On Wed, 2018-01-10 at 16:13 +0100, Andrea Arcangeli wrote: > > Can you also tell if IBRS must be written as a barrier to SPEC_CTRL in > return to userland (kernel exit) when ibrs_enabled 2? Generally we > wouldn't run a barrier there with ibrs_enabled 2, but absolutely > nothing is intuitive here so I need to ask explicitly.
Since it achieves nothing¹ but to make userspace run slower, there's no need to write it again on returning to userspace. It will perform that function just fine without doing so. (¹ that STIBP doesn't already do, and cheaper)
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