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

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Date:   Sat, 6 Jan 2018 20:47:19 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Dave Hansen <dave.hansen@...el.com>
cc:     "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, 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.

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