Re: [Qemu-devel] [PATCH 3/6] vmstate: Add support for alias ID

[Blue Swirl]

On 5/13/10, Jan Kiszka <address@hidden> wrote:
> From: Jan Kiszka <address@hidden>
>
>  Some legacy users (mostly PC devices) of vmstate_register manage
>  instance IDs on their own, and that unfortunately in a way that is
>  incompatible with automatically generated ones. This so far prevents
>  switching those users to vmstates that are registered by qdev.
>
>  To establish a migration path, this patch introduces the concept of
>  alias IDs. They can be passed to an extended vmstate registration
>  service, and qdev provides a set service to be used during device init.
>  find_se will consider the alias in addition to the default ID. We can
>  then start generating the default ID automatically and writing it on
>  vmsave, thus converting that format without breaking support for upward
>  migration.

If this is only for compatibility, I think the name should show it,
like vmstate_set_compat_instance_id(), or
vmstate_set_legacy_instance_id(). That way, if there happens to be an
incompatible version bump, the function name suggests that it can be
removed.

The function should also take a last_legacy_version_id parameter.
Consider for example that a vmstate format with the legacy ID is
currently used with version_id of 2. We also start using this
compatibility system. A new, compatible version 3 arrives but we only
want to support legacy ID for version 2, as indicated by
last_legacy_version_id=2. Then with a new version, let's say 5, which
is no longer compatible with 2 or 3, the legacy ID stuff can finally
be thrown away. qdev.c should check if last_legacy_id >=
minimum_version_id and complain otherwise.