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=-5.6 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,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 7B3AAC388F9 for ; Wed, 21 Oct 2020 15:37:46 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id EF5982225F for ; Wed, 21 Oct 2020 15:37:45 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1603294666; bh=gW9CXD+7+//SLGh446wzYYlErVq28FiErkaXTFxmtIs=; h=Date:From:To:Cc:Subject:References:In-Reply-To:List-ID:From; b=t2q5Kxb+LZlV+K8YDcsFAvKcguFyFuHGzdkmBYs++wAb96Ctzd9gqtrkrbQyMJgJp pyCsqoXY+5UAm4jQZ7jI/WFLVJJqgvB5ogD/uAbV1G6m1ktGeBctnGVkVY4iL46Tsp Nf7bpL0t81oL/qcewuOpD5liSq9jp0WsOL+9eP5E= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2502581AbgJUPhp (ORCPT ); Wed, 21 Oct 2020 11:37:45 -0400 Received: from mail.kernel.org ([198.145.29.99]:48746 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2502580AbgJUPhp (ORCPT ); Wed, 21 Oct 2020 11:37:45 -0400 Received: from willie-the-truck (236.31.169.217.in-addr.arpa [217.169.31.236]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 70DDF2098B; Wed, 21 Oct 2020 15:37:42 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1603294664; bh=gW9CXD+7+//SLGh446wzYYlErVq28FiErkaXTFxmtIs=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=EcqF3zE6m9aru7CEJJ6H4zErkxqvRL/Oc0tvlHv5OUegHp+t36U7dBTmeKPWnCWR/ i3nxw/IGDgzYM0aRXEyGrdCuStEhTtXxtEPUrXtrbScaBCD2b7hCyoF1OyooL3P+SQ oWYuL6qPJPgw3TXLpkNDacDOTE4pvuZsKHWTrl9Q= Date: Wed, 21 Oct 2020 16:37:38 +0100 From: Will Deacon To: Catalin Marinas Cc: Morten Rasmussen , Marc Zyngier , linux-arch@vger.kernel.org, "Peter Zijlstra (Intel)" , Greg Kroah-Hartman , James Morse , Linus Torvalds , Qais Yousef , linux-arm-kernel@lists.infradead.org Subject: Re: [RFC PATCH v2 4/4] arm64: Export id_aar64fpr0 via sysfs Message-ID: <20201021153738.GB18071@willie-the-truck> References: <20201021104611.2744565-1-qais.yousef@arm.com> <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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20201021151005.GF3976@gaia> User-Agent: Mutt/1.10.1 (2018-07-13) Precedence: bulk List-ID: X-Mailing-List: linux-arch@vger.kernel.org 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? > > > There's also the question on whether the kernel should allow an ELF32 to > > > be loaded (and potentially killed subsequently) if the user mask is not > > > correct on execve(). > > > > I don't see the point in distinguishing between "you did execve() on a core > > without 32-bit" and "you did execve() on a core with 32-bit and then > > migrated to a core without 32-bit". > > In the context of option 2 above, its more about whether execve() > returns -ENOEXEC or the process gets a SIGKILL immediately. I just don't see what we gain by returning -ENOEXEC except for extra code and behaviour in the ABI (and if you wanted consistentcy you'd also need to fail attempts to widen the affinity mask to include 64-bit-only cores from a 32-bit task). In other words, I don't think the kernel needs to hold userspace's hand for an opt-in feature that requires userspace to handle scheduling for optimal power/performance _anyway_. Allowing the affinity to be set arbitrarily and then killing the task if it ends up trying to run on the wrong CPU is both simple and sufficient. Will