Reducing the performance impact of 'Meltdown' CVE fixes for OpenStack guests with "PCID" CPU feature flag

Issue

Fixes for Meltdown security vulnerability variants can result in some performance slowdown for certain workloads, given the nature of those vulnerabilities and their fixes. This is well documented in the following related article:

• Speculative Execution Exploit Performance Impacts - Describing the performance impacts to security patches for CVE-2017-5754 CVE-2017-5753 and CVE-2017-5715

Is it possible to reduce the performance impact 'Meltdown' fixes have on OpenStack Platform guests? Yes, it is — by ensuring the presence of a particular CPU feature flag called 'PCID' (Process Context ID) within the guest. More details on that in the following sections.