liveupdate: document systemd support

systemd v261 will add native support for LUO via the file descriptor
store mechanism. Add a brief paragraph in the LUO userspace documentation
page to inform users.

Signed-off-by: Luca Boccassi <luca.boccassi@gmail.com>
Reviewed-by: Pasha Tatashin <pasha.tatashin@soleen.com>
Acked-by: Pratyush Yadav <pratyush@kernel.org>
Link: https://patch.msgid.link/20260519152957.548119-1-luca.boccassi@gmail.com
Signed-off-by: Pasha Tatashin <pasha.tatashin@soleen.com>
Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>

Author: bluca

Documentation/userspace-api/liveupdate.rst

@@ -14,6 +14,29 @@ ioctl uAPI
 ===========
 .. kernel-doc:: include/uapi/linux/liveupdate.h
 
+Userspace Integration
+=====================
+
+systemd (since version v261) uses LUO to preserve its per-service file
+descriptor store across a kexec-based live update. Services opt in by setting
+``FileDescriptorStoreMax=`` and ``FileDescriptorStorePreserve=`` in their unit,
+and push file descriptors with a name into the store via
+``sd_pid_notify_with_fds(... "FDSTORE=1\nFDNAME=foo")``.
+
+Services may also create their own LUO sessions (via ``/dev/liveupdate``) and
+push the resulting session fds into their file descriptor store like any other
+fd. systemd detects such session fds and handles them accordingly, and
+hands the re-retrieved session fd back to the service after kexec, using the
+existing file descriptor store service interface.
+
+For details, see:
+
+- `File Descriptor Store <https://systemd.io/FILE_DESCRIPTOR_STORE/>`_
+- `systemd.service(5) FileDescriptorStorePreserve=
+  <https://www.freedesktop.org/software/systemd/man/latest/systemd.service.html#FileDescriptorStorePreserve=>`_
+- `sd_pid_notify_with_fds(3)
+  <https://www.freedesktop.org/software/systemd/man/latest/sd_pid_notify_with_fds.html>`_
+
 See Also
 ========