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 13:47:07 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Tim Chen <tim.c.chen@...ux.intel.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 Fri, Jan 05, 2018 at 07:12:12PM -0800, Dave Hansen wrote:
> On 01/05/2018 06:12 PM, Tim Chen wrote:
> >  .macro ENABLE_IBRS
> > -	ALTERNATIVE "jmp .Lskip_\@", "", X86_FEATURE_SPEC_CTRL
> > +	testl	$1, dynamic_ibrs
> > +	jz	.Lskip_\@
> 
> There was an earlier suggestion to use STATIC_JUMP_IF_... here.  That's

Yes, and thanks for Cc'ing me... :/

> a good suggestion, but we encountered some issues with it either
> crashing the kernel at boot or not properly turning on/off.
> 
> We would love to do that minor really soon, but we figured everyone
> would rather see this version than wait for us to debug such a minor tweak.

Its not minor, you need that LFENCE without it. Which makes the whole
thing unconditionally expensive, which sucks.