linux-kernel - Re: [patch RFC 5/5] x86/speculation: Add basic speculation control code

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Message-ID: <20180110100344.GZ29822@worktop.programming.kicks-ass.net>
Date:   Wed, 10 Jan 2018 11:03:44 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     David Woodhouse <dwmw2@...radead.org>
Cc:     Dave Hansen <dave.hansen@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...uxfoundation.org>, x86@...nel.org,
        Borislav Petkov <bp@...en8.de>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        Arjan Van De Ven <arjan.van.de.ven@...el.com>
Subject: Re: [patch RFC 5/5] x86/speculation: Add basic speculation control
 code

On Wed, Jan 10, 2018 at 09:27:59AM +0000, David Woodhouse wrote:
> > The only question I have is if retpoline works at all on SKL (with ucode
> > update); BDW needs the ucode update for retpoline to work because of the
> > RSB fallback.
> 
> As I understand it, Skylake is never getting the IBRS_ATT (all the
> time) feature. That is for future CPUs only.

Ah, so then I got confused by your earlier email. So retpoline will work
on SKL, but it will, like BDW need the ucode update?

> I don't know why you're calling that 'IBRS=2'; are you getting confused
> by Andrea's distro horridness?

Yes, continuation of the nomenclature of that earlier thread.

Still, point remains, do we ever need that MSR fiddling dynamic IBRS
stuff now that we have retpolines? I would prefer to not have it if
there is no compelling reason for it.