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Date: Thu, 2 Nov 2017 09:03:45 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Ingo Molnar <mingo@kernel.org>, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, Andy Lutomirski <luto@kernel.org>,
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        Josh Poimboeuf <jpoimboe@redhat.com>,
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Subject: Re: [PATCH 00/23] KAISER: unmap most of the kernel from userspace
 page tables
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On Wed, Nov 01, 2017 at 03:14:11PM -0700, Dave Hansen wrote:
> On 11/01/2017 01:54 AM, Ingo Molnar wrote:
> > Beyond the inevitable cavalcade of (solvable) problems that will pop up during 
> > review, one major item I'd like to see addressed is runtime configurability: it 
> > should be possible to switch between a CR3-flushing and a regular syscall and page 
> > table model on the admin level, without restarting the kernel and apps. Distros 
> > really, really don't want to double the number of kernel variants they have.
> > 
> > The 'Kaiser off' runtime switch doesn't have to be as efficient as 
> > CONFIG_KAISER=n, at least initialloy, but at minimum it should avoid the most 
> > expensive page table switching paths in the syscall entry codepaths.
> 
> Due to popular demand, I went and implemented this today.  It's not the
> prettiest code I ever wrote, but it's pretty small.
> 
> Just in case anyone wants to play with it, I threw a snapshot of it up here:
> 
> > https://git.kernel.org/pub/scm/linux/kernel/git/daveh/x86-kaiser.git/log/?h=kaiser-dynamic-414rc6-20171101
> 
> 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 resent you don't think the NMI is performance critical ;-)