linux-kernel - RE: [PATCH v2 4/8] x86/spec_ctrl: Add sysctl knobs to enable/disable SPEC

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Date:   Sat, 6 Jan 2018 21:44:43 +0000
From:   "Van De Ven, Arjan" <arjan.van.de.ven@...el.com>
To:     Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
CC:     Thomas Gleixner <tglx@...utronix.de>,
        "Hansen, Dave" <dave.hansen@...el.com>,
        Konrad Rzeszutek Wilk <konrad@...nel.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        "Linus Torvalds" <torvalds@...ux-foundation.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        "Andi Kleen" <ak@...ux.intel.com>,
        David Woodhouse <dwmw@...zon.co.uk>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH v2 4/8] x86/spec_ctrl: Add sysctl knobs to
 enable/disable SPEC_CTRL feature


> > it's a mistake though... retpoline is what people should be using ....
> > ... and only in super exception cases should IBRS even be considered
> 
> Like on certain Skylake and Broadwell where using the retpoline won't suffice?

skylake is a bit more complex but that was discussed in earlier emails where a few more things are needed. No idea what you're inventing on broadwell where you would use ibrs but not retpoline


>  - The CPU can do retpoline, but MSRS are faster (is that even the case?)

that is never the case