Re: [PATCH v5 01/22] liveupdate: luo_core: luo_ioctl: Live Update Orchestrator

[Mike Rapoport <rppt@kernel.org>]

On Fri, Nov 07, 2025 at 04:02:59PM -0500, 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@soleen.com>
> ---

...

> @@ -0,0 +1,46 @@
> +/* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */
> +
> +/*
> + * Userspace interface for /dev/liveupdate
> + * Live Update Orchestrator
> + *
> + * Copyright (c) 2025, Google LLC.
> + * Pasha Tatashin <pasha.tatashin@soleen.com>
> + */
> +
> +#ifndef _UAPI_LIVEUPDATE_H
> +#define _UAPI_LIVEUPDATE_H
> +
> +#include <linux/ioctl.h>
> +#include <linux/types.h>
> +
> +/**
> + * DOC: General ioctl format
> + *

It seems it's not linked from Documentation/.../liveupdate.rst

> + * The ioctl interface follows a general format to allow for extensibility. Each
> + * ioctl is passed in a structure pointer as the argument providing the size of
> + * the structure in the first u32. The kernel checks that any structure space
> + * beyond what it understands is 0. This allows userspace to use the backward
> + * compatible portion while consistently using the newer, larger, structures.
> + *
> + * ioctls use a standard meaning for common errnos:
> + *
> + *  - ENOTTY: The IOCTL number itself is not supported at all
> + *  - E2BIG: The IOCTL number is supported, but the provided structure has
> + *    non-zero in a part the kernel does not understand.
> + *  - EOPNOTSUPP: The IOCTL number is supported, and the structure is
> + *    understood, however a known field has a value the kernel does not
> + *    understand or support.
> + *  - EINVAL: Everything about the IOCTL was understood, but a field is not
> + *    correct.
> + *  - ENOENT: A provided token does not exist.
> + *  - ENOMEM: Out of memory.
> + *  - EOVERFLOW: Mathematics overflowed.
> + *
> + * As well as additional errnos, within specific ioctls.

...

> --- a/kernel/liveupdate/Kconfig
> +++ b/kernel/liveupdate/Kconfig
> @@ -1,7 +1,34 @@
>  # SPDX-License-Identifier: GPL-2.0-only
> +#
> +# Copyright (c) 2025, Google LLC.
> +# Pasha Tatashin <pasha.tatashin@soleen.com>
> +#
> +# Live Update Orchestrator
> +#
>  
>  menu "Live Update and Kexec HandOver"
>  
> +config LIVEUPDATE
> +	bool "Live Update Orchestrator"
> +	depends on KEXEC_HANDOVER
> +	help
> +	  Enable the Live Update Orchestrator. Live Update is a mechanism,
> +	  typically based on kexec, that allows the kernel to be updated
> +	  while keeping selected devices operational across the transition.
> +	  These devices are intended to be reclaimed by the new kernel and
> +	  re-attached to their original workload without requiring a device
> +	  reset.
> +
> +	  Ability to handover a device from current to the next kernel depends
> +	  on specific support within device drivers and related kernel
> +	  subsystems.
> +
> +	  This feature primarily targets virtual machine hosts to quickly update
> +	  the kernel hypervisor with minimal disruption to the running virtual
> +	  machines.
> +
> +	  If unsure, say N.
> +

Not a big deal, but since LIVEUPDATE depends on KEXEC_HANDOVER, shouldn't
it go after KEXEC_HANDOVER?

>  config KEXEC_HANDOVER
>  	bool "kexec handover"
>  	depends on ARCH_SUPPORTS_KEXEC_HANDOVER && ARCH_SUPPORTS_KEXEC_FILE

-- 
Sincerely yours,
Mike.