Re: [RFC v2] migration: Add migrate-set-bitmap-node-mapping

[Eric Blake]

On 5/13/20 9:56 AM, Max Reitz wrote:
> This command allows mapping block node names to aliases for the purpose
> of block dirty bitmap migration.
>
> This way, management tools can use different node names on the source
> and destination and pass the mapping of how bitmaps are to be
> transferred to qemu (on the source, the destination, or even both with
> arbitrary aliases in the migration stream).
>
> Suggested-by: Vladimir Sementsov-Ogievskiy <address@hidden>
> Signed-off-by: Max Reitz <address@hidden>
> ---

> @@ -713,6 +731,44 @@ static bool dirty_bitmap_has_postcopy(void *opaque)
>       return true;
>   }
>   
> +void qmp_migrate_set_bitmap_node_mapping(MigrationBlockNodeMappingList *mapping,
> +                                         Error **errp)
> +{
> +    QDict *in_mapping = qdict_new();
> +    QDict *out_mapping = qdict_new();
> +
> +    for (; mapping; mapping = mapping->next) {
> +        MigrationBlockNodeMapping *entry = mapping->value;
> +
> +        if (qdict_haskey(out_mapping, entry->node_name)) {
> +            error_setg(errp, "Cannot map node name '%s' twice",
> +                       entry->node_name);
> +            goto fail;
> +        }

Can we call this command more than once?  Is it cumulative (call it once 
to set mapping for "a", second time to also set mapping for "b"), or 
should it reset (second call wipes out all mappings from first call, any 
mappings that must exist must be passed in the final call)?

The idea makes sense, and the interface seems usable.  It's nice that 
either source, destination, or both sides of migration can use it (which 
helps in upgrade vs. downgrade scenarios).

--
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org