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Message-ID: <aRnG8wDSSAtkEI_z@kernel.org>
Date: Sun, 16 Nov 2025 14:43:31 +0200
From: Mike Rapoport <rppt@...nel.org>
To: Pasha Tatashin <pasha.tatashin@...een.com>
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Subject: Re: [PATCH v6 02/20] liveupdate: luo_core: integrate with KHO
On Sat, Nov 15, 2025 at 06:33:48PM -0500, Pasha Tatashin wrote:
> Integrate the LUO with the KHO framework to enable passing LUO state
> across a kexec reboot.
>
> When LUO is transitioned to a "prepared" state, it tells KHO to
> finalize, so all memory segments that were added to KHO preservation
> list are getting preserved. After "Prepared" state no new segments
> can be preserved. If LUO is canceled, it also tells KHO to cancel the
> serialization, and therefore, later LUO can go back into the prepared
> state.
>
> This patch introduces the following changes:
> - During the KHO finalization phase allocate FDT blob.
This happens much earlier, isn't it?
> - Populate this FDT with a LUO compatibility string ("luo-v1").
>
> LUO now depends on `CONFIG_KEXEC_HANDOVER`. The core state transition
> logic (`luo_do_*_calls`) remains unimplemented in this patch.
>
> Signed-off-by: Pasha Tatashin <pasha.tatashin@...een.com>
> ---
> include/linux/liveupdate/abi/luo.h | 54 ++++++++++
> kernel/liveupdate/luo_core.c | 153 ++++++++++++++++++++++++++++-
> 2 files changed, 206 insertions(+), 1 deletion(-)
> create mode 100644 include/linux/liveupdate/abi/luo.h
>
> diff --git a/include/linux/liveupdate/abi/luo.h b/include/linux/liveupdate/abi/luo.h
> new file mode 100644
> index 000000000000..9483a294287f
> --- /dev/null
> +++ b/include/linux/liveupdate/abi/luo.h
> @@ -0,0 +1,54 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +
> +/*
> + * Copyright (c) 2025, Google LLC.
> + * Pasha Tatashin <pasha.tatashin@...een.com>
> + */
> +
> +/**
> + * DOC: Live Update Orchestrator ABI
> + *
> + * This header defines the stable Application Binary Interface used by the
> + * Live Update Orchestrator to pass state from a pre-update kernel to a
> + * post-update kernel. The ABI is built upon the Kexec HandOver framework
> + * and uses a Flattened Device Tree to describe the preserved data.
> + *
> + * This interface is a contract. Any modification to the FDT structure, node
> + * properties, compatible strings, or the layout of the `__packed` serialization
> + * structures defined here constitutes a breaking change. Such changes require
> + * incrementing the version number in the relevant `_COMPATIBLE` string to
> + * prevent a new kernel from misinterpreting data from an old kernel.
I'd add a sentence that stresses that ABI changes are possible as long they
include changes to the FDT version.
This is indeed implied by the last paragraph, but I think it's worth
spelling it explicitly.
Another thing that I think this should mention is that compatibility is
only guaranteed for the kernels that use the same ABI version.
> + *
> + * FDT Structure Overview:
> + * The entire LUO state is encapsulated within a single KHO entry named "LUO".
> + * This entry contains an FDT with the following layout:
> + *
> + * .. code-block:: none
> + *
> + * / {
> + * compatible = "luo-v1";
> + * liveupdate-number = <...>;
> + * };
> + *
> + * Main LUO Node (/):
> + *
> + * - compatible: "luo-v1"
> + * Identifies the overall LUO ABI version.
> + * - liveupdate-number: u64
> + * A counter tracking the number of successful live updates performed.
> + */
...
> +static int __init liveupdate_early_init(void)
> +{
> + int err;
> +
> + err = luo_early_startup();
> + if (err) {
> + pr_err("The incoming tree failed to initialize properly [%pe], disabling live update\n",
> + ERR_PTR(err));
How do we report this to the userspace?
I think the decision what to do in this case belongs there. Even if it's
down to choosing between plain kexec and full reboot, it's still a policy
that should be implemented in userspace.
> + luo_global.enabled = false;
> + }
> +
> + return err;
> +}
> +early_initcall(liveupdate_early_init);
--
Sincerely yours,
Mike.
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