Date: Thu, 4 Jan 2018 23:54:22 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Tim Chen <tim.c.chen@linux.intel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>,
        Andy Lutomirski <luto@kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Greg KH <gregkh@linuxfoundation.org>,
        Dave Hansen <dave.hansen@intel.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Andi Kleen <ak@linux.intel.com>,
        Arjan Van De Ven <arjan.van.de.ven@intel.com>,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH 6/7] x86/spec_ctrl: Add sysctl knobs to enable/disable
 SPEC_CTRL feature
Message-ID: <20180104225422.GG32035@hirez.programming.kicks-ass.net>
References: <cover.1515086770.git.tim.c.chen@linux.intel.com>
 <4d4b3752e8e533201c6983d8473eea95c747ea33.1515086770.git.tim.c.chen@linux.intel.com>
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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.