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

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Date:   Wed, 10 Jan 2018 11:58:54 +0000
From:   David Woodhouse <dwmw2@...radead.org>
To:     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 12:54 +0100, Andrea Arcangeli wrote:
> On Wed, Jan 10, 2018 at 09:27:59AM +0000, David Woodhouse wrote:
> > I don't know why you're calling that 'IBRS=2'; are you getting
> confused
> > by Andrea's distro horridness?
> 
> Eh, yes he's got confused. ibrs_enabled 2 simply means to leave IBRS
> set in SPEC_CTLR 100% of the time, except in guest mode.

On all current hardware, if you only set IBRS when you exit a guest,
then you are not protecting yourself from userspace at all. IBRS acts
as a *barrier* in all current hardware.

Future CPUs will have a new feature where you *can* do something like
this, but this is not available yet.
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