Re: x86/retpoline: Fill RSB on context switch for affected CPUs

[Andi Kleen]

> Shouldn't the RSB filling on context switch also be done on non-IBPB
> CPUs to protect (retpolined) user space tasks from other user space
> tasks?

The comment is actually incorrect. There's no risk to hit user space
addresses if we have KPTI and NX (which is fairly universal).

It's mainly needed on Skylake era CPUs.

Should fix the comment. I'll send a patch.

-Andi