Re: [RFC v2 05/16] luo: luo_core: integrate with KHO

[Jason Gunthorpe]

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 <pratyush@xxxxxxxxxx> 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