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

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Message-ID: <d7413d40-7581-a994-5185-9f11c014cb0f@intel.com>
Date:   Fri, 5 Jan 2018 19:12:12 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     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>
Cc:     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 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
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.