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We currently do not guarantee compatibility due to ongoing development, so mention this explicitly in the user-facing documentation. Signed-off-by: Luca Boccassi <luca.boccassi@gmail.com> Acked-by: Pratyush Yadav <pratyush@kernel.org> Link: https://patch.msgid.link/20260527153725.1770394-1-luca.boccassi@gmail.com Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
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52 lines
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.. SPDX-License-Identifier: GPL-2.0
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================
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Live Update uAPI
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================
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:Author: Pasha Tatashin <pasha.tatashin@soleen.com>
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ioctl interface
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===============
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.. kernel-doc:: kernel/liveupdate/luo_core.c
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:doc: LUO ioctl Interface
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ioctl uAPI
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===========
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.. kernel-doc:: include/uapi/linux/liveupdate.h
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Note on Compatibility
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=====================
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Note that the Live Update feature is still under development and subject to
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change, so compatibility is not guaranteed when kexec rebooting between
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different kernel versions. This is expected to change and stabilize in a future
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version.
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Userspace Integration
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=====================
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systemd (since version v261) uses LUO to preserve its per-service file
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descriptor store across a kexec-based live update. Services opt in by setting
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``FileDescriptorStoreMax=`` and ``FileDescriptorStorePreserve=`` in their unit,
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and push file descriptors with a name into the store via
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``sd_pid_notify_with_fds(... "FDSTORE=1\nFDNAME=foo")``.
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Services may also create their own LUO sessions (via ``/dev/liveupdate``) and
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push the resulting session fds into their file descriptor store like any other
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fd. systemd detects such session fds and handles them accordingly, and
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hands the re-retrieved session fd back to the service after kexec, using the
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existing file descriptor store service interface.
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For details, see:
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- `File Descriptor Store <https://systemd.io/FILE_DESCRIPTOR_STORE/>`_
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- `systemd.service(5) FileDescriptorStorePreserve=
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<https://www.freedesktop.org/software/systemd/man/latest/systemd.service.html#FileDescriptorStorePreserve=>`_
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- `sd_pid_notify_with_fds(3)
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<https://www.freedesktop.org/software/systemd/man/latest/sd_pid_notify_with_fds.html>`_
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See Also
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========
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- :doc:`Live Update Orchestrator </core-api/liveupdate>`
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