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

Peter Zijlstra
2018-01-04T22:54:33Z

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.