Re: [PATCH v3 2/3] migration: dirty-bitmap: Allow control of bitmap persistence

[Eric Blake]

On 2/12/21 11:34 AM, Peter Krempa wrote:
> Bitmap's source persistence is transported over the migration stream and
> the destination mirrors it. In some cases the destination might want to
> persist bitmaps which are not persistent on the source (e.g. the result
> of merge of bitmaps from a number of layers on the source when migrating

Sorry for not proposing this grammar tweak earlier, but
s/merge of/merging/

> into a squashed image) but currently it would need to create another set
> of persistent bitmaps and merge them.
> 
> This patch adds a 'transform' property to the alias map which allows to
> override the persistence of migrated bitmaps both on the source and

Once again, we encounter the non-idiomatic "allows to ${VERB}"; the
easiest solutions are "allows ${SUBJECT} to ${VERB}" or "allows
${VERB}ing". I'll go with the latter.

> destination sides.
> 
> Signed-off-by: Peter Krempa <pkrempa@redhat.com>
> ---
>  migration/block-dirty-bitmap.c | 30 +++++++++++++++++++++++++++---
>  qapi/migration.json            | 19 ++++++++++++++++++-
>  2 files changed, 45 insertions(+), 4 deletions(-)
> 

> @@ -806,7 +819,16 @@ static int dirty_bitmap_load_start(QEMUFile *f, 
> DBMLoadState *s)
>          return -EINVAL;
>      }
> 
> -    if (flags & DIRTY_BITMAP_MIG_START_FLAG_PERSISTENT) {
> +    if (s->bmap_inner &&
> +        s->bmap_inner->has_transform &&
> +        s->bmap_inner->transform &&

This leg of the conjunction is always true (if has_transform is set,
transform is necessarily non-NULL).

> +        s->bmap_inner->transform->has_persistent) {
> +        persistent = s->bmap_inner->transform->persistent;
> +    } else {
> +        persistent = flags & DIRTY_BITMAP_MIG_START_FLAG_PERSISTENT;
> +    }
> +
> +    if (persistent) {
>          bdrv_dirty_bitmap_set_persistence(s->bitmap, true);
>      }
> 

> @@ -544,12 +557,16 @@
>  # @alias: An alias name for migration (for example the bitmap name on
>  #         the opposite site).
>  #
> +# @transform: Allows to modify properties of the migrated bitmap.

Allows the modification of

I can make those tweaks while queuing.
Reviewed-by: Eric Blake <eblake@redhat.com>

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