From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751591AbdKYTyC (ORCPT ); Sat, 25 Nov 2017 14:54:02 -0500 Received: from mail-oi0-f67.google.com ([209.85.218.67]:39068 "EHLO mail-oi0-f67.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750819AbdKYTyA (ORCPT ); Sat, 25 Nov 2017 14:54:00 -0500 X-Google-Smtp-Source: AGs4zMbwexFPy24Jtn75xfuVwwnvjGE2z4eB/ieeayIUxXEVFrMsfw3O9A5FM1uW/GTuVAs/Is087g== Content-Type: text/plain; charset=us-ascii Mime-Version: 1.0 (1.0) Subject: Re: [PATCH 42/43] x86/mm/kaiser: Allow KAISER to be enabled/disabled at runtime From: Andy Lutomirski X-Mailer: iPhone Mail (15B150) In-Reply-To: Date: Sat, 25 Nov 2017 12:53:55 -0700 Cc: Ingo Molnar , linux-kernel@vger.kernel.org, Dave Hansen , "H . Peter Anvin" , Peter Zijlstra , Borislav Petkov , Linus Torvalds Message-Id: <47186975-1DCB-4767-8C4C-0816931E3595@amacapital.net> References: <20171124172411.19476-1-mingo@kernel.org> <20171124172411.19476-43-mingo@kernel.org> To: Thomas Gleixner Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Transfer-Encoding: 8bit X-MIME-Autoconverted: from quoted-printable to 8bit by nfs id vAPJs7TU031260 > 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. Keep in mind that, for a static_branch, actually setting the thing needs to be deferred, but that's straightforward.