[PATCH v5 5/5] docs: Document CPU hotplug

[Mykyta Poturai <Mykyta_Poturai@xxxxxxxx>]

Signed-off-by: Mykyta Poturai <mykyta_poturai@xxxxxxxx>
---

v4->v5:
* s/supported/implemented/
* update SUPPORT.md

v3->v4:
* update configuration section

v2->v3:
* patch introduced
---
 SUPPORT.md                |  1 +
 docs/misc/cpu-hotplug.txt | 50 +++++++++++++++++++++++++++++++++++++++
 2 files changed, 51 insertions(+)
 create mode 100644 docs/misc/cpu-hotplug.txt

diff --git a/SUPPORT.md b/SUPPORT.md
index d441bccf37..7b93ae69e7 100644
--- a/SUPPORT.md
+++ b/SUPPORT.md
@@ -52,6 +52,7 @@ For the Cortex A77 r0p0 - r1p0, see Errata 1508412.
 ### ACPI CPU Hotplug
 
     Status, x86: Experimental
+    Status, Arm64: Experimental
 
 ### Physical Memory
 
diff --git a/docs/misc/cpu-hotplug.txt b/docs/misc/cpu-hotplug.txt
new file mode 100644
index 0000000000..c34fc66361
--- /dev/null
+++ b/docs/misc/cpu-hotplug.txt
@@ -0,0 +1,50 @@
+CPU Hotplug
+===========
+
+CPU hotplug is a feature that allows pCPU cores to be added to or removed from 
a
+running system without requiring a reboot. It is implemented on x86 and Arm64
+architectures.
+
+Implementation Details
+----------------------
+
+CPU hotplug is implemented through the `XEN_SYSCTL_CPU_HOTPLUG_*` sysctl calls.
+The specific calls are:
+
+- `XEN_SYSCTL_CPU_HOTPLUG_ONLINE`: Brings a pCPU online
+- `XEN_SYSCTL_CPU_HOTPLUG_OFFLINE`: Takes a pCPU offline
+- `XEN_SYSCTL_CPU_HOTPLUG_SMT_ENABLE`: Enables SMT threads (x86 only)
+- `XEN_SYSCTL_CPU_HOTPLUG_SMT_DISABLE`: Disables SMT threads (x86 only)
+
+All cores can be disabled, assuming hardware support, except for the boot core.
+Sysctl calls are routed to the boot core before doing any actual up/down
+operations on other cores.
+
+Configuration
+-------------
+
+The presence of the feature is controlled by CONFIG_CPU_HOTPLUG option. It is
+enabled unconditionally on x86 architecture. On Arm64, the option is enabled by
+default when ITS, FFA, and TEE configs are disabled.
+xen-hptool userspace tool is built unconditionally.
+
+Usage
+-----
+
+Disable core:
+
+$ xen-hptool cpu-offline 2
+Prepare to offline CPU 2
+(XEN) Removing cpu 2 from runqueue 0
+CPU 2 offlined successfully
+
+Enable core:
+
+$ xen-hptool cpu-online 2
+Prepare to online CPU 2
+(XEN) Bringing up CPU2
+(XEN) GICv3: CPU2: Found redistributor in region 0 @00000a004005c000
+(XEN) CPU2: Guest atomics will try 1 times before pausing the domain
+(XEN) CPU 2 booted.
+(XEN) Adding cpu 2 to runqueue 0
+CPU 2 onlined successfully
-- 
2.51.2