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=-10.1 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY, SPF_HELO_NONE,SPF_PASS,USER_AGENT_GIT autolearn=unavailable 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 067B4C54FD0 for ; Tue, 21 Apr 2020 14:29:52 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id D67BC206B8 for ; Tue, 21 Apr 2020 14:29:51 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1587479391; bh=vq62htzQrEGkXQgbmBIdb3flO+yGFZQ1M31v4/nKJRM=; h=From:To:Cc:Subject:Date:In-Reply-To:References:List-ID:From; b=VjLJMaiDFUXvUXrlQQ3hhWn8jQfmhHYRQ3V+QsQpx1r9dBxYroCzPF7AjA7KjPhJV +5j+C1uo1iwukUNHj0kF56OE6nhJw3DaiKLznVktV6lTzWoYW1DYRpyoIgUm/2pr+r UMuvW590AVoTyt+wVp/DRBSqi31lpwSPaETm+03g= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729172AbgDUO3v (ORCPT ); Tue, 21 Apr 2020 10:29:51 -0400 Received: from mail.kernel.org ([198.145.29.99]:42638 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1729159AbgDUO3s (ORCPT ); Tue, 21 Apr 2020 10:29:48 -0400 Received: from localhost.localdomain (236.31.169.217.in-addr.arpa [217.169.31.236]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 5080C20724; Tue, 21 Apr 2020 14:29:45 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1587479387; bh=vq62htzQrEGkXQgbmBIdb3flO+yGFZQ1M31v4/nKJRM=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=hu8zjX8bct2a6eO9hoEe3hbPq+qhKJA3kLRvPBvyROaRtSdHsc0csp7/E9Jx7ZGCG gdbzQa31ODGBaEHFBB36vsimK/YYtIJ4KspcGBQq0hDjrjdUA2efBGuHIjQtGW1zwh uRyDvXHtb5D44+ytNrHc8bdNyUXfK2g4OVH++ZP8= From: Will Deacon To: linux-arm-kernel@lists.infradead.org, kvmarm@lists.cs.columbia.edu Cc: linux-kernel@vger.kernel.org, Will Deacon , Suzuki K Poulose , Mark Rutland , Marc Zyngier , Anshuman Khandual , Catalin Marinas , Sai Prakash Ranjan , Doug Anderson , kernel-team@android.com Subject: [PATCH v2 8/8] arm64: cpufeature: Add an overview comment for the cpufeature framework Date: Tue, 21 Apr 2020 15:29:22 +0100 Message-Id: <20200421142922.18950-9-will@kernel.org> X-Mailer: git-send-email 2.20.1 In-Reply-To: <20200421142922.18950-1-will@kernel.org> References: <20200421142922.18950-1-will@kernel.org> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Now that Suzuki isn't within throwing distance, I thought I'd better add a rough overview comment to cpufeature.c so that it doesn't take me days to remember how it works next time. Signed-off-by: Will Deacon --- arch/arm64/kernel/cpufeature.c | 50 ++++++++++++++++++++++++++++++++++ 1 file changed, 50 insertions(+) diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c index d63653d7c5d0..c1d44d127baa 100644 --- a/arch/arm64/kernel/cpufeature.c +++ b/arch/arm64/kernel/cpufeature.c @@ -3,6 +3,56 @@ * Contains CPU feature definitions * * Copyright (C) 2015 ARM Ltd. + * + * A note for the weary kernel hacker: the code here is confusing and hard to + * follow! That's partly because it's solving a nasty problem, but also because + * there's a little bit of over-abstraction that tends to obscure what's going + * on behind a maze of helper functions and macros. + * + * The basic problem is that hardware folks have started gluing together CPUs + * with distinct architectural features; in some cases even creating SoCs where + * user-visible instructions are available only on a subset of the available + * cores. We try to address this by snapshotting the feature registers of the + * boot CPU and comparing these with the feature registers of each secondary + * CPU when bringing them up. If there is a mismatch, then we update the + * snapshot state to indicate the lowest-common denominator of the feature, + * known as the "safe" value. This snapshot state can be queried to view the + * "sanitised" value of a feature register. + * + * The sanitised register values are used to decide which capabilities we + * have in the system. These may be in the form of traditional "hwcaps" + * advertised to userspace or internal "cpucaps" which are used to configure + * things like alternative patching and static keys. While a feature mismatch + * may result in a TAINT_CPU_OUT_OF_SPEC kernel taint, a capability mismatch + * may prevent a CPU from being onlined at all. + * + * Some implementation details worth remembering: + * + * - Mismatched features are *always* sanitised to a "safe" value, which + * usually indicates that the feature is not supported. + * + * - A mismatched feature marked with FTR_STRICT will cause a "SANITY CHECK" + * warning when onlining an offending CPU and the kernel will be tainted + * with TAINT_CPU_OUT_OF_SPEC. + * + * - Features marked as FTR_VISIBLE have their sanitised value visible to + * userspace. FTR_VISIBLE features in registers that are only visible + * to EL0 by trapping *must* have a corresponding HWCAP so that late + * onlining of CPUs cannot lead to features disappearing at runtime. + * + * - A "feature" is typically a 4-bit register field. A "capability" is the + * high-level description derived from the sanitised field value. + * + * - Read the Arm ARM (DDI 0487F.a) section D13.1.3 ("Principles of the ID + * scheme for fields in ID registers") to understand when feature fields + * may be signed or unsigned (FTR_SIGNED and FTR_UNSIGNED accordingly). + * + * - KVM exposes its own view of the feature registers to guest operating + * systems regardless of FTR_VISIBLE. This is typically driven from the + * sanitised register values to allow virtual CPUs to be migrated between + * arbitrary physical CPUs, but some features not present on the host are + * also advertised and emulated. Look at sys_reg_descs[] for the gory + * details. */ #define pr_fmt(fmt) "CPU features: " fmt -- 2.26.1.301.g55bc3eb7cb9-goog