Comment 2 for bug 1361207

The issue I'm referring to here is present since 331.67. The default NVIDIA driver in Trusty is 331.38.
This issue is characterized by CUDA applications failing during initialization with an error similar to this one:
CUDA Runtime API error 30: unknown error

For newer NVIDIA driver you need to install the nvidia-###-uvm package (### == major NVIDIA driver version).
After that you need to ensure after every reboot that the nvidia-uvm kernel module is loaded and /dev/nvidia-uvm is present.
If you have nvidia-modprobe installed then it takes care of these two conditions if you launch a CUDA application.
A CUDA application would also react similar to nvidia-modprobe if you would run it with root rights.
As a last resort you can run the script I've mentioned in the bug description to load the nvidia-uvm kernel module and create the /dev/nvidia-uvm character device after a reboot.

Of all solutions nvidia-modprobe is the most convenient one as everything works out of the box if needed but it might be considered a security concern as it is a setuid binary. You can install the nvidia-modprobe package from Utopic if you want to give it a try.