Message-ID: <mafs0ecpv4a4q.fsf@kernel.org>
Date: Tue, 18 Nov 2025 16:45:57 +0100
From: Pratyush Yadav <pratyush@...nel.org>
To: Pasha Tatashin <pasha.tatashin@...een.com>
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Subject: Re: [PATCH v6 01/20] liveupdate: luo_core: luo_ioctl: Live Update
 Orchestrator

On Sat, Nov 15 2025, Pasha Tatashin wrote:

> Introduce LUO, a mechanism intended to facilitate kernel updates while
> keeping designated devices operational across the transition (e.g., via
> kexec). The primary use case is updating hypervisors with minimal
> disruption to running virtual machines. For userspace side of hypervisor
> update we have copyless migration. LUO is for updating the kernel.
>
> This initial patch lays the groundwork for the LUO subsystem.
>
> Further functionality, including the implementation of state transition
> logic, integration with KHO, and hooks for subsystems and file
> descriptors, will be added in subsequent patches.
>
> Create a character device at /dev/liveupdate.
>
> A new uAPI header, <uapi/linux/liveupdate.h>, will define the necessary
> structures. The magic number for IOCTL is registered in
> Documentation/userspace-api/ioctl/ioctl-number.rst.
>
> Signed-off-by: Pasha Tatashin <pasha.tatashin@...een.com>
[...]
> diff --git a/kernel/liveupdate/luo_core.c b/kernel/liveupdate/luo_core.c
> new file mode 100644
> index 000000000000..0e1ab19fa1cd
> --- /dev/null
> +++ b/kernel/liveupdate/luo_core.c
> @@ -0,0 +1,86 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +/*
> + * Copyright (c) 2025, Google LLC.
> + * Pasha Tatashin <pasha.tatashin@...een.com>
> + */
> +
> +/**
> + * DOC: Live Update Orchestrator (LUO)
> + *
> + * Live Update is a specialized, kexec-based reboot process that allows a
> + * running kernel to be updated from one version to another while preserving
> + * the state of selected resources and keeping designated hardware devices
> + * operational. For these devices, DMA activity may continue throughout the
> + * kernel transition.
> + *
> + * While the primary use case driving this work is supporting live updates of
> + * the Linux kernel when it is used as a hypervisor in cloud environments, the
> + * LUO framework itself is designed to be workload-agnostic. Much like Kernel
> + * Live Patching, which applies security fixes regardless of the workload,
> + * Live Update facilitates a full kernel version upgrade for any type of system.

Nit: I think live update is very different from live patching. It has
very different limitations and advantages. In fact, I view live patching
and live update on two opposite ends of the "applying security patches"
spectrum. I think this line is going to mislead or confuse people.

I think it would better to either spend more lines explaining the
difference between the two, or just drop it from here.

> + *
> + * For example, a non-hypervisor system running an in-memory cache like
> + * memcached with many gigabytes of data can use LUO. The userspace service
> + * can place its cache into a memfd, have its state preserved by LUO, and
> + * restore it immediately after the kernel kexec.
> + *
> + * Whether the system is running virtual machines, containers, a
> + * high-performance database, or networking services, LUO's primary goal is to
> + * enable a full kernel update by preserving critical userspace state and
> + * keeping essential devices operational.
> + *
> + * The core of LUO is a mechanism that tracks the progress of a live update,
> + * along with a callback API that allows other kernel subsystems to participate
> + * in the process. Example subsystems that can hook into LUO include: kvm,
> + * iommu, interrupts, vfio, participating filesystems, and memory management.
> + *
> + * LUO uses Kexec Handover to transfer memory state from the current kernel to
> + * the next kernel. For more details see
> + * Documentation/core-api/kho/concepts.rst.
> + */
> +
[...]
> diff --git a/kernel/liveupdate/luo_ioctl.c b/kernel/liveupdate/luo_ioctl.c
> new file mode 100644
> index 000000000000..44d365185f7c
> --- /dev/null
> +++ b/kernel/liveupdate/luo_ioctl.c
[...]
> +MODULE_LICENSE("GPL");
> +MODULE_AUTHOR("Pasha Tatashin");
> +MODULE_DESCRIPTION("Live Update Orchestrator");
> +MODULE_VERSION("0.1");

Nit: do we really need the module version? I don't think LUO can even be
used as a module. What does this number mean then?

Other than these two nitpicks,

Reviewed-by: Pratyush Yadav <pratyush@...nel.org>

-- 
Regards,
Pratyush Yadav

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