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

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Date:   Wed, 10 Jan 2018 13:51:31 +0000
From:   "Van De Ven, Arjan" <arjan.van.de.ven@...el.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        David Woodhouse <dwmw2@...radead.org>
CC:     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

> 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.

I will answer for current cpus, so those not having IBRS_ATT
> 
> 1) Does IBRS=1 when set once act as a set-and-forget option ?

NO

 
>  1a) If the answer to #1 is yes, is it more secure than toggling it?

NO
>
>  1b) If the answer to #1 is yes, is retpoline required ?

NO - Once you have IBRS done PROPERLY (toggled) you don't need retpoline
> 
>  1c) If the answer to #1 is yes, is RSB stuffing required ?

Only for the VM exit case

> 
> 2) Does toggle mode of IBRS require retpoline ?

NO

> 
> 3) Does toggle mode of IBRS require RSB stuffing ?

Only for the VM exit case
> 
> 4) Exist CPUs which require IBRS to be selected automatically ?

I do not understand your question exactly