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[142.167.56.70]) by smtp.gmail.com with ESMTPSA id 6a1803df08f44-6fd09577929sm59926056d6.86.2025.06.24.11.31.59 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 24 Jun 2025 11:31:59 -0700 (PDT) Received: from jgg by wakko with local (Exim 4.97) (envelope-from ) id 1uU8R4-00000000hJ0-3C5H; Tue, 24 Jun 2025 15:31:58 -0300 Date: Tue, 24 Jun 2025 15:31:58 -0300 From: Jason Gunthorpe To: Pratyush Yadav Cc: Pasha Tatashin , Mike Rapoport , jasonmiu@google.com, graf@amazon.com, changyuanl@google.com, dmatlack@google.com, rientjes@google.com, corbet@lwn.net, rdunlap@infradead.org, ilpo.jarvinen@linux.intel.com, kanie@linux.alibaba.com, ojeda@kernel.org, aliceryhl@google.com, masahiroy@kernel.org, akpm@linux-foundation.org, tj@kernel.org, yoann.congal@smile.fr, mmaurer@google.com, roman.gushchin@linux.dev, chenridong@huawei.com, axboe@kernel.dk, mark.rutland@arm.com, jannh@google.com, vincent.guittot@linaro.org, hannes@cmpxchg.org, dan.j.williams@intel.com, david@redhat.com, joel.granados@kernel.org, rostedt@goodmis.org, anna.schumaker@oracle.com, song@kernel.org, zhangguopeng@kylinos.cn, linux@weissschuh.net, linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, linux-mm@kvack.org, gregkh@linuxfoundation.org, tglx@linutronix.de, mingo@redhat.com, bp@alien8.de, dave.hansen@linux.intel.com, x86@kernel.org, hpa@zytor.com, rafael@kernel.org, dakr@kernel.org, bartosz.golaszewski@linaro.org, cw00.choi@samsung.com, myungjoo.ham@samsung.com, yesanishhere@gmail.com, Jonathan.Cameron@huawei.com, quic_zijuhu@quicinc.com, aleksander.lobakin@intel.com, ira.weiny@intel.com, andriy.shevchenko@linux.intel.com, leon@kernel.org, lukas@wunner.de, bhelgaas@google.com, wagi@kernel.org, djeffery@redhat.com, stuart.w.hayes@gmail.com Subject: Re: [RFC v2 05/16] luo: luo_core: integrate with KHO Message-ID: <20250624183158.GF72557@ziepe.ca> References: Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: On Tue, Jun 24, 2025 at 06:12:14PM +0200, Pratyush Yadav wrote: > On Fri, Jun 20 2025, Pasha Tatashin wrote: > > > On Fri, Jun 20, 2025 at 11:28 AM Pratyush Yadav wrote: > >> On Thu, Jun 19 2025, Pasha Tatashin wrote: > [...] > >> Outside of hypervisor live update, I have a very clear use case in mind: > >> userspace memory handover (on guest side). Say a guest running an > >> in-memory cache like memcached with many gigabytes of cache wants to > >> reboot. It can just shove the cache into a memfd, give it to LUO, and > >> restore it after reboot. Some services that suffer from long reboots are > >> looking into using this to reduce downtime. Since it pretty much > >> overlaps with the hypervisor work for now, I haven't been talking about > >> it as much. > >> > >> Would you also call this use case "live update"? Does it also fit with > >> your vision of where LUO should go? > > > > Yes, absolutely. The use case you described (preserving a memcached > > instance via memfd) is a perfect fit for LUO's vision. > > > > While the primary use case driving this work is supporting the > > preservation of virtual machines on a hypervisor, the framework itself > > is not restricted to that scenario. We define "live update" as the > > process of updating the kernel from one version to another while > > preserving FD-based resources and keeping selected devices > > operational. The machine itself can be running storage, database, > > networking, containers, or anything else. > > > > A good parallel is Kernel Live Patching: we don't distinguish what > > workload is running on a machine when applying a security patch; we > > simply patch the running kernel. In the same way, Live Update is > > designed to be workload-agnostic. Whether the system is running an > > in-memory database, containers, or VMs, its primary goal is to enable > > a full kernel update while preserving the userspace-requested state. > > Okay, then we are on the same page and I can live with whatever name we > go with :-) > > BTW, I think it would be useful to make this clarification on the LUO > docs as well so the intended use case/audience of the API is clear. > Currently the doc string in luo_core.c only talks about hypervisors and > VMs. Just to be clear though - you used the word "reboot" and here we are really only talking about kexec. kexec is not really a reboot, but it is sort of close. LUO is a way to pass lots of different things across a kexec, and if you are happy to use kexec then you should be able to use it. Jason