Re: [RFC 1/6] migration: Add virtio-iterative capability

[Peter Xu <peterx@redhat.com>]

On Tue, Jul 22, 2025 at 12:41:22PM +0000, Jonah Palmer wrote:
> Adds a new migration capability 'virtio-iterative' that will allow
> virtio devices, where supported, to iteratively migrate configuration
> changes that occur during the migration process.
> 
> This capability is added to the validated capabilities list to ensure
> both the source and destination support it before enabling.
> 
> The capability defaults to off to maintain backward compatibility.
> 
> To enable the capability via HMP:
> (qemu) migrate_set_capability virtio-iterative on
> 
> To enable the capability via QMP:
> {"execute": "migrate-set-capabilities", "arguments": {
>      "capabilities": [
>         { "capability": "virtio-iterative", "state": true }
>      ]
>   }
> }
> 
> Signed-off-by: Jonah Palmer <jonah.palmer@oracle.com>
> ---
>  migration/savevm.c  | 1 +
>  qapi/migration.json | 7 ++++++-
>  2 files changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/migration/savevm.c b/migration/savevm.c
> index bb04a4520d..40a2189866 100644
> --- a/migration/savevm.c
> +++ b/migration/savevm.c
> @@ -279,6 +279,7 @@ static bool should_validate_capability(int capability)
>      switch (capability) {
>      case MIGRATION_CAPABILITY_X_IGNORE_SHARED:
>      case MIGRATION_CAPABILITY_MAPPED_RAM:
> +    case MIGRATION_CAPABILITY_VIRTIO_ITERATIVE:
>          return true;
>      default:
>          return false;
> diff --git a/qapi/migration.json b/qapi/migration.json
> index 4963f6ca12..8f042c3ba5 100644
> --- a/qapi/migration.json
> +++ b/qapi/migration.json
> @@ -479,6 +479,11 @@
>  #     each RAM page.  Requires a migration URI that supports seeking,
>  #     such as a file.  (since 9.0)
>  #
> +# @virtio-iterative: Enable iterative migration for virtio devices, if
> +#     the device supports it. When enabled, and where supported, virtio
> +#     devices will track and migrate configuration changes that may
> +#     occur during the migration process. (Since 10.1)
> +#

Having a migration capability to enable iterative support for a specific
type of device sounds wrong.

If virtio will be able to support iterative saves, it could provide the
save_live_iterate() function.  Any explanation why it needs to be a
migration capability?

>  # Features:
>  #
>  # @unstable: Members @x-colo and @x-ignore-shared are experimental.
> @@ -498,7 +503,7 @@
>             { 'name': 'x-ignore-shared', 'features': [ 'unstable' ] },
>             'validate-uuid', 'background-snapshot',
>             'zero-copy-send', 'postcopy-preempt', 'switchover-ack',
> -           'dirty-limit', 'mapped-ram'] }
> +           'dirty-limit', 'mapped-ram', 'virtio-iterative'] }
>  
>  ##
>  # @MigrationCapabilityStatus:
> -- 
> 2.47.1
> 

-- 
Peter Xu