linux-kernel - Re: [PATCH 00/23] KAISER: unmap most of the kernel from userspace page tables

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Date:   Wed, 1 Nov 2017 15:28:48 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Dave Hansen <dave.hansen@...ux.intel.com>
Cc:     Ingo Molnar <mingo@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>, Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>,
        "H. Peter Anvin" <hpa@...or.com>,
        borisBrian Gerst <brgerst@...il.com>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Thomas Garnier <thgarnie@...gle.com>,
        Kees Cook <keescook@...gle.com>
Subject: Re: [PATCH 00/23] KAISER: unmap most of the kernel from userspace
 page tables

On Wed, Nov 1, 2017 at 3:14 PM, Dave Hansen <dave.hansen@...ux.intel.com> wrote:
>
> I ran some quick tests.  When CONFIG_KAISER=y, but "echo 0 >
> kaiser-enabled", the tests that I ran were within the noise vs. a
> vanilla kernel, and that's with *zero* optimization.

I guess the optimal version just ends up switching between two
different entrypoints for the on/off case.

And the not-quite-as-aggressive, but almost-optimal version would just
be a two-byte asm alternative with an unconditional branch to the
movcr3 code and back, and is turned into a noop when it's off.

But since 99%+ of the cost is going to be that cr3 write, even the
stupid "just load value and branch over the cr3 conditionally" is
going to make things hard to measure.

                Linus