What are the implication of iommu=soft?

I've got new laptop at work (Lenovo A485) and there are few issues with it. It prints AMD-Vi: IOAPIC not in IVRS table and kernel panic after that. So far I've figured few ways to get the system up and running. * noapic - terrible performance and high temperature, so not really a good way to do it * amd_iommu=off - not ideal either * ivrs_ioapic=00:14.0 ivrs_ioapic=00:00.2 - this seems to work fine * iommu=soft My questions are about iommu=soft. I'm not sure what exactly it does. What are the implications of this mode? What is preferable, overriding the IVRS table or iommu=soft?