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Subject: Re: [PATCH 6/7] x86/spec_ctrl: Add sysctl knobs to enable/disable
 SPEC_CTRL feature
To: Borislav Petkov <bp@alien8.de>,
        Tim Chen <tim.c.chen@linux.intel.com>
References: <cover.1515086770.git.tim.c.chen@linux.intel.com>
 <4d4b3752e8e533201c6983d8473eea95c747ea33.1515086770.git.tim.c.chen@linux.intel.com>
 <20180104183345.od2o4hsfu2tv6nc4@pd.tnic>
Cc: Thomas Gleixner <tglx@linutronix.de>,
        Andy Lutomirski <luto@kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Greg KH <gregkh@linuxfoundation.org>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Andi Kleen <ak@linux.intel.com>,
        Arjan Van De Ven <arjan.van.de.ven@intel.com>,
        linux-kernel@vger.kernel.org
From: Dave Hansen <dave.hansen@intel.com>
Message-ID: <f66779eb-cba2-ea03-e84f-91d3512709ed@intel.com>
Date: Thu, 4 Jan 2018 10:36:30 -0800
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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.