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

[Dave Hansen]

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.