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 7D363C388F9 for ; Wed, 21 Oct 2020 15:23:19 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 16D2F22283 for ; Wed, 21 Oct 2020 15:23:19 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1603293799; bh=80+F/rS39OaAv82faW86LFYzPhStpfh5kVT+wc50cTo=; h=Date:From:To:Cc:Subject:References:In-Reply-To:List-ID:From; b=EihwYdZRVKbSms0A30hEeHpj6VFisCZ15wg/jlal5DP4jsEl8fOiF3EuLCQMLEcbH r/B2Jlz6PvWn9YPxAUmvEbrQlkmwU2jeAYHDbgKwV8LIebsiex4m45IqHx7spddMyj 1X7iRIxswwndIMmiHBTM6sFGqPs6Vlhwbo+sPsvU= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2502329AbgJUPXS (ORCPT ); Wed, 21 Oct 2020 11:23:18 -0400 Received: from mail.kernel.org ([198.145.29.99]:45710 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2502303AbgJUPXS (ORCPT ); Wed, 21 Oct 2020 11:23:18 -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 798252177B; Wed, 21 Oct 2020 15:23:15 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1603293797; bh=80+F/rS39OaAv82faW86LFYzPhStpfh5kVT+wc50cTo=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=ne6+cBPy7G0QjkpVV/vRlit89BKDZur0WUpbxJqgk0c6u5vuOVw71i+SsDVbpF5uP QX1OQBDkBRc5P+27/Y7bJ7SS2ej+gbo9584uXnefw6bBUAmpURIYr1FivQXSysBX2f T0mhfYcuwUAmBbBd7YLWB8gj+w24BVGrvjGrAq1g= Date: Wed, 21 Oct 2020 16:23:12 +0100 From: Will Deacon To: Qais Yousef Cc: Catalin Marinas , Marc Zyngier , "Peter Zijlstra (Intel)" , Morten Rasmussen , Greg Kroah-Hartman , Linus Torvalds , James Morse , linux-arm-kernel@lists.infradead.org, linux-arch@vger.kernel.org Subject: Re: [RFC PATCH v2 4/4] arm64: Export id_aar64fpr0 via sysfs Message-ID: <20201021152310.GA18071@willie-the-truck> References: <20201021104611.2744565-1-qais.yousef@arm.com> <20201021104611.2744565-5-qais.yousef@arm.com> <63fead90e91e08a1b173792b06995765@kernel.org> <20201021121559.GB3976@gaia> <20201021144112.GA17912@willie-the-truck> <20201021150313.ecxawwxsowweye43@e107158-lin> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20201021150313.ecxawwxsowweye43@e107158-lin> 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 04:03:13PM +0100, Qais Yousef wrote: > On 10/21/20 15:41, Will Deacon wrote: > > On Wed, Oct 21, 2020 at 01:15:59PM +0100, Catalin Marinas wrote: > > > On Wed, Oct 21, 2020 at 12:09:58PM +0100, Marc Zyngier wrote: > > > > On 2020-10-21 11:46, Qais Yousef wrote: > > > > > Example output. I was surprised that the 2nd field (bits[7:4]) is > > > > > printed out > > > > > although it's set as FTR_HIDDEN. > > > > > > > > > > # cat /sys/devices/system/cpu/cpu*/regs/identification/id_aa64pfr0 > > > > > 0x0000000000000011 > > > > > 0x0000000000000011 > > > > > 0x0000000000000011 > > > > > 0x0000000000000011 > > > > > 0x0000000000000011 > > > > > 0x0000000000000011 > > > > > > > > > > # echo 1 > /proc/sys/kernel/enable_asym_32bit > > > > > > > > > > # cat /sys/devices/system/cpu/cpu*/regs/identification/id_aa64pfr0 > > > > > 0x0000000000000011 > > > > > 0x0000000000000011 > > > > > 0x0000000000000012 > > > > > 0x0000000000000012 > > > > > 0x0000000000000011 > > > > > 0x0000000000000011 > > > > > > > > This looks like a terrible userspace interface. It exposes unrelated > > > > features, > > > > > > Not sure why the EL1 field ended up in here, that's not relevant to the > > > user. > > > > > > > and doesn't expose the single useful information that the kernel has: > > > > the cpumask describing the CPUs supporting AArch32 at EL0. Why not expose > > > > this synthetic piece of information which requires very little effort from > > > > userspace and doesn't spit out unrelated stuff? > > > > > > I thought the whole idea is to try and avoid the "very little effort" > > > part ;). > > > > > > > Not to mention the discrepancy with what userspace gets while reading > > > > the same register via the MRS emulation. > > > > > > > > Granted, the cpumask doesn't fit the cpu*/regs/identification hierarchy, > > > > but I don't think this fits either. > > > > > > We already expose MIDR and REVIDR via the current sysfs interface. We > > > can expand it to include _all_ the other ID_* regs currently available > > > to user via the MRS emulation and we won't have to debate what a new > > > interface would look like. The MRS emulation and the sysfs info should > > > probably match, though that means we need to expose the > > > ID_AA64PFR0_EL1.EL0 field which we currently don't. > > > > > > I do agree that an AArch32 cpumask is an easier option both from the > > > kernel implementation perspective and from the application usability > > > one, though not as easy as automatic task placement by the scheduler (my > > > first preference, followed by the id_* regs and the aarch32 mask, though > > > not a strong preference for any). > > > > If a cpumask is easier to implement and easier to use, then I think that's > > what we should do. It's also then dead easy to disable if necessary by > > just returning 0. The only alternative I would prefer is not having to > > expose this information altogether, but I'm not sure that figuring this > > out from MIDR/REVIDR alone is reliable. > > I did suggest this before, but I'll try gain. If we want to assume a custom > bootloader and custom user space, we can make them provide the mask. Who mentioned a custom bootloader? In the context of Android, we're talking about a user-space that already manages scheduling affinity. > For example, the new sysctl_enable_asym_32bit could be a cpumask instead of > a bool as it currently is. Or we can make it a cmdline parameter too. > In both cases some admin (bootloader or init process) has to ensure to fill it > correctly for the target platform. The bootloader should be able to read the > registers to figure out the mask. So more weight to make it a cmdline param. I think this is adding complexity for the sake of it. I'm much more in favour of keeping the implementation and ABI as simple as possible: expose the fact that the system is heterogenous, have an opt-in for userspace to say it can handle that and let it handle it. Will