Dave Hansen from Intel posted a patch and said, "Intel is considering adding a new bit to the IA32_ARCH_CAPABILITIES MSR (Model-Specific Register) to tell when RSB (Return Stack Buffer) underflow might be happening. Feedback on this would be greatly appreciated before the specification is finalized." He explained that RSB:

Efforts to work around serious hardware flaws in Intel chips are ongoing. Nadav Amit posted a patch to improve compatibility mode with respect to Intel's Meltdown flaw. Compatibility mode is when the system emulates an older CPU in order to provide a runtime environment that supports an older piece of software that relies on the features of that CPU. The thing to be avoided is to emulate massive security holes created by hardware flaws in that older chip as well.

For weeks, the world's been talking about severe Intel design flaws affecting many CPUs and forcing operating systems to look for sometimes costly workarounds. Linux patches for these issues are in a state of ongoing development. Security is always the first priority, at the expense of any other feature. Next would probably be the general speed of a running system for the average user. After that, the developers might begin piecing together any features that had been pulled as part of the initial security fix.

Red Hat's Spectre remediation currently requires new microcode for a complete fix, which leaves most x86 processors vulnerable as they lack this update. Oracle has released new retpoline kernels that completely remediate Meltdown and Spectre on all compatible CPUs, which I show how to install and test on CentOS here. The Red Hat community has patiently awaited a retpoline kernel implementation that remediates CVE-2017-5715 (Spectre v2) and closes all Meltdown and Spectre vulnerabilities that have captured headlines this year.