The original design came from thinking about systems where using the iommu
was mandatory. I think we almost always reserve memory below 1G for the kexec
on panic kernel so it really shouldn't be an issue in that case. Except
we need to pass an option to force not using the iommu. I don't think
noiommu or swiotlb is going to make any real difference.
So I'm totally in favor of turning off features if we don't need them and we
don't take a tremendous performance hit. (People get grumpy when writing
all of memory to disk takes completely unreasonable amounts of time).