From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751569AbdKYTSF (ORCPT ); Sat, 25 Nov 2017 14:18:05 -0500 Received: from Galois.linutronix.de ([146.0.238.70]:50286 "EHLO Galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750775AbdKYTSE (ORCPT ); Sat, 25 Nov 2017 14:18:04 -0500 Date: Sat, 25 Nov 2017 20:18:02 +0100 (CET) From: Thomas Gleixner To: Ingo Molnar cc: linux-kernel@vger.kernel.org, Dave Hansen , Andy Lutomirski , "H . Peter Anvin" , Peter Zijlstra , Borislav Petkov , Linus Torvalds Subject: Re: [PATCH 42/43] x86/mm/kaiser: Allow KAISER to be enabled/disabled at runtime In-Reply-To: <20171124172411.19476-43-mingo@kernel.org> Message-ID: References: <20171124172411.19476-1-mingo@kernel.org> <20171124172411.19476-43-mingo@kernel.org> User-Agent: Alpine 2.20 (DEB 67 2015-01-07) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII X-Linutronix-Spam-Score: -1.0 X-Linutronix-Spam-Level: - X-Linutronix-Spam-Status: No , -1.0 points, 5.0 required, ALL_TRUSTED=-1,SHORTCIRCUIT=-0.0001 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, 24 Nov 2017, Ingo Molnar wrote: > From: Dave Hansen > > 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