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 19:52:23 +0000
From:   "Lu, Hongjiu" <hongjiu.lu@...el.com>
To:     "Woodhouse, David" <dwmw@...zon.co.uk>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "tim.c.chen@...ux.intel.com" <tim.c.chen@...ux.intel.com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Van De Ven, Arjan" <arjan.van.de.ven@...el.com>,
        "torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "ak@...ux.intel.com" <ak@...ux.intel.com>,
        "aarcange@...hat.com" <aarcange@...hat.com>,
        "luto@...nel.org" <luto@...nel.org>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        "Hansen, Dave" <dave.hansen@...el.com>
Subject: RE: [PATCH v2 4/8] x86/spec_ctrl: Add sysctl knobs to
 enable/disable SPEC_CTRL feature

> 
> On Mon, 2018-01-08 at 18:42 +0100, 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.
> 
> I think HJ said he was planning to backport as far as 4.9. When was asm-
> goto added?

It was added to GCC 4.5.

H.J.