Comment 40 for bug 1561019

(In reply to Zeeshan Ali from comment #20)
> From Boxes bug https://bugzilla.gnome.org/show_bug.cgi?id=720798
>
> "When I run qemu with the '-cpu host' parameter instead, GNOME Shell starts
> correctly."
>
> Shouldn't libvirt just use that option of qemu for 'host-model' config?

host-passthrough maps to the qemu '-cpu host' parameter.

host-model is as convoluted as it is so that live migration can
be supported. ie. You start a guest on host A with host-model,
then migrate it to host B. What you DON'T want to happen is
that -cpu host is used on host B, since likely host A and host B
have different CPUs, so the guest will see a sudden change in CPU
capabilities and probably crash / get inconsistent results.
Instead host-model tries to create a full description of host A's
CPU, and after live migration the guest sees the same (host A)
CPU features.

AIUI host-passthrough disables or prevents live migration in some way.