defaults to Y while recommends N? That doesn't make too much sense...:
Enable DMA Remapping Devices by default (DMAR_DEFAULT_ON) [Y/n/?] (NEW) ?
Selecting this option will enable a DMAR device at boot time if
one is found. If this option is not selected, DMAR support can
be enabled by passing intel_iommu=on to the kernel. It is
recommended you say N here while the DMAR code remains
experimental.
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i.