Re: [PATCH v2 4/8] x86/spec_ctrl: Add sysctl knobs to enable/disable SPEC_CTRL feature

[Dave Hansen]

On 01/06/2018 09:41 AM, Van De Ven, Arjan wrote:
>>>>  .macro DISABLE_IBRS
>>>> -	ALTERNATIVE "jmp .Lskip_\@", "", X86_FEATURE_SPEC_CTRL
>>>> +	testl	$1, dynamic_ibrs
>>> On every system call we end up hammering on this 'dynamic_ibrs'
>>> variable. And it looks like it can be flipped via the IPI mechanism.
>>>
>>> Would it make sense for this to be per-cpu?
>>
>> It's probably better to either just make it __read_mostly or get the
>> static branches that folks were suggesting actually working.
> 
> I still wonder if this isn't just better as a boot command line

It's simpler that way.  But, ideally, we want to make it runtime
switchable to match the implementation in the distros.