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=-5.5 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED,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 18AB1C388F9 for ; Wed, 21 Oct 2020 14:42:40 +0000 (UTC) Received: from merlin.infradead.org (merlin.infradead.org [205.233.59.134]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id 8C910208B8 for ; Wed, 21 Oct 2020 14:42:39 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=lists.infradead.org header.i=@lists.infradead.org header.b="y8cRpqee"; dkim=fail reason="signature verification failed" (1024-bit key) header.d=kernel.org header.i=@kernel.org header.b="Y7RZqUjS" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 8C910208B8 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=kernel.org Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=merlin.20170209; h=Sender:Content-Transfer-Encoding: Content-Type:Cc:List-Subscribe:List-Help:List-Post:List-Archive: List-Unsubscribe:List-Id:In-Reply-To:MIME-Version:References:Message-ID: Subject:To:From:Date:Reply-To:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Owner; bh=Rm1IhCCLCi9HKMqUR9cQdCIvXa5cd8O1ZW2ccdEKQRY=; b=y8cRpqeen73fh2wyU86vYYV11 gNHlk90jgfmQJhvvxoBaJrI58u0trI4HDQ6IUt3F8KDcelLL8hzsIVbQURNJdP3ILOovBaI9+hTpG FdZmczrvxZnuwI71cU2T9NfD7Vuqu+xYrpZkxIb3AxUdYhUvfa5rF7gc3quM+zEoRfO6hEwhOnQdJ 0njU69pj9lo4/N4GOKU9qVYh0xu+jUV3FNYou1jpLxz4smvHIwcTDzj0D303sw5QpWirpCo+jkuUb GpnpPytfa14be8P9nJVd+AoPpEKkVMmW3rl0bGIoX3QYJeUKKyObLVJipHUuDSshT229saoeQCCi/ YpJ+CAKPA==; Received: from localhost ([::1] helo=merlin.infradead.org) by merlin.infradead.org with esmtp (Exim 4.92.3 #3 (Red Hat Linux)) id 1kVFIp-0006sA-C4; Wed, 21 Oct 2020 14:41:23 +0000 Received: from mail.kernel.org ([198.145.29.99]) by merlin.infradead.org with esmtps (Exim 4.92.3 #3 (Red Hat Linux)) id 1kVFIl-0006mf-Vm for linux-arm-kernel@lists.infradead.org; Wed, 21 Oct 2020 14:41:20 +0000 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 24EB52224E; Wed, 21 Oct 2020 14:41:16 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1603291279; bh=ZkXHEiFNy+OsEciCgVHW7rrheEkonWr79tZKybOgUUI=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=Y7RZqUjSodQFOY2O8qndgLqVSlFveJtsKXVSRUAUI9UXlO0g9WW8BZQKjSgs6GtiK CIKy7/l8tmnXQylqXG5tU247DrlM+5kBAQrp1ZKg0nSiYY3EE+VlFc+/6aWoqzWLxl 81upOn4jnjtSE1PY93rXan+njOkS7NFcKz89YxVQ= Date: Wed, 21 Oct 2020 15:41:13 +0100 From: Will Deacon To: Catalin Marinas Subject: Re: [RFC PATCH v2 4/4] arm64: Export id_aar64fpr0 via sysfs Message-ID: <20201021144112.GA17912@willie-the-truck> References: <20201021104611.2744565-1-qais.yousef@arm.com> <20201021104611.2744565-5-qais.yousef@arm.com> <63fead90e91e08a1b173792b06995765@kernel.org> <20201021121559.GB3976@gaia> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: <20201021121559.GB3976@gaia> User-Agent: Mutt/1.10.1 (2018-07-13) X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20201021_104120_187740_41C07DCD X-CRM114-Status: GOOD ( 26.38 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: linux-arch@vger.kernel.org, "Peter Zijlstra \(Intel\)" , Marc Zyngier , Morten Rasmussen , James Morse , Greg Kroah-Hartman , Linus Torvalds , Qais Yousef , linux-arm-kernel@lists.infradead.org Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org 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. Will _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel