1 .. SPDX-License-Identifier: GPL-2.0
2 .. _cpuhp_index:
3
4 ====================
5 CPU Hotplug and ACPI
6 ====================
7
8 CPU hotplug in the arm64 world is commonly use
9 CPUs online/offline using PSCI. This document
10 CPUs that were not available during boot to be
11
12 ``possible`` and ``present`` refer to the stat
13
14
15 CPU Hotplug on physical systems - CPUs not pre
16 ----------------------------------------------
17
18 Physical systems need to mark a CPU that is ``
19 being ``present``. An example would be a dual
20 in one of the sockets can be replaced while th
21
22 This is not supported.
23
24 In the arm64 world CPUs are not a single devic
25 There are no systems that support the physical
26 while the system is running, and ACPI is not a
27 them.
28
29 e.g. New CPUs come with new caches, but the pl
30 described in a static table, the PPTT. How cac
31 not discoverable, and must be described by fir
32
33 e.g. The GIC redistributor for each CPU must b
34 boot to discover the system wide supported fea
35 structures can describe a redistributor associ
36 can't describe whether the redistributor is ac
37 'always on'.
38
39 arm64's ACPI tables assume that everything des
40
41
42 CPU Hotplug on virtual systems - CPUs not enab
43 ----------------------------------------------
44
45 Virtual systems have the advantage that all th
46 ever have can be described at boot. There are
47 as such devices are emulated.
48
49 CPU Hotplug on virtual systems is supported. I
50 CPU Hotplug as all resources are described as
51 marked as disabled by firmware. Only the CPU's
52 influenced by firmware. An example is where a
53 single CPU, and additional CPUs are added once
54 the workload.
55
56 For a virtual machine, the VMM (e.g. Qemu) pla
57
58 Virtual hotplug is implemented as a firmware p
59 brought online. Firmware can enforce its polic
60 ``DENIED``.
61
62 The ACPI tables must describe all the resource
63 that firmware wishes to disable either from bo
64 ``enabled`` in the MADT GICC structures, but s
65 bit set, to indicate they can be enabled later
66 ``enabled``. The 'always on' GICR structure m
67 redistributors.
68
69 CPUs described as ``online capable`` but not `
70 by the DSDT's Processor object's _STA method.
71 must always report the CPU as ``present``. Cha
72 be notified to the OS via device-check or ejec
73
74 CPUs described as ``enabled`` in the static ta
75 modified dynamically by firmware. Soft-restart
76 re-read the static properties of the system fr
77 may malfunction if these no longer describe th
78 re-discover the dynamic properties of the syst
79 during boot.
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