I'll add that I resolved my ARCH issue as well with the kernel setting prior to this thread popping up, and offered help because of my experience with the ARCH SPECIFIC problem. This issue cropped with a new Gigabyte board and ONLY on Arch. Three other distros had no issue, whatsoever, with IOMMU one way or another....Arch did.
Well, then that completely changes things. Should have corrected me earlier! If it indeed is an "Arch does something differently" issue, then my earlier comments just aren't relevant. Glad you and the OP found a solution, and that you were able to help him out.