linux-kernel - Re: [patch RFC 5/5] x86/speculation: Add basic speculation control code

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Date:   Wed, 10 Jan 2018 14:12:07 +0100
From:   Andrea Arcangeli <aarcange@...hat.com>
To:     David Woodhouse <dwmw2@...radead.org>
Cc:     Jiri Kosina <jikos@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...uxfoundation.org>, x86@...nel.org,
        Borislav Petkov <bp@...en8.de>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        Arjan Van De Ven <arjan.van.de.ven@...el.com>
Subject: Re: [patch RFC 5/5] x86/speculation: Add basic speculation control
 code

On Wed, Jan 10, 2018 at 02:10:10PM +0100, Andrea Arcangeli wrote:
> It's still incredibly faster to shutdown part of the CPU temporarily
> than to flush its internal state as a whole with IBPB. If it wouldn't
> be the case ibrs_enabled 0 ibpb_enabled 2 special mode would perform
> better (but that's only enabled by default if SPEC_CTRL is not
> available and only IBPB_SUPPORT is).

Yet another bit of perhaps useful info: if you run 100% kernel
intensive computation (i.e. only interrupts on idle task or a kernel
driver doing all in kernel) ibrs_enabled 0 ibpb_enabled 2 of course is
much faster than ibrs_enabled 1 ibpb_enabled 1. As long as you don't
have frequent ring change, ibrs_enabled 0 ibpb_enabled 2 is faster.