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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>
References: <cover.1515086770.git.tim.c.chen@linux.intel.com>
 <4d4b3752e8e533201c6983d8473eea95c747ea33.1515086770.git.tim.c.chen@linux.intel.com>
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 <20180104185219.wji527snw6xzggak@pd.tnic>
Cc: Tim Chen <tim.c.chen@linux.intel.com>,
        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: <e83ce989-5f4d-b35a-aeed-34b8eb949bea@intel.com>
Date: Thu, 4 Jan 2018 10:59:35 -0800
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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.