linux-kernel - Re: [PATCH v2 4/8] x86/spec_ctrl: Add sysctl knobs to enable/disable SPEC

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Message-ID: <20180106213227.GJ19213@char.us.oracle.com>
Date:   Sat, 6 Jan 2018 16:32:27 -0500
From:   Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Dave Hansen <dave.hansen@...el.com>,
        "Van De Ven, Arjan" <arjan.van.de.ven@...el.com>,
        Konrad Rzeszutek Wilk <konrad@...nel.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andi Kleen <ak@...ux.intel.com>,
        David Woodhouse <dwmw@...zon.co.uk>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 4/8] x86/spec_ctrl: Add sysctl knobs to enable/disable
 SPEC_CTRL feature

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
>