Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S965081AbeALReI (ORCPT + 1 other); Fri, 12 Jan 2018 12:34:08 -0500 Received: from mail-it0-f52.google.com ([209.85.214.52]:36857 "EHLO mail-it0-f52.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S934069AbeALReG (ORCPT ); Fri, 12 Jan 2018 12:34:06 -0500 X-Google-Smtp-Source: ACJfBovJZvFL5cE8xx7P9H4ZucqNrOFCu6GmXt5fR6MQRCaa9Dn94V5OjheBPy3wVQxgQ+NXQqDsRxQXLttukp7GJew= MIME-Version: 1.0 In-Reply-To: References: <20180110233252.GA14739@amd> <20180112110624.GA13254@amd> From: Linus Torvalds Date: Fri, 12 Jan 2018 09:34:03 -0800 X-Google-Sender-Auth: W6YIdx644AbZ-HGhiGTS6-h3Umc Message-ID: Subject: Re: Linux 4.15-rc7 To: Arnd Bergmann Cc: Pavel Machek , Olivier Galibert , Linux Kernel Mailing List , jikos@suse.cz Content-Type: text/plain; charset="UTF-8" Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Return-Path: On Fri, Jan 12, 2018 at 5:23 AM, Arnd Bergmann wrote: > > However, there is an important difference between KPTI and X86_4G: > The former unmaps the kernel pages from the user space page tables, > but keeps both the linear mapping and the user pages visible in > kernel mode, while the latter must have also unmapped user space > pages from kernel mode, requiring a more expensive get_user/put_user > implementation. Indeed. And I think that the 4G:4G patches do things wrong. People are already complaining about the PTI costs. Separating user space entirely is much much worse, and makes all user accesses from kernel space too painful for words. Honestly, I didn't merge the old 4G:4G patches originally, and I'm not going to merge them this time around either. Linus