From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751630AbdKYUF2 (ORCPT ); Sat, 25 Nov 2017 15:05:28 -0500 Received: from Galois.linutronix.de ([146.0.238.70]:50375 "EHLO Galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750819AbdKYUF1 (ORCPT ); Sat, 25 Nov 2017 15:05:27 -0500 Date: Sat, 25 Nov 2017 21:05:20 +0100 (CET) From: Thomas Gleixner To: Andy Lutomirski cc: Ingo Molnar , linux-kernel@vger.kernel.org, Dave Hansen , "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: <47186975-1DCB-4767-8C4C-0816931E3595@amacapital.net> Message-ID: References: <20171124172411.19476-1-mingo@kernel.org> <20171124172411.19476-43-mingo@kernel.org> <47186975-1DCB-4767-8C4C-0816931E3595@amacapital.net> 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 Sat, 25 Nov 2017, Andy Lutomirski wrote: > > On Nov 25, 2017, at 12:18 PM, Thomas Gleixner wrote: > >> 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. > > > > Nah, let's do it right: use either X86_FEATURE_WHATEVER or a > static_branch. We have nice asm support for both. Agreed. > Keep in mind that, for a static_branch, actually setting the thing needs > to be deferred, but that's straightforward. That's not an issue during boot. That would be an issue for a run time switch. Thanks, tglx