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

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Date:   Wed, 10 Jan 2018 15:24:17 +0000
From:   David Woodhouse <dwmw2@...radead.org>
To:     Andrea Arcangeli <aarcange@...hat.com>,
        Dave Hansen <dave.hansen@...el.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        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 Wed, 2018-01-10 at 16:13 +0100, Andrea Arcangeli wrote:
> 
> Can you also tell if IBRS must be written as a barrier to SPEC_CTRL in
> return to userland (kernel exit) when ibrs_enabled 2? Generally we
> wouldn't run a barrier there with ibrs_enabled 2, but absolutely
> nothing is intuitive here so I need to ask explicitly.

Since it achieves nothing¹ but to make userspace run slower, there's no
need to write it again on returning to userspace. It will perform that
function just fine without doing so.


(¹ that STIBP doesn't already do, and cheaper)
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