I’ve just recently noticed these lines in the dmesg output:
Please enable the IOMMU option in the BIOS setup This costs you 64 MB of RAM
they may have been there forever, but I never noticed them before. This system is an AMD Phenom-II X2 CPU on a Gigabyte MA770-UD3 motherboard.
I have no idea which BIOS option I need to change to solve this, I see nothing in the setup that refers to IOMMU.