From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753311AbbFQKNg (ORCPT ); Wed, 17 Jun 2015 06:13:36 -0400 Received: from mail-yk0-f181.google.com ([209.85.160.181]:34464 "EHLO mail-yk0-f181.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751146AbbFQKN2 (ORCPT ); Wed, 17 Jun 2015 06:13:28 -0400 MIME-Version: 1.0 In-Reply-To: <20150617094857.GB3940@gmail.com> References: <20150617094857.GB3940@gmail.com> Date: Wed, 17 Jun 2015 12:13:27 +0200 Message-ID: Subject: Re: [RFC/INCOMPLETE 00/13] x86: Rewrite exit-to-userspace code From: Richard Weinberger To: Ingo Molnar Cc: Andy Lutomirski , "x86@kernel.org" , LKML , =?UTF-8?B?RnLDqWTDqXJpYyBXZWlzYmVja2Vy?= , Rik van Riel , Oleg Nesterov , Denys Vlasenko , Borislav Petkov , Kees Cook , Brian Gerst , Linus Torvalds , Denys Vlasenko Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Jun 17, 2015 at 11:48 AM, Ingo Molnar wrote: > > * Andy Lutomirski wrote: > >> This is incomplete, but it's finally good enough that I think it's >> time to get other opinions on it. It is a complete rewrite of the >> slow path code that handles exits to user mode. > > Modulo the small comments I made about the debug checks interface plus naming > details the structure and intention of this series gives me warm fuzzy feelings. > >> The exit-to-usermode code is copied in several places and is written in a nasty >> combination of asm and C. It's not at all clear what it's supposed to do, and >> the way it's structured makes it very hard to work with. For example, it's not >> even clear why syscall exit hooks are called only once per syscall right now. >> (It seems to be a side effect of the way that rdi and rdx are handled in the asm >> loop, and it seems reliable, but it's still pointlessly complicated.) The >> existing code also makes context tracking overly complicated and hard to >> understand. Finally, it's nearly impossible for anyone to change what happens >> on exit to usermode, since the existing code is so fragile. > > Amen. > >> I tried to clean it up incrementally, but I decided it was too hard. Instead, >> this series just replaces the code. It seems to work. > > Any known bugs beyond UML build breakage? > >> Context tracking in particular works very differently now. The low-level entry >> code checks that we're in CONTEXT_USER and switches to CONTEXT_KERNEL. The exit >> code does the reverse. There is no need to track what CONTEXT_XYZ state we came >> from, because we already know. Similarly, SCHEDULE_USER is gone, since we can >> reschedule if needed by simply calling schedule() from C code. >> >> The main things that are missing are that I haven't done the 32-bit parts >> (anyone want to help?) and therefore I haven't deleted the old C code. I also >> think this may break UML for trivial reasons. >> >> Because I haven't converted the 32-bit code yet, all of the now-unnecessary >> unnecessary calls to exception_enter are still present in traps.c. >> >> IRQ context tracking is still duplicated. We should probably clean it up by >> changing the core code to supply something like >> irq_enter_we_are_already_in_context_kernel. >> >> Thoughts? > > So assuming you fix the UML build I'm inclined to go for it, even in this > incomplete form, to increase testing coverage. Andy, can you please share the build breakage you're facing? I'll happily help you fixing it. -- Thanks, //richard