Re: [PATCH 2/2] migrate: More sophisticated versioning

[Stefano Brivio <sbrivio@redhat.com>]

On Fri, 31 Jan 2025 15:52:15 +1100
David Gibson <david@gibson.dropbear.id.au> wrote:

> At present the header for our device state migration stream is sent as
> a buffer in the sections array, like anything else.  It contains both
> version information, and details on the source ABI which are specific to
> v1 of the migration protocol.  Alter this for greater flexibility:
> 
>  * We separate out a minimal fixed header, which we will need to keep for
>    every future version, from a version specific header, containing (for
>    v1) the ABI data

What's the purpose of this if there should be no ABI data?

I have this for the moment:

	/* Immutable part of header structure: keep these two sections at the
	 * beginning, because they are enough to identify a version regardless
	 * of metadata.
	 */
	.magic		= MIGRATE_MAGIC,
	.version	= htonl_constant(MIGRATE_VERSION),
	/* End of immutable part of header structure */

...which to be honest already felt like wasted effort, despite being
just two comments.

>  * Handle both the headers separately from the data sections making for
>    better symmetry between the source and target sides
>  * Add a "compat_version" field.  This will allow us to make future
>    protocol extensions which are backwards compatible with older targets,
>    while retaining the ability to also make breaking protocol extensions.
> 
> This establishes a minimal header with fixed representation to maintain
> for all future versions.
> 
> Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
> ---
>  migrate.c | 157 ++++++++++++++++++++++++++++++++++++++++--------------
>  migrate.h |  26 ++++++---
>  2 files changed, 138 insertions(+), 45 deletions(-)
                    ^^^

...is this really a good idea if we want to drop this right away? I
might be missing something but I think it would more sense to focus on
the "targeted" data transfer instead.

-- 
Stefano