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SubjectRe: [patch 21/24] x86/speculation: Prepare arch_smt_update() for PRCTL mode
On Fri, 23 Nov 2018, Thomas Gleixner wrote:

> > So I'm wondering, shouldn't firmware_restrict_branch_speculation_start()/_end()
> > also enable/disable STIBP? It already enabled/disables IBRS.
>
> IBRS includes STIBP.

True.

> We don't use IBRS in the kernel otherwise because you'd have to do more
> MSR writes on the protection boundaries.

Just for the record -- we do have an option for IBRS in our distro kernel
on SKL+ systems.

There definitely is a measurable performance impact, but the MSR writes on
protection boundaries are totally cheap compared to the actual IBRS
runtime operation effect.

--
Jiri Kosina
SUSE Labs

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