Skylake with retpoline needs additional protection against return stack underflows. While full coverage is more involved, there are some low hanging fruits that are relatively easy. We already did stuffing on context switch earlier, this adds the next low hanging fruit: stuffing on interrupts and exception return to the kernel.
We only need to stuff when returning to the kernel so there is no impact on anything interrupting user space directly. Also the additional stuffing is only enabled on Skylake. Git tree available here git://git.kernel.org/pub/scm/linux/kernel/git/ak/linux-misc.git spec/interrupt-stuff-1 v1: Initial post, but based on the earlier skl-rsb code.