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

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Message-ID: <1610a587-fe9e-28bd-dcd1-b9ec940c07ef@intel.com>
Date:   Wed, 10 Jan 2018 06:59:54 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Andrea Arcangeli <aarcange@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>
Cc:     David Woodhouse <dwmw2@...radead.org>,
        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>,
        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 01/10/2018 06:10 AM, Andrea Arcangeli wrote:
> Tim and Dave please comment too, Tim you originally wrote that code
> that leaves IBRS always on and never toggles it in the kernel entry
> point so you must know full well if Arjan is correct that you must
> toggle IBRS every time you enter kernel and in turn ibrs_enabled 2
> isn't valid mode.

Hi Andrea,

The "writing IBRS=1 acts as a barrier when it is already IBRS=1"
behavior is something which I misunderstood in the past.  Thanks, Arjan,
for clearing it up.