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

[Konrad Rzeszutek Wilk]

On Sat, Jan 06, 2018 at 08:47:19PM +0100, Thomas Gleixner wrote:
> On Sat, 6 Jan 2018, Dave Hansen wrote:
> 
> > 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.
> 
> Stop this silly argument please. The distros shipped lots of crap which we
> dont want to have at all.
> 
> I told you folks yesterday what I want to see and the sysctl thing is the
> least on that list and it's not needed for getting the important thing -
> the protection - to work.

I agree. But this is what customers are told to inspect to see if they
are impacted. And if in the future versions this goes away or such - they
will freak out and cause needless escalations.

> 
> Can we pretty please do the basics and worry about that sysctl or whatever
> people have on their wishlist once the dust settled.
> 
> Thanks,
> 
> 	tglx
>