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=-5.2 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 autolearn=no 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 10E55C4363A for ; Thu, 22 Oct 2020 09:55:55 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id AF7FD2417D for ; Thu, 22 Oct 2020 09:55:54 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2896136AbgJVJzy (ORCPT ); Thu, 22 Oct 2020 05:55:54 -0400 Received: from foss.arm.com ([217.140.110.172]:52820 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2896132AbgJVJzy (ORCPT ); Thu, 22 Oct 2020 05:55:54 -0400 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 2D827D6E; Thu, 22 Oct 2020 02:55:53 -0700 (PDT) Received: from e123083-lin (usa-sjc-imap-foss1.foss.arm.com [10.121.207.14]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 3E2A43F66E; Thu, 22 Oct 2020 02:55:51 -0700 (PDT) Date: Thu, 22 Oct 2020 11:55:48 +0200 From: Morten Rasmussen To: Will Deacon Cc: Catalin Marinas , linux-arch@vger.kernel.org, "Peter Zijlstra (Intel)" , Marc Zyngier , Qais Yousef , surenb@google.com, James Morse , Greg Kroah-Hartman , Linus Torvalds , linux-arm-kernel@lists.infradead.org, balejs@google.com Subject: Re: [RFC PATCH v2 4/4] arm64: Export id_aar64fpr0 via sysfs Message-ID: <20201022095548.GH8004@e123083-lin> References: <20201021104611.2744565-5-qais.yousef@arm.com> <63fead90e91e08a1b173792b06995765@kernel.org> <20201021121559.GB3976@gaia> <20201021133316.GF8004@e123083-lin> <20201021140945.GD3976@gaia> <20201021144542.GB17912@willie-the-truck> <20201021151005.GF3976@gaia> <20201021153738.GB18071@willie-the-truck> <20201021161836.GG3976@gaia> <20201021171945.GE18071@willie-the-truck> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20201021171945.GE18071@willie-the-truck> User-Agent: Mutt/1.9.4 (2018-02-28) Precedence: bulk List-ID: X-Mailing-List: linux-arch@vger.kernel.org On Wed, Oct 21, 2020 at 06:19:48PM +0100, Will Deacon wrote: > On Wed, Oct 21, 2020 at 05:18:37PM +0100, Catalin Marinas wrote: > > On Wed, Oct 21, 2020 at 04:37:38PM +0100, Will Deacon wrote: > > > On Wed, Oct 21, 2020 at 04:10:06PM +0100, Catalin Marinas wrote: > > > > On Wed, Oct 21, 2020 at 03:45:43PM +0100, Will Deacon wrote: > > > > > On Wed, Oct 21, 2020 at 03:09:46PM +0100, Catalin Marinas wrote: > > > > > > Anyway, if the task placement is entirely off the table, the next thing > > > > > > is asking applications to set their own mask and kill them if they do > > > > > > the wrong thing. Here I see two possibilities for killing an app: > > > > > > > > > > > > 1. When it ends up scheduled on a non-AArch32-capable CPU > > > > > > > > > > That sounds fine to me. If we could do the exception return and take a > > > > > SIGILL, that's what we'd do, but we can't so we have to catch it before. > > > > > > > > Indeed, the illegal ERET doesn't work for this scenario. > > > > > > > > > > 2. If the user cpumask (bar the offline CPUs) is not a subset of the > > > > > > aarch32_mask > > > > > > > > > > > > Option 1 is simpler but 2 would be slightly more consistent. > > > > > > > > > > I disagree -- if we did this for something like fpsimd, then the consistent > > > > > behaviour would be to SIGILL on the cores without the instructions. > > > > > > > > For fpsimd it makes sense since the main ISA is still available and the > > > > application may be able to do something with the signal. But here we > > > > can't do much since the entire AArch32 mode is not supported. That's why > > > > we went for SIGKILL instead of SIGILL but thinking of it, after execve() > > > > the signals are reset to SIG_DFL so SIGILL cannot be ignored. > > > > > > > > I think it depends on whether you look at this fault as a part of ISA > > > > not being available or as the overall application not compatible with > > > > the system it is running on. If the latter, option 2 above makes more > > > > sense. > > > > > > Hmm, I'm not sure I see the distinction in practice: you still have a binary > > > application that cannot run on all CPUs in the system. Who cares if some of > > > the instructions work? > > > > The failure would be more predictable rather than the app running for a > > while and randomly getting SIGKILL. If it only fails on execve or > > sched_setaffinity, it may be easier to track down (well, there's the CPU > > hotplug as well that can change the cpumask intersection outside the > > user process control). Migration between cpusets is another failure scenario where the app can get SIGKILL randomly. > But it's half-baked, because the moment the 32-bit task changes its affinity > mask then you're back in the old situation. That's why I'm saying this > doesn't add anything, because the rest of the series is designed entirely > around delivering SIGKILL at the last minute rather than preventing us > getting to that situation in the first place. The execve() case feels to me > like we're considering doing something because we can, rather than because > it's actually useful. Agree.