Re: [PATCH 6/7] x86/spec_ctrl: Add sysctl knobs to enable/disable SPEC_CTRL feature

[Peter Zijlstra]

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.