Date: Wed, 10 Jan 2018 14:46:22 +0100 (CET)
From: Thomas Gleixner <tglx@linutronix.de>
To: David Woodhouse <dwmw2@infradead.org>
cc: Andrea Arcangeli <aarcange@redhat.com>,
        Jiri Kosina <jikos@kernel.org>,
        "asit.k.mallick" <asit.k.mallick@intel.com>,
        "Van De Ven, Arjan" <arjan.van.de.ven@intel.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Dave Hansen <dave.hansen@intel.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Linus Torvalds <torvalds@linuxfoundation.org>, x86@kernel.org,
        Borislav Petkov <bp@alien8.de>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Andi Kleen <ak@linux.intel.com>,
        Greg KH <gregkh@linuxfoundation.org>,
        Andy Lutomirski <luto@kernel.org>
Subject: Re: [patch RFC 5/5] x86/speculation: Add basic speculation control
 code
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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