docs: sysctl: Add documentation for /proc/sys/xen/

Add documentation for the Xen hypervisor sysctl controls in
/proc/sys/xen/balloon/.

Documents the hotplug_unpopulated tunable (available when
CONFIG_XEN_BALLOON_MEMORY_HOTPLUG is enabled) which controls
whether unpopulated memory regions are automatically hotplugged
when the Xen balloon driver needs to reclaim memory.

The documentation is based on source code analysis of
drivers/xen/balloon.c.

Signed-off-by: Shubham Chakraborty <chakrabortyshubham66@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <20260304150419.16738-1-chakrabortyshubham66@gmail.com>

diff --git a/Documentation/admin-guide/sysctl/index.rst b/Documentation/admin-guide/sysctl/index.rst
index e153c96119df2729ea99c010aafca8af5bab1ea2..50f00514f0ffd4d6976a5747d91afeae1551e4b5 100644 (file)
--- a/Documentation/admin-guide/sysctl/index.rst
+++ b/Documentation/admin-guide/sysctl/index.rst
@@ -84,7 +84,7 @@ sunrpc/               SUN Remote Procedure Call (NFS)
 user/          Per user namespace limits
 vm/            memory management tuning
                buffer and cache management
-xen/           <undocumented>
+xen/           Xen hypervisor controls
 =============== ===============================================================
 
 These are the subdirs I have on my system or have been discovered by
@@ -104,3 +104,4 @@ it :-)
    sunrpc
    user
    vm
+   xen

diff --git a/Documentation/admin-guide/sysctl/xen.rst b/Documentation/admin-guide/sysctl/xen.rst
new file mode 100644 (file)
index 0000000..6c5edc3
--- /dev/null
+++ b/Documentation/admin-guide/sysctl/xen.rst
@@ -0,0 +1,31 @@
+===============
+/proc/sys/xen/
+===============
+
+Copyright (c) 2026, Shubham Chakraborty <chakrabortyshubham66@gmail.com>
+
+For general info and legal blurb, please look in
+Documentation/admin-guide/sysctl/index.rst.
+
+------------------------------------------------------------------------------
+
+These files show up in ``/proc/sys/xen/``, depending on the
+kernel configuration:
+
+.. contents:: :local:
+
+balloon/hotplug_unpopulated
+===========================
+
+This flag controls whether unpopulated memory ranges are automatically
+hotplugged as system RAM.
+
+- ``0``: Unpopulated ranges are not hotplugged (default).
+- ``1``: Unpopulated ranges are automatically hotplugged.
+
+When enabled, the Xen balloon driver will add memory regions that are
+marked as unpopulated in the Xen memory map to the system as usable RAM.
+This allows for dynamic memory expansion in Xen guest domains.
+
+This option is only available when the kernel is built with
+``CONFIG_XEN_BALLOON_MEMORY_HOTPLUG`` enabled.