linux-kernel - Re: [PATCH] x86/speculation: Support Enhanced IBRS on future CPUs

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Date:   Mon, 30 Jul 2018 18:58:16 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Dave Hansen <dave.hansen@...el.com>
cc:     Sai Praneeth Prakhya <sai.praneeth.prakhya@...el.com>,
        x86@...nel.org, linux-kernel@...r.kernel.org,
        Tim C Chen <tim.c.chen@...el.com>,
        Ravi Shankar <ravi.v.shankar@...el.com>,
        Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH] x86/speculation: Support Enhanced IBRS on future CPUs

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@...el.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