Message-ID: <CA+CK2bDfxMhNQKjZD1uRSrg+wJYcFamji_oBvGBn+bnsb4Bbog@mail.gmail.com>
Date: Tue, 18 Nov 2025 11:11:18 -0500
From: Pasha Tatashin <pasha.tatashin@...een.com>
To: Pratyush Yadav <pratyush@...nel.org>
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Subject: Re: [PATCH v6 01/20] liveupdate: luo_core: luo_ioctl: Live Update Orchestrator

On Tue, Nov 18, 2025 at 10:46 AM Pratyush Yadav <pratyush@...nel.org> wrote:
>
> 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.

I removed mentioning live-patching.

>
> > + *
> > + * 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?

Removed the above and also removed liveupdate_exit(). Also changed:
module_init(liveupdate_ioctl_init); to late_initcall(liveupdate_ioctl_init);

> Other than these two nitpicks,
>
> Reviewed-by: Pratyush Yadav <pratyush@...nel.org>

Thank you!

Pasha

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