Bug#889990: gnome-boxes: Unable to start VMs because CPU mode 'custom' for x86_64 kvm domain on x86_64 host is not supported

[Guido Günther]

On Thu, Feb 15, 2018 at 09:45:03AM -0800, Josh Triplett wrote:
> Package: gnome-boxes
> Version: 3.26.2-3
> Followup-For: Bug #889990
> 
> I've encountered this issue as well, with the current version in
> unstable.

Likely your user is not in the libvirt group and so the list of CPUs
available for KVM is empty. Could you try 

    adduser <youruser> kvm

If that helps we should probably give rw permissions to the currently
logged in user via logind (as done with the audio and video devices
already).

Cheers,
 -- Guido