[coreboot] IOMMU on Skylake / Chell?

[Trammell Hudson]

I've successfully built a coreboot firmware and Linux bootloader payload
for the Chell Chromebook with Skylake, which then kexec's Xen / Qubes
from the eMMC.  Both of them are reporting that the IOMMU is not in use,
and there is no DMAR entry in the ACPI table, which I believe is what
they are looking for for enabling the IOMMU.

The Xen console prints:
> (XEN) I/O virtualization disabled

And qubes-hcl reports:
> I/O MMU: Not active

What's involved in enabling it?  How soon in the boot process can it
be turned on?