Comment 8 for bug 1837877

So:

nova/virt/libvirt/guest.py
===========
    def launch(self, pause=False):
        """Starts a created guest.

        :param pause: Indicates whether to start and pause the guest
        """
        flags = pause and libvirt.VIR_DOMAIN_START_PAUSED or 0
        try:
            return self._domain.createWithFlags(flags)
        except Exception:
            with excutils.save_and_reraise_exception():
                LOG.error('Error launching a defined domain '
                          'with XML: %s',
                          self._encoded_xml, errors='ignore')
===========

This bubbles up any exceptions from libvirt, the function that calls it..

nova/virt/libvirt/guest.py
===========
    def _create_domain(self, xml=None, domain=None,
                       power_on=True, pause=False, post_xml_callback=None):
        """Create a domain.

        Either domain or xml must be passed in. If both are passed, then
        the domain definition is overwritten from the xml.

        :returns guest.Guest: Guest just created
        """
        if xml:
            guest = libvirt_guest.Guest.create(xml, self._host)
            if post_xml_callback is not None:
                post_xml_callback()
        else:
            guest = libvirt_guest.Guest(domain)

        if power_on or pause:
            guest.launch(pause=pause)

        if not utils.is_neutron():
            guest.enable_hairpin()

        return guest
===========

This also handles no exceptions, so it goes up to _create_domain_and_network() which has a case to grab all generic exceptions

===========
        except Exception:
            # Any other error, be sure to clean up
            LOG.error('Failed to start libvirt guest', instance=instance)
            with excutils.save_and_reraise_exception():
                self._cleanup_failed_start(context, instance, network_info,
                                           block_device_info, guest,
                                           destroy_disks_on_failure)
===========

but that still bubbles up.. in this case, up to _hard_reboot() which has no exception handling, which is called by reboot() that has no exception handling, which means at this point, we've sent a *libvirt* exception up to the compute manager.. more specifically reboot_instance() which is decorated by the following:

===========
    @wrap_exception()
    @reverts_task_state
    @wrap_instance_event(prefix='compute')
    @wrap_instance_fault
===========

- wrap_exception: this seems to emit notifications on exceptions
- reverts_task_state: pretty self explanitory
- wrap_instance_event: this seems like it adds an instance action to the log
- wrap_instance_fault:L