Re: [PATCH v2 08/15] block: Add blockdev-set-active QMP command

[Eric Blake]

On Thu, Jan 30, 2025 at 06:12:39PM +0100, Kevin Wolf wrote:
> The system emulator tries to automatically activate and inactivate block
> nodes at the right point during migration. However, there are still
> cases where it's necessary that the user can do this manually.
> 
> Images are only activated on the destination VM of a migration when the
> VM is actually resumed. If the VM was paused, this doesn't happen
> automatically. The user may want to perform some operation on a block
> device (e.g. taking a snapshot or starting a block job) without also
> resuming the VM yet. This is an example where a manual command is
> necessary.
> 
> Another example is VM migration when the image files are opened by an
> external qemu-storage-daemon instance on each side. In this case, the
> process that needs to hand over the images isn't even part of the
> migration and can't know when the migration completes. Management tools
> need a way to explicitly inactivate images on the source and activate
> them on the destination.
> 
> This adds a new blockdev-set-active QMP command that lets the user
> change the status of individual nodes (this is necessary in
> qemu-storage-daemon because it could be serving multiple VMs and only
> one of them migrates at a time). For convenience, operating on all
> devices (like QEMU does automatically during migration) is offered as an
> option, too, and can be used in the context of single VM.
> 
> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
> ---
>  qapi/block-core.json               | 32 ++++++++++++++++++++++++++++++
>  include/block/block-global-state.h |  3 +++
>  block.c                            | 21 ++++++++++++++++++++
>  blockdev.c                         | 32 ++++++++++++++++++++++++++++++
>  4 files changed, 88 insertions(+)
> 
> diff --git a/qapi/block-core.json b/qapi/block-core.json
> index 6029e54889..2ffb2efbc7 100644
> --- a/qapi/block-core.json
> +++ b/qapi/block-core.json
> @@ -4945,6 +4945,38 @@
>  { 'command': 'blockdev-del', 'data': { 'node-name': 'str' },
>    'allow-preconfig': true }
>  
> +##
> +# @blockdev-set-active:
> +#
> +# Activate or inactive a block device. Use this to manage the handover of 
> block

s/inactive/inactivate/

> +# devices on migration with qemu-storage-daemon.
> +#
> +# Activating a node automatically activates all of its child nodes first.
> +# Inactivating a node automatically inactivates any of its child nodes that 
> are
> +# not in use by a still active node.
> +#

With that minor fix, you can add:

Reviewed-by: Eric Blake <eblake@redhat.com>

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.
Virtualization:  qemu.org | libguestfs.org