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

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Date:   Wed, 10 Jan 2018 12:51:13 +0000
From:   David Woodhouse <dwmw2@...radead.org>
To:     Jiri Kosina <jikos@...nel.org>,
        Andrea Arcangeli <aarcange@...hat.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        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>,
        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, 2018-01-10 at 13:47 +0100, Jiri Kosina wrote:
> On Wed, 10 Jan 2018, Andrea Arcangeli wrote:
> 
> > Perhaps the confusing come from "less privileged prediction mode" and
> > you thought that meant "less privileged ring mode". It says "predction
> > mode" not ring 3.
> 
> Well, prediction mode is defined by "CPL3 vs CPL0-2" and "VMX root vs VMX 
> non-root", with obvious ordering of privileges.
> 
> So if IBRS is set, branch predictor will not allow the predicted target to 
> be influenced by code that executed in less privileged prediction mode 
> before value of '1' IBRS mode was last written to, and that's pretty much 
> it.

The operative words in that sentence being, "before the IBRS mode was
last written with a value of 1".

If it worked as Andrea suggests, then there would be absolutely no
point in the patches we've seen which add the IBRS-frobbing on syscall
entry and vmexit.

The "IBRS all the time" feature is something we get on *future*
hardware, not current hardware.
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