Without actually reading current kernel source code, I have found on the 
web:

iommu=soft. It deactivates the hardware IOMMU (which causes numerous DMA 
errors on my system) and uses a software IOMMU instead.

A colleague had to use iommu=soft with a UEFI enabled linux on a 
motherboard that did not have SecureBoot enabled in order for the UEFI 
enable linux to work with the USB 2 port.  Evidently, USB 2 is not 
regarded as UEFI "secure" and has to enabled/used in a particular method 
for a USB 2 port to be usable on a UEFI machine.  USB 3 works fine.

However, the software IOMMU evidently runs noticeable "slower" than the 
hardware IOMMU (that does not function).  Has anyone experienced this 
issue and does anyone have a suggested workaround?

Yasha Karant