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=-0.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS 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 43E75C3A5A9 for ; Mon, 4 May 2020 15:50:20 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 29FE82073B for ; Mon, 4 May 2020 15:50:20 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729479AbgEDPuT (ORCPT ); Mon, 4 May 2020 11:50:19 -0400 Received: from foss.arm.com ([217.140.110.172]:47742 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726641AbgEDPuS (ORCPT ); Mon, 4 May 2020 11:50:18 -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 880281FB; Mon, 4 May 2020 08:50:18 -0700 (PDT) Received: from C02TD0UTHF1T.local (unknown [10.57.4.172]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id B41913F68F; Mon, 4 May 2020 08:50:16 -0700 (PDT) Date: Mon, 4 May 2020 16:50:13 +0100 From: Mark Rutland To: Anshuman Khandual Cc: linux-arm-kernel@lists.infradead.org, Catalin Marinas , Will Deacon , Mark Brown , Suzuki Poulose , linux-kernel@vger.kernel.org Subject: Re: [PATCH] arm64/cpuinfo: Drop boot_cpu_data Message-ID: <20200504155013.GG73375@C02TD0UTHF1T.local> References: <1588595400-4560-1-git-send-email-anshuman.khandual@arm.com> <20200504124321.GA73375@C02TD0UTHF1T.local> <224296d1-086a-5516-95a8-8f4ad5c533d9@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <224296d1-086a-5516-95a8-8f4ad5c533d9@arm.com> Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, May 04, 2020 at 08:23:08PM +0530, Anshuman Khandual wrote: > > > On 05/04/2020 06:13 PM, Mark Rutland wrote: > > On Mon, May 04, 2020 at 06:00:00PM +0530, Anshuman Khandual wrote: > >> A global boot_cpu_data is not really required. Lets drop this. > > > > I don't think it's true that this isn't required today. > > > > One reason that we have both boot_cpu_data and a cpu_data variable for > > CPU0 is that CPU0 itself can be hotplugged out then back in, and this > > allows us to detect if CPU0's features have changed (e.g. due to FW > > failing to configure it appropriately, or real physical hotplug > > occurring). > > Understood. After hotplug, CPU0 will come back via secondary_start_kernel() > where it's current register values will be checked against earlier captured > values i.e boot_cpu_data. > > But wondering why should CPU0 be treated like any other secondary CPU. IOW > in case the fresh boot CPU register values dont match with boot_cpu_data, > should not the online process just be declined ? AFAICS, current approach > will let the kernel run with taint in case of a mismatch. I don't follow. When CPU0 is hotplguged back in it'll follow the secondary boot path, so it can be rejected as with any other secondary CPU. If I'm missing a case, could you please point that out more specifically? > > So NAK to the patch as it stands. If we're certain we capture all of > > those details even without boot_cpu_data, then we should make other > > changes to make that clear (e.g. removing it as an argument to > > update_cpu_features()). > > There might not be another way, unless we can override CPU0's cpu_data > variable when the boot CPU comes back in after vetting against existing > values. Is there any particular reason to store the very first boot CPU0 > info for ever ? The reason is so that we can log the values for comparison. Otherwise we'll have to choose some arbitrary CPU's value in order to do so. > Passing on CPU0's cpu_data variable in update_cpu_features() for secondary > CPUs during boot still make sense. It helps in finalizing register values. > Re-entering CPU0's test against boot_cpu_data seems different. I think that practically this means we should leave this as-is. If we need to keep it around for CPU, then we may as well keep it around and use it consitently for all secondary CPUs. I'd prefer to leave this as-is given it's simple to reason about. Thanks, Mark.