From patchwork Wed Apr 17 13:19:08 2024 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Jonathan Cameron X-Patchwork-Id: 791902 Received: from frasgout.his.huawei.com (frasgout.his.huawei.com [185.176.79.56]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 87BA313E414; Wed, 17 Apr 2024 13:26:53 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=185.176.79.56 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1713360415; cv=none; b=LPF8JAT12eLBvyjoK8U7tlU4P935V+Ow+MXj8Y9eTZGwmoJNg68k4REl5DRTfR3KIrpNrpxrzIhXH23wJK6GVqYHjDFxFYwuykOi/EYI/7HIWELEtqY8Q33MfbeakIpB3JzObOIeWdHVToBJF88ZeM6Sb52goiaG6Vci4ZS55Q8= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1713360415; c=relaxed/simple; bh=at/D2TnqgbwauUxHhWlbNw1UdgkgeRlnps3s3gggvsQ=; h=From:To:CC:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version:Content-Type; b=lXJCudqIMTNa7eUF6sg4q9yYd0cx8KG7TH1LiazgaJEssFSMWA+9p/hHKvGFaJFXbrgYpGBMbUc0X1T8VFsYLE9x6gUhhdGeoGognuLQ6aOBn+CoAFjl5RMycvWRsnsg7eENLELvyk/kpkFTsJyTGS+ciSmzMVWSZ1rV5/osW1I= ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=huawei.com; spf=pass smtp.mailfrom=huawei.com; arc=none smtp.client-ip=185.176.79.56 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=huawei.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=huawei.com Received: from mail.maildlp.com (unknown [172.18.186.231]) by frasgout.his.huawei.com (SkyGuard) with ESMTP id 4VKM8M2tHNz6JBK9; Wed, 17 Apr 2024 21:24:51 +0800 (CST) Received: from lhrpeml500005.china.huawei.com (unknown [7.191.163.240]) by mail.maildlp.com (Postfix) with ESMTPS id 52F29140B38; Wed, 17 Apr 2024 21:26:51 +0800 (CST) Received: from SecurePC-101-06.china.huawei.com (10.122.247.231) by lhrpeml500005.china.huawei.com (7.191.163.240) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.1.2507.35; Wed, 17 Apr 2024 14:26:50 +0100 From: Jonathan Cameron To: Thomas Gleixner , Peter Zijlstra , , , , , , , , , Russell King , "Rafael J . Wysocki" , Miguel Luis , James Morse , Salil Mehta , Jean-Philippe Brucker , Catalin Marinas , Will Deacon CC: Ingo Molnar , Borislav Petkov , Dave Hansen , , , Subject: [PATCH v6 15/16] arm64: document virtual CPU hotplug's expectations Date: Wed, 17 Apr 2024 14:19:08 +0100 Message-ID: <20240417131909.7925-16-Jonathan.Cameron@huawei.com> X-Mailer: git-send-email 2.39.2 In-Reply-To: <20240417131909.7925-1-Jonathan.Cameron@huawei.com> References: <20240417131909.7925-1-Jonathan.Cameron@huawei.com> Precedence: bulk X-Mailing-List: linux-pm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-ClientProxiedBy: lhrpeml500001.china.huawei.com (7.191.163.213) To lhrpeml500005.china.huawei.com (7.191.163.240) From: James Morse Add a description of physical and virtual CPU hotplug, explain the differences and elaborate on what is required in ACPI for a working virtual hotplug system. Signed-off-by: James Morse Reviewed-by: Jonathan Cameron Signed-off-by: Russell King (Oracle) Signed-off-by: Jonathan Cameron --- v5: No change. --- Documentation/arch/arm64/cpu-hotplug.rst | 79 ++++++++++++++++++++++++ Documentation/arch/arm64/index.rst | 1 + 2 files changed, 80 insertions(+) diff --git a/Documentation/arch/arm64/cpu-hotplug.rst b/Documentation/arch/arm64/cpu-hotplug.rst new file mode 100644 index 000000000000..76ba8d932c72 --- /dev/null +++ b/Documentation/arch/arm64/cpu-hotplug.rst @@ -0,0 +1,79 @@ +.. SPDX-License-Identifier: GPL-2.0 +.. _cpuhp_index: + +==================== +CPU Hotplug and ACPI +==================== + +CPU hotplug in the arm64 world is commonly used to describe the kernel taking +CPUs online/offline using PSCI. This document is about ACPI firmware allowing +CPUs that were not available during boot to be added to the system later. + +``possible`` and ``present`` refer to the state of the CPU as seen by linux. + + +CPU Hotplug on physical systems - CPUs not present at boot +---------------------------------------------------------- + +Physical systems need to mark a CPU that is ``possible`` but not ``present`` as +being ``present``. An example would be a dual socket machine, where the package +in one of the sockets can be replaced while the system is running. + +This is not supported. + +In the arm64 world CPUs are not a single device but a slice of the system. +There are no systems that support the physical addition (or removal) of CPUs +while the system is running, and ACPI is not able to sufficiently describe +them. + +e.g. New CPUs come with new caches, but the platform's cache toplogy is +described in a static table, the PPTT. How caches are shared between CPUs is +not discoverable, and must be described by firmware. + +e.g. The GIC redistributor for each CPU must be accessed by the driver during +boot to discover the system wide supported features. ACPI's MADT GICC +structures can describe a redistributor associated with a disabled CPU, but +can't describe whether the redistributor is accessible, only that it is not +'always on'. + +arm64's ACPI tables assume that everything described is ``present``. + + +CPU Hotplug on virtual systems - CPUs not enabled at boot +--------------------------------------------------------- + +Virtual systems have the advantage that all the properties the system will +ever have can be described at boot. There are no power-domain considerations +as such devices are emulated. + +CPU Hotplug on virtual systems is supported. It is distinct from physical +CPU Hotplug as all resources are described as ``present``, but CPUs may be +marked as disabled by firmware. Only the CPU's online/offline behaviour is +influenced by firmware. An example is where a virtual machine boots with a +single CPU, and additional CPUs are added once a cloud orchestrator deploys +the workload. + +For a virtual machine, the VMM (e.g. Qemu) plays the part of firmware. + +Virtual hotplug is implemented as a firmware policy affecting which CPUs can be +brought online. Firmware can enforce its policy via PSCI's return codes. e.g. +``DENIED``. + +The ACPI tables must describe all the resources of the virtual machine. CPUs +that firmware wishes to disable either from boot (or later) should not be +``enabled`` in the MADT GICC structures, but should have the ``online capable`` +bit set, to indicate they can be enabled later. The boot CPU must be marked as +``enabled``. The 'always on' GICR structure must be used to describe the +redistributors. + +CPUs described as ``online capable`` but not ``enabled`` can be set to enabled +by the DSDT's Processor object's _STA method. On virtual systems the _STA method +must always report the CPU as ``present``. Changes to the firmware policy can +be notified to the OS via device-check or eject-request. + +CPUs described as ``enabled`` in the static table, should not have their _STA +modified dynamically by firmware. Soft-restart features such as kexec will +re-read the static properties of the system from these static tables, and +may malfunction if these no longer describe the running system. Linux will +re-discover the dynamic properties of the system from the _STA method later +during boot. diff --git a/Documentation/arch/arm64/index.rst b/Documentation/arch/arm64/index.rst index d08e924204bf..78544de0a8a9 100644 --- a/Documentation/arch/arm64/index.rst +++ b/Documentation/arch/arm64/index.rst @@ -13,6 +13,7 @@ ARM64 Architecture asymmetric-32bit booting cpu-feature-registers + cpu-hotplug elf_hwcaps hugetlbpage kdump