Re: [PATCH 2/6] migration: introduce savevm, loadvm, delvm QMP commands

["Denis V. Lunev" <den@virtuozzo.com>]

On 7/2/20 8:57 PM, Daniel P. Berrangé wrote:
> savevm, loadvm and delvm are some of the few commands that have never
> been converted to use QMP. The primary reason for this lack of
> conversion is that they block execution of the thread for as long as
> they run.
>
> Despite this downside, however, libvirt and applications using libvirt
> has used these commands for as long as QMP has existed, via the
> "human-monitor-command" passthrough command. IOW, while it is clearly
> desirable to be able to fix the blocking problem, this is not an
> immediate obstacle to real world usage.
>
> Meanwhile there is a need for other features which involve adding new
> parameters to the commands. This is possible with HMP passthrough, but
> it provides no reliable way for apps to introspect features, so using
> QAPI modelling is highly desirable.
>
> This patch thus introduces trival savevm, loadvm, delvm commands
> to QMP that are functionally identical to the HMP counterpart, including
> the blocking problem.
>
> Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
> ---
>  migration/savevm.c  | 27 ++++++++++++++++
>  monitor/hmp-cmds.c  | 18 ++---------
>  qapi/migration.json | 76 +++++++++++++++++++++++++++++++++++++++++++++
>  3 files changed, 106 insertions(+), 15 deletions(-)
>
> diff --git a/migration/savevm.c b/migration/savevm.c
> index 72dbad95ed..53586a6406 100644
> --- a/migration/savevm.c
> +++ b/migration/savevm.c
> @@ -2943,3 +2943,30 @@ bool vmstate_check_only_migratable(const VMStateDescription *vmsd)
>  
>      return !(vmsd && vmsd->unmigratable);
>  }
> +
> +void qmp_savevm(const char *tag, Error **errp)
> +{
> +    save_snapshot(tag, errp);
> +}
> +
> +void qmp_loadvm(const char *tag, Error **errp)
> +{
> +    int saved_vm_running  = runstate_is_running();
> +
> +    vm_stop(RUN_STATE_RESTORE_VM);
> +
> +    if (load_snapshot(tag, errp) == 0 && saved_vm_running) {
> +        vm_start();
> +    }
> +}
> +
> +void qmp_delvm(const char *tag, Error **errp)
> +{
> +    BlockDriverState *bs;
> +
> +    if (bdrv_all_delete_snapshot(tag, &bs, errp) < 0) {
> +        error_prepend(errp,
> +                      "deleting snapshot on device '%s': ",
> +                      bdrv_get_device_or_node_name(bs));
> +    }
> +}
> diff --git a/monitor/hmp-cmds.c b/monitor/hmp-cmds.c
> index 2b0b58a336..26a5a1a701 100644
> --- a/monitor/hmp-cmds.c
> +++ b/monitor/hmp-cmds.c
> @@ -1089,15 +1089,9 @@ void hmp_balloon(Monitor *mon, const QDict *qdict)
>  
>  void hmp_loadvm(Monitor *mon, const QDict *qdict)
>  {
> -    int saved_vm_running  = runstate_is_running();
> -    const char *name = qdict_get_str(qdict, "name");
>      Error *err = NULL;
>  
> -    vm_stop(RUN_STATE_RESTORE_VM);
> -
> -    if (load_snapshot(name, &err) == 0 && saved_vm_running) {
> -        vm_start();
> -    }
> +    qmp_loadvm(qdict_get_str(qdict, "name"), &err);
>      hmp_handle_error(mon, err);
>  }
>  
> @@ -1105,21 +1099,15 @@ void hmp_savevm(Monitor *mon, const QDict *qdict)
>  {
>      Error *err = NULL;
>  
> -    save_snapshot(qdict_get_try_str(qdict, "name"), &err);
> +    qmp_savevm(qdict_get_try_str(qdict, "name"), &err);
>      hmp_handle_error(mon, err);
>  }
>  
>  void hmp_delvm(Monitor *mon, const QDict *qdict)
>  {
> -    BlockDriverState *bs;
>      Error *err = NULL;
> -    const char *name = qdict_get_str(qdict, "name");
>  
> -    if (bdrv_all_delete_snapshot(name, &bs, &err) < 0) {
> -        error_prepend(&err,
> -                      "deleting snapshot on device '%s': ",
> -                      bdrv_get_device_name(bs));
> -    }
> +    qmp_delvm(qdict_get_str(qdict, "name"), &err);
>      hmp_handle_error(mon, err);
>  }
>  
> diff --git a/qapi/migration.json b/qapi/migration.json
> index d5000558c6..849de38fb0 100644
> --- a/qapi/migration.json
> +++ b/qapi/migration.json
> @@ -1621,3 +1621,79 @@
>  ##
>  { 'event': 'UNPLUG_PRIMARY',
>    'data': { 'device-id': 'str' } }
> +
> +##
> +# @savevm:
> +#
> +# Save a VM snapshot
> +#
> +# @tag: name of the snapshot to create. If it already
> +# exists it will be replaced.
> +#
> +# Note that execution of the VM will be paused during the time
> +# it takes to save the snapshot
> +#
> +# Returns: nothing
> +#
> +# Example:
> +#
> +# -> { "execute": "savevm",
> +#      "data": {
> +#         "tag": "my-snap"
> +#      }
> +#    }
> +# <- { "return": { } }
> +#
> +# Since: 5.2
> +##
> +{ 'command': 'savevm',
> +  'data': { 'tag': 'str' } }
> +
> +##
> +# @loadvm:
> +#
> +# Load a VM snapshot
> +#
> +# @tag: name of the snapshot to load.
> +#
> +# Returns: nothing
> +#
> +# Example:
> +#
> +# -> { "execute": "loadvm",
> +#      "data": {
> +#         "tag": "my-snap"
> +#      }
> +#    }
> +# <- { "return": { } }
> +#
> +# Since: 5.2
> +##
> +{ 'command': 'loadvm',
> +  'data': { 'tag': 'str' } }
> +
> +##
> +# @delvm:
> +#
> +# Delete a VM snapshot
> +#
> +# @tag: name of the snapshot to delete.
> +#
> +# Note that execution of the VM will be paused during the time
> +# it takes to delete the snapshot
> +#
> +# Returns: nothing
> +#
> +# Example:
> +#
> +# -> { "execute": "delvm",
> +#      "data": {
> +#         "tag": "my-snap"
> +#      }
> +#    }
> +# <- { "return": { } }
> +#
> +# Since: 5.2
> +##
> +{ 'command': 'delvm',
> +  'data': { 'tag': 'str' } }
The interface proposed is not good from the interface point of view.
The problem is that savevm and loadvm are VERY lengthy operations
especially for VM with big amount of RAM. Thus they can run
for hours f.e. with 1 Tb RAM.

This "feature" was available in the old legacy HMP interface, but
this should be fixed in the new and clear one.

Den