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SubjectRe: [RFC 09/10] x86/enter: Create macros to restrict/unrestrict Indirect Branch Speculation
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@infradead.org> 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

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