Thanks to Mike Pagano who enabled kdbus support in Gentoo kernel sources almost 2 weeks ago. Which gives us the choice to test it. As described in Mikes blog post you will need to enable the use flags kdbus and experimental on sys-kernel/gentoo-sources and kdbus on sys-apps/systemd.
root # echo "sys-kernel/gentoo-sources kdbus experimental" >> /etc/portage/package.use/kdbus
If you are running >=sys-apps/systemd-221 kdbus is already enabled by default otherwise you have to enable it.
root # echo "sys-apps/systemd kdbus" >> /etc/portage/package.use/kdbus
Any packages affected by the change need to be rebuilt.
root # emerge -avuND @world
Enable kdbus option in kernel.
General setup --->
<*> kdbus interprocess communication
Build the kernel, install it and reboot. Now we can check if kdbus is enabled properly. systemd should automatically mask dbus.service and start systemd-bus-proxyd.service instead (Thanks to eliasp for the info).
root # systemctl status dbus
● dbus.service
Loaded: masked (/dev/null)
Active: inactive (dead)
root # systemctl status systemd-bus-proxyd
● systemd-bus-proxyd.service - Legacy D-Bus Protocol Compatibility Daemon
Loaded: loaded (/usr/lib64/systemd/system/systemd-bus-proxyd.service; static; vendor preset: enabled)
Active: active (running) since Fr 2015-07-10 22:42:16 CEST; 16min ago
Main PID: 317 (systemd-bus-pro)
CGroup: /system.slice/systemd-bus-proxyd.service
└─317 /usr/lib/systemd/systemd-bus-proxyd --address=kernel:path=/sys/fs/kdbus/0-system/bus
Plasma 5 starts fine here using sddm as login manager. On Plasma 4 you may be interested in Bug #553460.
Looking forward when Plasma 5 will get user session support.
Have fun!