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From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Wed, 1 Nov 2017 15:28:48 -0700
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Subject: Re: [PATCH 00/23] KAISER: unmap most of the kernel from userspace
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To: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Ingo Molnar <mingo@kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-mm <linux-mm@kvack.org>, Andy Lutomirski <luto@kernel.org>,
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        Peter Zijlstra <a.p.zijlstra@chello.nl>,
        "H. Peter Anvin" <hpa@zytor.com>, borisBrian Gerst <brgerst@gmail.com>,
        Denys Vlasenko <dvlasenk@redhat.com>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
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On Wed, Nov 1, 2017 at 3:14 PM, Dave Hansen <dave.hansen@linux.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