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Date:   Mon, 30 Jul 2018 19:11:05 +0200 (CEST)
From:   Thomas Gleixner <tglx@linutronix.de>
To:     "Prakhya, Sai Praneeth" <sai.praneeth.prakhya@intel.com>
cc:     "Hansen, Dave" <dave.hansen@intel.com>,
        "x86@kernel.org" <x86@kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "Chen, Tim C" <tim.c.chen@intel.com>,
        "Shankar, Ravi V" <ravi.v.shankar@intel.com>,
        Ingo Molnar <mingo@kernel.org>
Subject: RE: [PATCH] x86/speculation: Support Enhanced IBRS on future CPUs
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On Mon, 30 Jul 2018, Prakhya, Sai Praneeth 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.
> 
> Sure! Makes sense. Would you like me to send a V2 with updated change log?

Of course.