Starting with UEK R5U1, IOMMU functionality is enabled by default in the x86_64 kernel. This change better facilitates single root input-output virtualization (SR-IOV) and other virtualization extensions; but, is also known to result in boot failure issues on certain hardware that cannot complete discovery when IOMMU is enabled. The status of this feature no longer appears in /proc/cmd reporting as iommu=on and may need to be explicitly disabled as a kernel cmdline option if boot failure occurs. As an alternate workaround, you can disable IOMMU or Intel-Vtd in your system ROM by following your vendor instructions.

These boot failure issues have been observed on equipment with certain Broadcom network devices, such HP Gen8 servers. For more detailed information, see https://support.hpe.com/hpsc/doc/public/display?docId=emr_na-c04565693 .