Retpoline Backported To Linux 4.9, Linux 4.14 Kernels
Greg Kroah-Hartman has sent out the latest patches for the Linux 4.9 and 4.14 point releases, which now include the Retpoline support.
This X86_FEATURE_RETPOLINE is enabled for all AMD/Intel CPUs. For full support you also need to be building the kernel with a newer GCC compiler containing -mindirect-branch=thunk-extern support. The GCC changes landed in GCC 8.0 yesterday and is in the process of potentially being back-ported to GCC 7.3.
Those wanting to disable the Retpoline support can boot the patched kernels with noretpoline.
This Retpoline support will be found in the soon-to-be-released Linux 4.9.77 and 4.14.14 kernels.
Greg did send out new patches today for the Linux 4.4 and 3.18 stable series today, but they do not contain Retpoline support. But at least does include the sysfs additions for noting if the CPU is vulnerable to Spectre.
Linux 4.15 has Retpoline support as of a few days ago.