This mainly applies to 970 chipset mainboards, so your mileage may vary.
Symptom, no USB 3 , or Network, or USB 2 using Ubutnu 14.04 (actually from 12.04 up) 64 bit editions. As this is a kernel problem this should affect most 64 bit linux distros, not just Ubuntu.
There seems to be a problem with all recent 64 bit linux distros and 970 chipset mainboards.
There are 3 options.
1) Use a 32 bit distro.
2) In your BIOS enable IOMMU. This will fix your network and USB 2. You probably wont have USB 3 though 🙁
3) Enable IOMMU in BIOS and add iommu=soft to your grub config. This gets rid of the stream of warnings you get at boot with IOMMU enabled in BIOS. USBB 3 should work too.
sudo nano /etc/default/grub
Change the line
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash iommu=soft"
This should make the USB 2 and 3 work as well as networking.
Let me know what chipsets/mainboards this helped you with.
Changed option 3 as noted by Anthony Barnett. Many hanks for the feedback Anthony.