Merge branch 'for-next/vcpu-hotplug' into for-next/core

* for-next/vcpu-hotplug: (21 commits)
  : arm64 support for virtual CPU hotplug (ACPI)
  irqchip/gic-v3: Fix 'broken_rdists' unused warning when !SMP and !ACPI
  arm64: Kconfig: Fix dependencies to enable ACPI_HOTPLUG_CPU
  cpumask: Add enabled cpumask for present CPUs that can be brought online
  arm64: document virtual CPU hotplug's expectations
  arm64: Kconfig: Enable hotplug CPU on arm64 if ACPI_PROCESSOR is enabled.
  arm64: arch_register_cpu() variant to check if an ACPI handle is now available.
  arm64: psci: Ignore DENIED CPUs
  irqchip/gic-v3: Add support for ACPI's disabled but 'online capable' CPUs
  irqchip/gic-v3: Don't return errors from gic_acpi_match_gicc()
  arm64: acpi: Harden get_cpu_for_acpi_id() against missing CPU entry
  arm64: acpi: Move get_cpu_for_acpi_id() to a header
  ACPI: Add post_eject to struct acpi_scan_handler for cpu hotplug
  ACPI: scan: switch to flags for acpi_scan_check_and_detach()
  ACPI: processor: Register deferred CPUs from acpi_processor_get_info()
  ACPI: processor: Add acpi_get_processor_handle() helper
  ACPI: processor: Move checks and availability of acpi_processor earlier
  ACPI: processor: Fix memory leaks in error paths of processor_add()
  ACPI: processor: Return an error if acpi_processor_get_info() fails in processor_add()
  ACPI: processor: Drop duplicated check on _STA (enabled + present)
  cpu: Do not warn on arch_register_cpu() returning -EPROBE_DEFER
  ...

3 files changed, 86 insertions, 0 deletions

diff --git a/Documentation/ABI/testing/sysfs-devices-system-cpu b/Documentation/ABI/testing/sysfs-devices-system-cpu
index e7e160954e79..53ed1a803422 100644
--- a/Documentation/ABI/testing/sysfs-devices-system-cpu
+++ b/Documentation/ABI/testing/sysfs-devices-system-cpu
@@ -694,3 +694,9 @@ Description:
 		(RO) indicates whether or not the kernel directly supports
 		modifying the crash elfcorehdr for CPU hot un/plug and/or
 		on/offline changes.
+
+What:		/sys/devices/system/cpu/enabled
+Date:		Nov 2022
+Contact:	Linux kernel mailing list <linux-kernel@vger.kernel.org>
+Description:
+		(RO) the list of CPUs that can be brought online.
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