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Date:   Sat, 25 Nov 2017 20:18:02 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Ingo Molnar <mingo@...nel.org>
cc:     linux-kernel@...r.kernel.org,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...capital.net>,
        "H . Peter Anvin" <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Borislav Petkov <bp@...en8.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 42/43] x86/mm/kaiser: Allow KAISER to be enabled/disabled
 at runtime

On Fri, 24 Nov 2017, Ingo Molnar wrote:

> From: Dave Hansen <dave.hansen@...ux.intel.com>
> 
> The KAISER CR3 switches are expensive for many reasons.  Not all systems
> benefit from the protection provided by KAISER.  Some of them can not
> pay the high performance cost.
> 
> This patch adds a debugfs file.  To disable KAISER, you do:
> 
> 	echo 0 > /sys/kernel/debug/x86/kaiser-enabled
> 
> and to re-enable it, you can:
> 
> 	echo 1 > /sys/kernel/debug/x86/kaiser-enabled
> 
> This is a *minimal* implementation.  There are certainly plenty of
> optimizations that can be done on top of this by using ALTERNATIVES
> among other things.

It's not only minimal. It's naive and broken. That thing explodes when
toggled in the wrong moment. I did not even attempt to debug that, because
I think the approach is wrong.

If you really want to make it runtime switchable, then:

 - the shadow tables need to be updated unconditionally. I did not check
   whether thats done right now, but explosions are simpler to achieve when
   switching it back on. Though switching it off crashes as well.

 - you need to make sure that no task is in user space or on the way to it.
   The much I hate stop_machine(), that's probably the right tool.
   Once everything is in stomp_machine() the switch can be flipped.

 - the poisoning/unpoisoning of the kernel tables does not need to be done
   from stop_machine(). That can be done from regular context with a TIF
   flag, so you can make sure that every task is up to date before
   returning to user space. Though that needs a lot of thought.

For now I really want to see that removed entirely and replaced by a simple
boot time switch. We can use the global variable for now and optimize it
later on.

Thanks,

	tglx

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