Comment 28 for bug 203537

Some more details:
Prior to enabling the "iommu=soft" kernel option, this is what dmesg shows in regards to IOMMU:

/var/log$ zgrep IOMMU dmesg.1.gz
[ 22.839979] Please enable the IOMMU option in the BIOS setup
[ 23.667358] PCI-DMA: using GART IOMMU.
[ 23.667365] PCI-DMA: Reserving 64MB of IOMMU area in the AGP aperture

After enabling the kernel option:
/var/log$ grep IOMMU dmesg

No more IOMMU messages.