It seems that interrupt mapping is done by BIOS (or Libreboot in my case) first, then the linux kernel remaps things for whatever reason [0]. I know nothing about this low-level stuff, but because the laptop is working despite the DMAR failure, I ventured a guess that the interrupt remapping done by the kernel is not strictly necessary.
Therefore, I looked at the complete list of kernel boot parameters [1] and tried all the ones that sounded promising. I found that intremap=off makes the boot-time DMAR error go away! I tested all peripherals and everything seems to be working fine. Relevant lines from dmesg here: http://pastebin.com/KCGVaf8U In some cases, this DMAR error is related to faulty chipsets [2]. I guess the question that remains is whether my T400 (or its Libreboot) is defective in some way or is the DMAR failure something that is expected to happen on every T400 with Libreboot? If there is nothing wrong with my hardware or my Libreboot flash, is booting with intremap=off ill-advised for any reason? [0] https://unix.stackexchange.com/questions/84880/what-is-meant-by-interrupt-remapping-in-linux [1] https://www.kernel.org/doc/Documentation/kernel-parameters.txt [2] https://www.netiq.com/support/kb/doc.php?id=7014344
