Stuffing the return stack buffer

> RSB stuffing relies on retpolines for Spectre v2 mitigation.

FWIW, it's vice versa: retpolines rely on RSB stuffing to make them less broken on Skylake.

But yeah, the general sentiment of that email is that apparently retpolines would be unsafe on Skylake even if RSB stuffing were added in all cases when the RSB might become empty.