Re: [RFC 09/10] x86/enter: Create macros to restrict/unrestrict Indirect Branch Speculation

[Andi Kleen]

On Fri, Jan 26, 2018 at 09:19:09AM -0800, Linus Torvalds wrote:
> On Fri, Jan 26, 2018 at 1:11 AM, David Woodhouse <dwmw2@xxxxxxxxxxxxx> wrote:
> >
> > Do we need to look again at the fact that we've disabled the RSB-
> > stuffing for SMEP?
> 
> Absolutely. SMEP helps make people a lot less worried about things,
> but it doesn't fix the "BTB only contains partial addresses" case.
> 
> But did we do that "disable stuffing with SMEP"? I'm not seeing it. In
> my tree, it's only conditional on X86_FEATURE_RETPOLINE.

For Skylake we need RSB stuffing even with SMEP to avoid falling back to the
BTB on underflow.

It's also always needed with virtualization.

-Andi