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 19:57:12 +0100
From:   Andrea Arcangeli <aarcange@...hat.com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Dave Hansen <dave.hansen@...el.com>,
        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>,
        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 07:52:19PM +0100, Borislav Petkov wrote:
> So why not "IBRS always" or off? No need for the "IBRS only in the
> kernel" setting.

Because it's slower (or much slower depending on how much stuff the
microcode has to disable in the CPU to provide IBSR) and you only need
that kind of protection in kernel if you've PTI enabled already.

ibrs 1 (not 2) is the current default because of that.