linux-kernel - Re: [PATCH 6/7] x86/spec_ctrl: Add sysctl knobs to enable/disable SPEC

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Date:   Thu, 4 Jan 2018 23:54:22 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Tim Chen <tim.c.chen@...ux.intel.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Andy Lutomirski <luto@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Arjan Van De Ven <arjan.van.de.ven@...el.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 6/7] x86/spec_ctrl: Add sysctl knobs to enable/disable
 SPEC_CTRL feature

On Thu, Jan 04, 2018 at 09:56:47AM -0800, Tim Chen wrote:
>  .macro ENABLE_IBRS
> -	ALTERNATIVE "jmp 10f", "", X86_FEATURE_SPEC_CTRL
> +	testl $SPEC_CTRL_IBRS_INUSE, spec_ctrl_ibrs
> +	jz .Lskip_\@
> +
>  	PUSH_MSR_REGS
>  	WRMSR_ASM $MSR_IA32_SPEC_CTRL, $SPEC_CTRL_FEATURE_ENABLE_IBRS
>  	POP_MSR_REGS
> -10:
> +
> +	jmp .Ldone_\@
> +.Lskip_\@:
> +	/*
> +	 * prevent speculation beyond here as we could want to
> +	 * stop speculation by enabling IBRS
> +	 */
> +	lfence
> +.Ldone_\@:
>  .endm


Yeah no. We have jump labels for this stuff. There is no reason what so
ever to do dynamic tests for a variable that _never_ changes.