It would be nice if kdm would provide a systemd service file. I currently use the one from systemd-overlay  and it works fine. As I learned, gentoo-kde people are not excited about systemd but afaik it only requires this one file, it would help people that like to test systemd and it should not interfere with startup on non-systemd systems.
Which package should install this file and where should it be installed to?
If it is working for you we could send it upstream.
It would be nice if kde-base/kdm would install http://git.overlays.gentoo.org/gitweb/?p=user/systemd.git;a=blob;f=sys-apps/systemd-units/files/services-desktop/kdm.service;h=412ffa394ff565432a3a8eb63c2db303af0aee42;hb=HEAD as "kdm.service" into the default directory "/usr/lib/systemd/system/".
There is a related bug #391339 but I can not confirm this behavior on my AMD64 system. Here, it works fine.
Generally, it should be kde-base/kdm that provides its own service file, installed with the systemd eclass.
Now, since I don't use kdm myself, I don't know how correct or current the service file in the systemd-overlay is, but you are welcome to submit it upstream.
Created attachment 339000 [details]
Added from Unoficial Gentoo Wiki
(In reply to comment #4)
> Created attachment 339000 [details]
> Added from Unoficial Gentoo Wiki
That is exactly the one available in sys-apps/systemd-units-5 from the systemd-overlay.
Could be great if kdm-4.10.2 had this file included. It doesn't require much work and it fixes actual issue (and maintain all kdm files together).
So, please do it.
FYI, the systemd-love overlay has a kde-base/kdm ebuild containing kdm.service.
This file added in 4.10.2-r1 and later. Please file new bugs if problems occur.