Comment 56 for bug 1759920

>> And adding 'noibpb' kernel boot parameter helps, at least in my case.

However, according to:
   https://wiki.ubuntu.com/SecurityTeam/KnowledgeBase/SpectreAndMeltdown/MitigationControls
doing that,
"Disabling these features removes mitigations for CVE-2017-5715 (aka Spectre / Variant 2).", which sort-of defeats what the microcode fix is for.

However, the rest of the thread for Bug #1746418 does explain why my 18.04beta system is OK - it's running 4.15.0-xxx