Re: [PATCH v6 01/20] liveupdate: luo_core: luo_ioctl: Live Update Orchestrator
From: Pratyush Yadav <pratyush@kernel.org>
Date: 2025-11-18 15:46:13
Also in:
linux-doc, linux-fsdevel, linux-mm, lkml
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@soleen.com>
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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@soleen.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. + */ +
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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
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+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@kernel.org> -- Regards, Pratyush Yadav