Messages in this thread | | | Date | Mon, 30 Jul 2018 18:58:16 +0200 (CEST) | From | Thomas Gleixner <> | Subject | Re: [PATCH] x86/speculation: Support Enhanced IBRS on future CPUs |
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On Mon, 30 Jul 2018, Dave Hansen wrote: > On 07/30/2018 05:25 AM, Thomas Gleixner wrote: > > On Tue, 24 Jul 2018, Sai Praneeth Prakhya wrote: > >> From: Sai Praneeth <sai.praneeth.prakhya@intel.com> > >> Some future Intel processors may support "Enhanced IBRS" which is an > >> "always on" mode i.e. IBRS bit in SPEC_CTRL MSR is enabled once and > >> never disabled. According to specification[1], this should simplify > >> software enabling and improve performance. > > SHOULD is not really helpful. The question is whether it does improve > > performance in practice or not. You really want to add numbers comparing > > retpoutine and enhanced IBRS. > > One thing to remember from Intel's retpoline paper: > > > Retpoline is known to be an effective branch target injection > > (Spectre variant 2) mitigation on Intel processors belonging to > > family 6 (enumerated by the CPUID instruction) that do not have > > support for enhanced IBRS. On processors that support enhanced IBRS, > > it should be used for mitigation instead of retpoline. > > That's both a statement of "Intel would like you to use enhanced IBRS > over retpoline where available" and "retpoline provides less mitigation > on processors with enhanced IBRS compared to those without". > > In other words, we can _do_ performance deltas, but they won't be as > meaningful because they won't really have apples-to-apples mitigation > properties.
Fair enough, but this wants to be spelled out in the change log explicitely instead of unspecific blurbs.
Thanks,
tglx
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