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

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Date:   Wed, 10 Jan 2018 13:47:22 +0100 (CET)
From:   Jiri Kosina <jikos@...nel.org>
To:     Andrea Arcangeli <aarcange@...hat.com>
cc:     David Woodhouse <dwmw2@...radead.org>,
        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, 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.

-- 
Jiri Kosina
SUSE Labs