From: Jonathan Cameron <Jonathan.Cameron@huawei.com>
To: Thomas Gleixner <tglx@linutronix.de>,
Peter Zijlstra <peterz@infradead.org>, <linux-pm@vger.kernel.org>,
<loongarch@lists.linux.dev>, <linux-acpi@vger.kernel.org>,
<linux-arch@vger.kernel.org>, <linux-kernel@vger.kernel.org>,
<linux-arm-kernel@lists.infradead.org>, <kvmarm@lists.linux.dev>,
<x86@kernel.org>, Russell King <linux@armlinux.org.uk>,
"Rafael J . Wysocki" <rafael@kernel.org>,
Miguel Luis <miguel.luis@oracle.com>,
James Morse <james.morse@arm.com>,
Salil Mehta <salil.mehta@huawei.com>,
Jean-Philippe Brucker <jean-philippe@linaro.org>,
Catalin Marinas <catalin.marinas@arm.com>,
Will Deacon <will@kernel.org>
Cc: Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
Dave Hansen <dave.hansen@linux.intel.com>, <linuxarm@huawei.com>,
<justin.he@arm.com>, <jianyong.wu@arm.com>
Subject: [PATCH v7 15/16] arm64: document virtual CPU hotplug's expectations
Date: Thu, 18 Apr 2024 14:54:11 +0100 [thread overview]
Message-ID: <20240418135412.14730-16-Jonathan.Cameron@huawei.com> (raw)
In-Reply-To: <20240418135412.14730-1-Jonathan.Cameron@huawei.com>
From: James Morse <james.morse@arm.com>
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 <james.morse@arm.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
---
v7: 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
--
2.39.2
next prev parent reply other threads:[~2024-04-18 14:01 UTC|newest]
Thread overview: 67+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-04-18 13:53 [PATCH v7 00/16] ACPI/arm64: add support for virtual cpu hotplug Jonathan Cameron
2024-04-18 13:53 ` [PATCH v7 01/16] ACPI: processor: Simplify initial onlining to use same path for cold and hotplug Jonathan Cameron
2024-04-22 18:46 ` Rafael J. Wysocki
2024-04-23 6:18 ` Hanjun Guo
2024-04-26 9:23 ` Gavin Shan
2024-04-18 13:53 ` [PATCH v7 02/16] cpu: Do not warn on arch_register_cpu() returning -EPROBE_DEFER Jonathan Cameron
2024-04-23 6:22 ` Hanjun Guo
2024-04-26 9:20 ` Gavin Shan
2024-04-18 13:53 ` [PATCH v7 03/16] ACPI: processor: Drop duplicated check on _STA (enabled + present) Jonathan Cameron
2024-04-22 18:48 ` Rafael J. Wysocki
2024-04-23 6:49 ` Hanjun Guo
2024-04-23 9:31 ` Rafael J. Wysocki
2024-04-23 11:13 ` Hanjun Guo
2024-04-26 9:24 ` Gavin Shan
2024-04-18 13:54 ` [PATCH v7 04/16] ACPI: processor: Move checks and availability of acpi_processor earlier Jonathan Cameron
2024-04-22 18:56 ` Rafael J. Wysocki
2024-04-24 16:53 ` Jonathan Cameron
2024-04-23 11:53 ` Hanjun Guo
2024-04-24 17:18 ` Jonathan Cameron
2024-04-25 1:20 ` Hanjun Guo
2024-04-18 13:54 ` [PATCH v7 05/16] ACPI: processor: Add acpi_get_processor_handle() helper Jonathan Cameron
2024-04-22 18:59 ` Rafael J. Wysocki
2024-04-26 9:15 ` Gavin Shan
2024-04-18 13:54 ` [PATCH v7 06/16] ACPI: processor: Register deferred CPUs from acpi_processor_get_info() Jonathan Cameron
2024-04-22 19:02 ` Rafael J. Wysocki
2024-04-23 11:58 ` Hanjun Guo
2024-04-26 9:18 ` Gavin Shan
2024-04-18 13:54 ` [PATCH v7 07/16] ACPI: scan: switch to flags for acpi_scan_check_and_detach() Jonathan Cameron
2024-04-22 19:05 ` Rafael J. Wysocki
2024-04-23 12:02 ` Hanjun Guo
2024-04-26 9:25 ` Gavin Shan
2024-04-18 13:54 ` [PATCH v7 08/16] ACPI: Add post_eject to struct acpi_scan_handler for cpu hotplug Jonathan Cameron
2024-04-22 19:10 ` Rafael J. Wysocki
2024-04-23 12:06 ` Hanjun Guo
2024-04-26 11:48 ` Jonathan Cameron
2024-04-18 13:54 ` [PATCH v7 09/16] arm64: acpi: Move get_cpu_for_acpi_id() to a header Jonathan Cameron
2024-04-22 10:46 ` Jonathan Cameron
2024-04-23 12:10 ` Hanjun Guo
2024-04-18 13:54 ` [PATCH v7 10/16] irqchip/gic-v3: Don't return errors from gic_acpi_match_gicc() Jonathan Cameron
2024-04-22 10:39 ` Jonathan Cameron
2024-04-18 13:54 ` [PATCH v7 11/16] irqchip/gic-v3: Add support for ACPI's disabled but 'online capable' CPUs Jonathan Cameron
2024-04-22 10:40 ` Jonathan Cameron
2024-04-23 12:01 ` Marc Zyngier
2024-04-24 12:54 ` Jonathan Cameron
2024-04-24 15:33 ` Marc Zyngier
2024-04-24 16:35 ` Salil Mehta
2024-04-24 17:08 ` Jonathan Cameron
2024-04-25 10:23 ` Jonathan Cameron
2024-04-25 12:31 ` Jonathan Cameron
2024-04-25 15:00 ` Jonathan Cameron
2024-04-25 16:55 ` Jonathan Cameron
2024-04-26 12:41 ` Marc Zyngier
2024-04-25 9:28 ` Jonathan Cameron
2024-04-25 9:56 ` Jonathan Cameron
2024-04-25 10:13 ` Jonathan Cameron
2024-04-18 13:54 ` [PATCH v7 12/16] arm64: psci: Ignore DENIED CPUs Jonathan Cameron
2024-04-22 10:44 ` Jonathan Cameron
2024-04-26 9:36 ` Gavin Shan
2024-04-26 9:57 ` Jonathan Cameron
2024-04-18 13:54 ` [PATCH v7 13/16] arm64: arch_register_cpu() variant to check if an ACPI handle is now available Jonathan Cameron
2024-04-18 13:54 ` [PATCH v7 14/16] arm64: Kconfig: Enable hotplug CPU on arm64 if ACPI_PROCESSOR is enabled Jonathan Cameron
2024-04-24 17:24 ` Jonathan Cameron
2024-04-18 13:54 ` Jonathan Cameron [this message]
2024-04-18 13:54 ` [PATCH v7 16/16] cpumask: Add enabled cpumask for present CPUs that can be brought online Jonathan Cameron
2024-04-18 19:50 ` [PATCH v7 00/16] ACPI/arm64: add support for virtual cpu hotplug Rafael J. Wysocki
2024-04-22 19:16 ` Rafael J. Wysocki
2024-04-19 15:39 ` Miguel Luis
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