For those who came in late, Google developed software-based mitigation for Spectre Variant 2 called Retpoline that restricts the speculative execution behaviour enough to mitigate an attack.
What made the Google patch useful was that it had a negligible effect on performance. For this reason, Retpoline was implemented by Linux distributions such as Red Hat, SUSE, Oracle Linux 6 and 7.
Microsoft's kernel engineers have confirmed that Retpoline will be part of the next version of Windows 10, 19H1, which is due out next year. Vole said that its own kernel modifications have reduced the performance impact to "noise level".
Microsoft's Windows and Azure kernel team’s Mehmet Iyigun said that Retpoline was enabled by default in our 19H1 flights along with what we call 'import optimization' to further reduce perf impact due to indirect calls in kernel-mode.
“Combined, these reduce the perf impact of Spectre v2 mitigations to noise-level for most scenarios", wrote Iyigun.
Sadly Microsoft didn't include the Retpoline fix in the latest Windows 10 October 2018 Update Redstone 5, or RS5, release.