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

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Message-ID: <alpine.DEB.2.20.1801101452100.1919@nanos>
Date:   Wed, 10 Jan 2018 14:53:28 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     "Van De Ven, Arjan" <arjan.van.de.ven@...el.com>
cc:     David Woodhouse <dwmw2@...radead.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Jiri Kosina <jikos@...nel.org>,
        "Mallick, Asit K" <asit.k.mallick@...el.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "Hansen, Dave" <dave.hansen@...el.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...uxfoundation.org>,
        "x86@...nel.org" <x86@...nel.org>, Borislav Petkov <bp@...en8.de>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        Andy Lutomirski <luto@...nel.org>
Subject: RE: [patch RFC 5/5] x86/speculation: Add basic speculation control
 code

On Wed, 10 Jan 2018, Van De Ven, Arjan wrote:
> > So here is the simple list of questions all to be answered with YES or
> > NO. I don't want to see any of the 'but, though ...'. We all know by now
> > that it's CPU dependent and slow and whatever and that IBRS_ATT will be in
> > future CPUs. So get your act together and tell a clear YES or NO.

> > 2) Does toggle mode of IBRS require retpoline ?
> 
> NO
> 
> > 
> > 3) Does toggle mode of IBRS require RSB stuffing ?
> 
> Only for the VM exit case

Ok.

> > 
> > 4) Exist CPUs which require IBRS to be selected automatically ?
> 
> I do not understand your question exactly

Whether the kernel should enable IBRS by default on certain models. And if
yes, which ones.

Thanks,

	tglx