linux-kernel - Re: [PATCH v2 06/10] PCI/LUO: Save and restore driver name

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Message-ID: <20251002132315.GC3195829@ziepe.ca>
Date: Thu, 2 Oct 2025 10:23:15 -0300
From: Jason Gunthorpe <jgg@...pe.ca>
To: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc: Pasha Tatashin <pasha.tatashin@...een.com>,
	Chris Li <chrisl@...nel.org>, Bjorn Helgaas <bhelgaas@...gle.com>,
	"Rafael J. Wysocki" <rafael@...nel.org>,
	Danilo Krummrich <dakr@...nel.org>, Len Brown <lenb@...nel.org>,
	linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org,
	linux-acpi@...r.kernel.org, David Matlack <dmatlack@...gle.com>,
	Pasha Tatashin <tatashin@...gle.com>,
	Jason Miu <jasonmiu@...gle.com>, Vipin Sharma <vipinsh@...gle.com>,
	Saeed Mahameed <saeedm@...dia.com>,
	Adithya Jayachandran <ajayachandra@...dia.com>,
	Parav Pandit <parav@...dia.com>, William Tu <witu@...dia.com>,
	Mike Rapoport <rppt@...nel.org>, Leon Romanovsky <leon@...nel.org>
Subject: Re: [PATCH v2 06/10] PCI/LUO: Save and restore driver name

On Thu, Oct 02, 2025 at 08:09:11AM +0200, Greg Kroah-Hartman wrote:
> > However, it is my hope that we will
> > eventually stabilize this process and only allow breakages between,
> > for example, versions 6.n and 6.n+2, and eventually from one stable
> > release to stable+2. This would create a well-defined window for
> > safely removing deprecated data formats and the code that handles them
> > from the kernel.
> 
> How are you going to define this?  We can not break old users when they
> upgrade, and so you are going to have to support this "upgrade path" for
> forever.

I think the realistic proposal for LUO/kexec version compatability is
more like eBPF. Expressly saying it is not ABI, not stable, but here
are a bunch of tools and it is still useful.

> Just keeping a device "alive" while rebooting into the same exact kernel
> image seems odd to me given that this is almost never what people
> actually do.

This feature has a lot of development to go. Right now the baseline
for upstream is no ABI promise. You can live update between any two
kernel versions that don't change the LUO kexec ABI. In practice that
will be a lot of version pairs.

The downstreams are going to take this raw capability and choose
specific downstream version pairs, patch in support for certain ABI
versions that they need, and test.

When things mature and the project is more complete then the kernel
community may have a discussion about what upstream version pairs
should be supported by the community.

I don't think this would be as broad as every combination of linux
versions ever, but ideas like sequential pairs of stable
releases, sequential pairs of main release and so on are worth
exploring.

Jason