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

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Message-ID: <FFF73D592F13FD46B8700F0A279B802F475225F1@ORSMSX114.amr.corp.intel.com>
Date:   Mon, 30 Jul 2018 17:07:17 +0000
From:   "Prakhya, Sai Praneeth" <sai.praneeth.prakhya@...el.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        "Hansen, Dave" <dave.hansen@...el.com>
CC:     "x86@...nel.org" <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Chen, Tim C" <tim.c.chen@...el.com>,
        "Shankar, Ravi V" <ravi.v.shankar@...el.com>,
        Ingo Molnar <mingo@...nel.org>
Subject: RE: [PATCH] x86/speculation: Support Enhanced IBRS on future CPUs

> > >> 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.

Sure! Makes sense. Would you like me to send a V2 with updated change log?

Regards,
Sai