Re: [RFC v2 00/16] Live Update Orchestrator

[Mike Rapoport <rppt@kernel.org>]

(cc'ing linux-api)

On Thu, May 15, 2025 at 06:23:04PM +0000, Pasha Tatashin wrote:
> This v2 series introduces the LUO, a kernel subsystem designed to
> facilitate live kernel updates with minimal downtime,
> particularly in cloud delplyoments aiming to update without fully
> disrupting running virtual machines.
> 
> This series builds upon KHO framework [1] by adding programmatic
> control over KHO's lifecycle and leveraging KHO for persisting LUO's
> own metadata across the kexec boundary. The git branch for this series
> can be found at:
> https://github.com/googleprodkernel/linux-liveupdate/tree/luo/rfc-v2
> 
> Changelog from v1:
> - Control Interface: Shifted from sysfs-based control
>   (/sys/kernel/liveupdate/{prepare,finish}) to an ioctl interface
>   (/dev/liveupdate). Sysfs is now primarily for monitoring the state.
> - Event/State Renaming: LIVEUPDATE_REBOOT event/phase is now
>   LIVEUPDATE_FREEZE.
> - FD Preservation: A new component for preserving file descriptors.
>   Subsystem Registration: A formal mechanism for kernel subsystems
>   to participate.
> - Device Layer: removed device list handling from this series, it is
>   going to be added separately.
> - Selftests: Kernel-side selftest hooks and userspace selftests are
>   now included.
> KHO Enhancements:
> - KHO debugfs became optional, and kernel APIs for finalize/abort
>   were added (driven by LUO's needs).
> - KHO unpreserve functions were also added.
> 
> What is Live Update?
> Live Update is a specialized reboot process where selected kernel
> resources (memory, file descriptors, and eventually devices) are kept
> operational or their state preserved across a kernel transition (e.g.,
> via kexec). For certain resources, DMA and interrupt activity might
> continue with minimal interruption during the kernel reboot.
> 
> LUO v2 Overview:
> LUO v2 provides a framework for coordinating live updates. It features:
> State Machine: Manages the live update process through states:
> NORMAL, PREPARED, FROZEN, UPDATED.
> 
> KHO Integration:
> 
> LUO programmatically drives KHO's finalization and abort sequences.
> KHO's debugfs interface is now optional configured via
> CONFIG_KEXEC_HANDOVER_DEBUG.
> 
> LUO preserves its own metadata via KHO's kho_add_subtree and
> kho_preserve_phys() mechanisms.
> 
> Subsystem Participation: A callback API liveupdate_register_subsystem()
> allows kernel subsystems (e.g., KVM, IOMMU, VFIO, PCI) to register
> handlers for LUO events (PREPARE, FREEZE, FINISH, CANCEL) and persist a
> u64 payload via the LUO FDT.
> 
> File Descriptor Preservation: Infrastructure
> liveupdate_register_filesystem, luo_register_file, luo_retrieve_file to
> allow specific types of file descriptors (e.g., memfd, vfio) to be
> preserved and restored.
> 
> Handlers for specific file types can be registered to manage their
> preservation and restoration, storing a u64 payload in the LUO FDT.
> 
> Example WIP for memfd preservation can be found here [2].
> 
> User-space Interface:
> 
> ioctl (/dev/liveupdate): The primary control interface for
> triggering LUO state transitions (prepare, freeze, finish, cancel)
> and managing the preservation/restoration of file descriptors.
> Access requires CAP_SYS_ADMIN.
> 
> sysfs (/sys/kernel/liveupdate/state): A read-only interface for
> monitoring the current LUO state. This allows userspace services to
> track progress and coordinate actions.
> 
> Selftests: Includes kernel-side hooks and userspace selftests to
> verify core LUO functionality, particularly subsystem registration and
> basic state transitions.
> 
> LUO State Machine and Events:
> 
> NORMAL:   Default operational state.
> PREPARED: Initial preparation complete after LIVEUPDATE_PREPARE
>           event. Subsystems have saved initial state.
> FROZEN:   Final "blackout window" state after LIVEUPDATE_FREEZE
>           event, just before kexec. Workloads must be suspended.
> UPDATED:  Next kernel has booted via live update. Awaiting restoration
>           and LIVEUPDATE_FINISH.
> 
> Events:
> LIVEUPDATE_PREPARE: Prepare for reboot, serialize state.
> LIVEUPDATE_FREEZE:  Final opportunity to save state before kexec.
> LIVEUPDATE_FINISH:  Post-reboot cleanup in the next kernel.
> LIVEUPDATE_CANCEL:  Abort prepare or freeze, revert changes.
> 
> [1] https://lore.kernel.org/all/20250509074635.3187114-1-changyuanl@google.com
>     https://github.com/googleprodkernel/linux-liveupdate/tree/luo/kho-v8
> [2] https://github.com/googleprodkernel/linux-liveupdate/tree/luo/memfd-v0.1
> 
> RFC v1: https://lore.kernel.org/all/20250320024011.2995837-1-pasha.tatashin@soleen.com
> 
> Changyuan Lyu (1):
>   kho: add kho_unpreserve_folio/phys
> 
> Pasha Tatashin (15):
>   kho: make debugfs interface optional
>   kho: allow to drive kho from within kernel
>   luo: luo_core: Live Update Orchestrator
>   luo: luo_core: integrate with KHO
>   luo: luo_subsystems: add subsystem registration
>   luo: luo_subsystems: implement subsystem callbacks
>   luo: luo_files: add infrastructure for FDs
>   luo: luo_files: implement file systems callbacks
>   luo: luo_ioctl: add ioctl interface
>   luo: luo_sysfs: add sysfs state monitoring
>   reboot: call liveupdate_reboot() before kexec
>   luo: add selftests for subsystems un/registration
>   selftests/liveupdate: add subsystem/state tests
>   docs: add luo documentation
>   MAINTAINERS: add liveupdate entry
> 
>  .../ABI/testing/sysfs-kernel-liveupdate       |  51 ++
>  Documentation/admin-guide/index.rst           |   1 +
>  Documentation/admin-guide/liveupdate.rst      |  62 ++
>  .../userspace-api/ioctl/ioctl-number.rst      |   1 +
>  MAINTAINERS                                   |  14 +-
>  drivers/misc/Kconfig                          |   1 +
>  drivers/misc/Makefile                         |   1 +
>  drivers/misc/liveupdate/Kconfig               |  60 ++
>  drivers/misc/liveupdate/Makefile              |   7 +
>  drivers/misc/liveupdate/luo_core.c            | 547 +++++++++++++++
>  drivers/misc/liveupdate/luo_files.c           | 664 ++++++++++++++++++
>  drivers/misc/liveupdate/luo_internal.h        |  59 ++
>  drivers/misc/liveupdate/luo_ioctl.c           | 203 ++++++
>  drivers/misc/liveupdate/luo_selftests.c       | 283 ++++++++
>  drivers/misc/liveupdate/luo_selftests.h       |  23 +
>  drivers/misc/liveupdate/luo_subsystems.c      | 413 +++++++++++
>  drivers/misc/liveupdate/luo_sysfs.c           |  92 +++
>  include/linux/kexec_handover.h                |  27 +
>  include/linux/liveupdate.h                    | 214 ++++++
>  include/uapi/linux/liveupdate.h               | 324 +++++++++
>  kernel/Kconfig.kexec                          |  10 +
>  kernel/Makefile                               |   1 +
>  kernel/kexec_handover.c                       | 343 +++------
>  kernel/kexec_handover_debug.c                 | 237 +++++++
>  kernel/kexec_handover_internal.h              |  74 ++
>  kernel/reboot.c                               |   4 +
>  tools/testing/selftests/Makefile              |   1 +
>  tools/testing/selftests/liveupdate/.gitignore |   1 +
>  tools/testing/selftests/liveupdate/Makefile   |   7 +
>  tools/testing/selftests/liveupdate/config     |   6 +
>  .../testing/selftests/liveupdate/liveupdate.c | 440 ++++++++++++
>  31 files changed, 3933 insertions(+), 238 deletions(-)
>  create mode 100644 Documentation/ABI/testing/sysfs-kernel-liveupdate
>  create mode 100644 Documentation/admin-guide/liveupdate.rst
>  create mode 100644 drivers/misc/liveupdate/Kconfig
>  create mode 100644 drivers/misc/liveupdate/Makefile
>  create mode 100644 drivers/misc/liveupdate/luo_core.c
>  create mode 100644 drivers/misc/liveupdate/luo_files.c
>  create mode 100644 drivers/misc/liveupdate/luo_internal.h
>  create mode 100644 drivers/misc/liveupdate/luo_ioctl.c
>  create mode 100644 drivers/misc/liveupdate/luo_selftests.c
>  create mode 100644 drivers/misc/liveupdate/luo_selftests.h
>  create mode 100644 drivers/misc/liveupdate/luo_subsystems.c
>  create mode 100644 drivers/misc/liveupdate/luo_sysfs.c
>  create mode 100644 include/linux/liveupdate.h
>  create mode 100644 include/uapi/linux/liveupdate.h
>  create mode 100644 kernel/kexec_handover_debug.c
>  create mode 100644 kernel/kexec_handover_internal.h
>  create mode 100644 tools/testing/selftests/liveupdate/.gitignore
>  create mode 100644 tools/testing/selftests/liveupdate/Makefile
>  create mode 100644 tools/testing/selftests/liveupdate/config
>  create mode 100644 tools/testing/selftests/liveupdate/liveupdate.c
> 
> -- 
> 2.49.0.1101.gccaa498523-goog
> 

-- 
Sincerely yours,
Mike.