linux-kernel - Re: [PATCH 6/7] x86/spec_ctrl: Add sysctl knobs to enable/disable SPEC

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Date:   Thu, 4 Jan 2018 20:06:45 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Tim Chen <tim.c.chen@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        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>,
        Arjan Van De Ven <arjan.van.de.ven@...el.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 6/7] x86/spec_ctrl: Add sysctl knobs to enable/disable
 SPEC_CTRL feature

On Thu, Jan 04, 2018 at 10:59:35AM -0800, Dave Hansen wrote:
> On 01/04/2018 10:52 AM, Borislav Petkov wrote:
> >> Paranoid people want "IBRS always" aka "ibrs 2".
> > 
> > So why not "IBRS always" or off? No need for the "IBRS only in the
> > kernel" setting.
> 
> IBRS=1 slows execution down.  If it's on all the time, you pay a
> performance cost in userspace.  The assumption is that the user/kernel
> boundary switching cost is below the cost of having it on all the time.

Ok, so we need to make this distinction visible to users so that they
are aware.

Also, adding a common, combined option which is called something like

make_my_kernel_supa_dupa_secure

or so would make sense too, for all those paranoid people. Because I see
the mess coming:

ibrs= ibpb= pti= ...

A helluva confusion that would be.

Thx.

-- 
Regards/Gruss,
    Boris.

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