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

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Date:   Tue, 9 Jan 2018 11:40:44 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Peter Zijlstra <peterz@...radead.org>
cc:     Tim Chen <tim.c.chen@...ux.intel.com>,
        Dave Hansen <dave.hansen@...el.com>,
        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, 8 Jan 2018, Peter Zijlstra wrote:
> 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.

No, ifs and buts, really.

This wants to be a jump label and we set the requirements here. I'm tired
of this completely bogus crap just to support some archaic version of GCC.

This is messy enough and no, this whole we need a control variable nonsense
is just not going to happen.

Thanks,

	tglx