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 10:36:30 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Borislav Petkov <bp@...en8.de>,
        Tim Chen <tim.c.chen@...ux.intel.com>
Cc:     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 01/04/2018 10:33 AM, Borislav Petkov wrote:
>> 2. At run time
>>     echo 0 > /sys/kernel/debug/ibrs_enabled will turn off IBRS
>>     echo 1 > /sys/kernel/debug/ibrs_enabled will turn on IBRS in kernel
>>     echo 2 > /sys/kernel/debug/ibrs_enabled will turn on IBRS in both userspace and kernel
> I am not sure that tristate is really needed. What's wrong with on/off
> only?

Lots of things:

Distros have the tri-state already deployed.
Paranoid people want "IBRS always" aka "ibrs 2".
Future CPUs may have cheap-enough IBRS to not be worth switching it.