From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757168AbbFQOUK (ORCPT ); Wed, 17 Jun 2015 10:20:10 -0400 Received: from mail-lb0-f171.google.com ([209.85.217.171]:35738 "EHLO mail-lb0-f171.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757015AbbFQOTq (ORCPT ); Wed, 17 Jun 2015 10:19:46 -0400 MIME-Version: 1.0 In-Reply-To: References: <20150617094857.GB3940@gmail.com> From: Andy Lutomirski Date: Wed, 17 Jun 2015 07:19:24 -0700 Message-ID: Subject: Re: [RFC/INCOMPLETE 00/13] x86: Rewrite exit-to-userspace code To: Richard Weinberger Cc: Ingo Molnar , 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 3:13 AM, Richard Weinberger wrote: > 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. > The do_signal declaration in arch/um/include/shared/kern_util.h conflicts with the one I added to arch/x86/include/asm/signal.h. The latter shouldn't really be included in UML, I think, but I didn't see how to fix it. Just renaming one of the functions would resolve the conflict. --Andy