpu>
  <os>
    <type arch='x86_64' machine='pc-i440fx-7.0'>hvm</type>
    <boot dev='hd'/>
    <loader type='rom'>/usr/share/edk2-ovmf/OVMF_CODE.secboot.fd</loader>
    <nvram>/usr/share/edk2-ovmf/OVMF_VARS.fd</nvram>
  </os>
  <features>
    <acpi/>
    <apic/>
    <pae/>
  </features>
  <cpu mode='host-model' check='partial'/>
  <clock offset='utc'/>
  <on_poweroff>destroy</on_poweroff>
  <on_reboot>restart</on_reboot>
  <on_crash>destroy</on_crash>
  <devices>
    <emulator>/usr/bin/qemu-system-x86_64</emulator>
    <disk type='file' device='disk'>
      <driver name='qemu' type='qcow2'/>
      <source file='/var/lib/libvirt/images/vagrant-libvirt_default.img'/>
      <target dev='vda' bus='virtio'/>
      <alias name='ua-box-volume-0'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x03' function='0x0'/>
    </disk>
    <controller type='usb' index='0' model='piix3-uhci'>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x01' function='0x2'/>
    </controller>
    <controller type='pci' index='0' model='pci-root'/>
    <interface type='network'>
      <mac address='52:54:00:a9:7e:c5'/>
      <source network='vagrant-libvirt'/>
      <model type='virtio'/>
      <alias name='ua-net-0'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x05' function='0x0'/>
    </interface>
    <serial type='pty'>
      <target type='isa-serial' port='0'>
        <model name='isa-serial'/>
      </target>
    </serial>
    <console type='pty'>
      <target type='serial' port='0'/>
    </console>
    <input type='mouse' bus='ps2'/>
    <input type='keyboard' bus='ps2'/>
    <graphics type='vnc' port='-1' autoport='yes' listen='127.0.0.1' keymap='en-us'>
      <listen type='address' address='127.0.0.1'/>
    </graphics>
    <audio id='1' type='none'/>
    <video>
      <model type='cirrus' vram='256' heads='1' primary='yes'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x02' function='0x0'/>
    </video>
    <memballoon model='virtio'>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x04' function='0x0'/>
    </memballoon>
  </devices>
</domain>

Shows an issue where XML that has an nvram tag without a loader tag and an update to the vcpu tag will result in only the vcpu number being modified while the nvram tag is ignored. Expectation is this should result in an error and both changes should be ignored.

Once the loader tag is present as well, the definition applies as expected.

Output of virsh -c qemu:///system version:

Compiled against library: libvirt 8.2.0
Using library: libvirt 8.2.0
Using API: QEMU 8.2.0
Running hypervisor: QEMU 7.0.0