Issue#

When attempting to start any virtual machines, the following critical error occurs:

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Error starting domain: operation failed: Unable to find 'efi' firmware that is compatible with the current configuration

This prevents all VMs from starting. The full traceback from virt-manager is shown below:

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Error starting domain: operation failed: Unable to find 'efi' firmware that is compatible with the current configuration
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Traceback (most recent call last):
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  File "/nix/store/pwsbb36h2jj1vq1k8dvwxsbmf9kqnpx3-virt-manager-5.0.0/share/virt-manager/virtManager/asyncjob.py", line 71, in cb_wrapper
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    callback(asyncjob, *args, **kwargs)
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    ~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^
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  File "/nix/store/pwsbb36h2jj1vq1k8dvwxsbmf9kqnpx3-virt-manager-5.0.0/share/virt-manager/virtManager/asyncjob.py", line 107, in tmpcb
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    callback(*args, **kwargs)
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    ~~~~~~~~^^^^^^^^^^^^^^^^^
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  File "/nix/store/pwsbb36h2jj1vq1k8dvwxsbmf9kqnpx3-virt-manager-5.0.0/share/virt-manager/virtManager/object/libvirtobject.py", line 57, in newfn
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    ret = fn(self, *args, **kwargs)
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  File "/nix/store/pwsbb36h2jj1vq1k8dvwxsbmf9kqnpx3-virt-manager-5.0.0/share/virt-manager/virtManager/object/domain.py", line 1384, in startup
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    self._backend.create()
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    ~~~~~~~~~~~~~~~~~~~~^^
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  File "/nix/store/yhwvyjphkjmwwlfs2li36rxj9zkka7jp-python3.13-libvirt-11.6.0/lib/python3.13/site-packages/libvirt.py", line 1390, in create
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    raise libvirtError('virDomainCreate() failed')
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libvirt.libvirtError: operation failed: Unable to find 'efi' firmware that is compatible with the current configuration

Cause#

After a flake update, the path to qemu-system-x86_64 changed. The VM XML configuration still pointed to the old /nix/store/… firmware files, so libvirt failed to locate the EFI firmware.

Here is an example of the old XML snippet (no longer valid):

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<os firmware="efi">
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  <type arch="x86_64" machine="pc-q35-10.0">hvm</type>
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  <firmware>
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    <feature enabled="no" name="enrolled-keys"/>
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    <feature enabled="yes" name="secure-boot"/>
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  </firmware>
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  <loader readonly="yes" secure="yes" type="pflash" format="raw">
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    /nix/store/gc3xfs807jz08hd88mjy055fyhiifhxy-qemu-host-cpu-only-10.0.2/share/qemu/edk2-x86_64-secure-code.fd
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  </loader>
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  <nvram template="/nix/store/gc3xfs807jz08hd88mjy055fyhiifhxy-qemu-host-cpu-only-10.0.2/share/qemu/edk2-i386-vars.fd"
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         templateFormat="raw"
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         format="raw">
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    /var/lib/libvirt/qemu/nvram/win11-ltsc_VARS.fd
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  </nvram>
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  <boot dev="hd"/>
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</os>

Solution#

Find the current path of qemu-system-x86_64 in your Nix store:

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nix-store -q $(which qemu-system-x86_64)

Update the VM XML so that the <loader> and <nvram> entries point to the new paths:

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<os firmware="efi">
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  <type arch="x86_64" machine="pc-q35-10.0">hvm</type>
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  <firmware>
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    <feature enabled="no" name="enrolled-keys"/>
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    <feature enabled="yes" name="secure-boot"/>
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  </firmware>
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  <loader readonly="yes" secure="yes" type="pflash" format="raw">
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    /nix/store/8fg38774hh8mysi8f0a259762i4mcgzx-qemu-host-cpu-only-10.0.3/share/qemu/edk2-x86_64-secure-code.fd
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  </loader>
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  <nvram template="/nix/store/8fg38774hh8mysi8f0a259762i4mcgzx-qemu-host-cpu-only-10.0.3/share/qemu/edk2-i386-vars.fd"
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         templateFormat="raw"
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         format="raw">
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    /var/lib/libvirt/qemu/nvram/win11-ltsc_VARS.fd
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  </nvram>
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  <boot dev="hd"/>
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</os>