[CentOS] intel_iommu=on => No root device found

Tue Jan 29 21:16:45 UTC 2013
Nils Caspar <ncaspar at me.com>

Hi all,

for some reaseon, my message wasn't published. Second try, this time without the giant attachment.

I have an Intel server (SR2600URBRPR) in front of me. The disks in my server are connected via PCI card (Adaptec RAID 3405).

Now I would like to passthrough a PCI-E graphics card to a KVM guest. As this requires Intel VT-d, I configured the following options in the BIOS:

Intel VT for Directed I/O  [Enabled]
 Interrupt Remapping       [Enabled]
 Coherency Support         [Disabled]
 ATS Support               [Enabled]
 Pass-through DMA Support  [Enabled]

Everything works so far, but when I add intel_iommu=on to the kernel line of my grub.conf, Dracut gives my a kernel panic while booting. I added "rdshell" to the command line and extracted the boot messages via dmesg (http://nilscaspar.ch/centos-dmesg.log). I think the main problem is this:

dracut Warning: No root device "block:/dev/disk/by-uuid/280234d1-dff5-4e04-b1ef-93d54cdcd825" found

But I doesn't make sense to me... As soon as I remove intel_iommu=on, everything works again (except the passthrough :P).