From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.4 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS,USER_AGENT_MUTT autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id BBF49C07E85 for ; Tue, 11 Dec 2018 11:53:02 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 899BB20811 for ; Tue, 11 Dec 2018 11:53:02 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 899BB20811 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=arm.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726357AbeLKLxB (ORCPT ); Tue, 11 Dec 2018 06:53:01 -0500 Received: from foss.arm.com ([217.140.101.70]:45512 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726135AbeLKLxB (ORCPT ); Tue, 11 Dec 2018 06:53:01 -0500 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.72.51.249]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 51DEFEBD; Tue, 11 Dec 2018 03:53:00 -0800 (PST) Received: from arrakis.emea.arm.com (arrakis.cambridge.arm.com [10.1.196.113]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id A50BC3F6A8; Tue, 11 Dec 2018 03:52:57 -0800 (PST) Date: Tue, 11 Dec 2018 11:52:55 +0000 From: Catalin Marinas To: Florian Weimer Cc: Arnd Bergmann , Andy Lutomirski , "H.J. Lu" , the arch/x86 maintainers , Linux Kernel Mailing List , Linux API , "H. Peter Anvin" , Peter Zijlstra , Borislav Petkov , vapier@gentoo.org, Rich Felker , x32@buildd.debian.org, Will Deacon , Linus Torvalds Subject: Re: Can we drop upstream Linux x32 support? Message-ID: <20181211115254.GC35824@arrakis.emea.arm.com> References: <20181211113230.GB35824@arrakis.emea.arm.com> <87efaoxpix.fsf@oldenburg2.str.redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <87efaoxpix.fsf@oldenburg2.str.redhat.com> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Dec 11, 2018 at 12:37:42PM +0100, Florian Weimer wrote: > * Catalin Marinas: > > On Tue, Dec 11, 2018 at 10:02:45AM +0100, Arnd Bergmann wrote: > >> On Tue, Dec 11, 2018 at 6:35 AM Andy Lutomirski wrote: > >> > I tried to understand what's going on. As far as I can tell, most of > >> > the magic is the fact that __kernel_long_t and __kernel_ulong_t are > >> > 64-bit as seen by x32 user code. This means that a decent number of > >> > uapi structures are the same on x32 and x86_64. Syscalls that only > >> > use structures like this should route to the x86_64 entry points. But > >> > the implementation is still highly dubious -- in_compat_syscall() will > >> > be *true* in such system calls, > >> > >> I think the fundamental issue was that the intention had always been > >> to use only the 64-bit entry points for system calls, but the most > >> complex one we have -- ioctl() -- has to use the compat entry point > >> because device drivers define their own data structures using 'long' > >> and pointer members and they need translation, as well as > >> matching in_compat_syscall() checks. This in turn breaks down > >> again whenever a driver defines an ioctl command that takes > >> a __kernel_long_t or a derived type like timespec as its argument. > > > > With arm64 ILP32 we tried to avoid the ioctl() problem by having > > __kernel_long_t 32-bit, IOW mimicking the arm32 ABI (compat). The > > biggest pain point is signals where the state is completely different > > from arm32 (more, wider registers) and can't be dealt with by the compat > > layer. > > I would expect to approach this from the opposite direction: use 64-bit > types in places where the 64-bit kernel interface uses 64-bit types. > After all, not everyone who is interested in ILP32 has a companion > 32-bit architecture which could serve as a model for the application > ABI. I fully agree with you that if someone wants ILP32 for a 64-bit only architecture, they should use the 64-bit kernel interface and ensure POSIX is adjusted. In the arm64 context, both options were discussed with the libc community complaining that a partial 64-bit syscall ABI breaks POSIX while the potential users were just asking for a 32-bit ABI to run their existing software stack on ARMv8 machines without native 32-bit support (until they complete the migration to 64-bit). > (If there are conflicts with POSIX, then POSIX needs to be fixed to > support this.) This would have been nice but no-one volunteered and, more importantly, there was no conclusive argument that ARM ILP32 is better than LP64 (well, apart from a minority of benchmarks) and something that people would want to migrate to. Given that the only credible case made was about legacy code, we decided to go ahead with a (mostly) compat 32-bit ABI. -- Catalin