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

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Date:   Wed, 10 Jan 2018 14:46:22 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     David Woodhouse <dwmw2@...radead.org>
cc:     Andrea Arcangeli <aarcange@...hat.com>,
        Jiri Kosina <jikos@...nel.org>,
        "asit.k.mallick" <asit.k.mallick@...el.com>,
        "Van De Ven, Arjan" <arjan.van.de.ven@...el.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Dave Hansen <dave.hansen@...el.com>,
        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>,
        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, David Woodhouse wrote:
> Andrea, what you're saying is directly contradicting what I've heard
> from Intel.
> 
> The documentation already distinguishes between IBRS on current
> hardware, and IBRS_ATT on future hardware. If it was the case that IBRS
> on current hardware is a set-and-forget option and completely disables
> branch prediction, then they would say that. Rather than explicitly
> saying the *opposite*, specifically for the case of current hardware,
> as they do.
> 
> Rather than continuing to debate it, perhaps it's best just to wake for
> the US to wake up, and Intel to give a definitive answer.

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.

1) Does IBRS=1 when set once act as a set-and-forget option ?

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

 1b) If the answer to #1 is yes, is retpoline required ?

 1c) If the answer to #1 is yes, is RSB stuffing required ?

2) Does toggle mode of IBRS require retpoline ?

3) Does toggle mode of IBRS require RSB stuffing ?

4) Exist CPUs which require IBRS to be selected automatically ?

   4b) If yes, provide the list as a separate answer please

Thanks,

	tglx