linux-kernel - Re: [PATCH v2 4/8] x86/spec_ctrl: Add sysctl knobs to enable/disable SPEC

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Date:   Mon, 8 Jan 2018 18:42:24 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Tim Chen <tim.c.chen@...ux.intel.com>
Cc:     Dave Hansen <dave.hansen@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andy Lutomirski <luto@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Arjan Van De Ven <arjan.van.de.ven@...el.com>,
        David Woodhouse <dwmw@...zon.co.uk>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 4/8] x86/spec_ctrl: Add sysctl knobs to enable/disable
 SPEC_CTRL feature

On Mon, Jan 08, 2018 at 09:28:12AM -0800, Tim Chen wrote:
> On 01/08/2018 08:14 AM, Peter Zijlstra wrote:
> > On Mon, Jan 08, 2018 at 01:47:07PM +0100, Peter Zijlstra wrote:
> >>> a good suggestion, but we encountered some issues with it either
> >>> crashing the kernel at boot or not properly turning on/off.
> > 
> > The below boots, but I lack stuff to test the enabling.
> 
> Peter,
> 
> Thanks.  Will give it a spin.  One other concern is if
> JUMP_LABEL is not configured, this may not work, and
> we may still need fall back to a control variable.

Urgh yes.. I always forget about that case. Will the retpoline crud be
backported to a GCC old enough to not have asm-goto? If not, we could
make all of this simply require asm-goto.