Re: [Qemu-devel] [PATCH] savevm: Ignore minimum_version_id_old if there is no load_state_old

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Mar 31, 2014 at 05:37:06PM +0100, Peter Maydell wrote:
> At the moment we require vmstate definitions to set minimum_version_id_old
> to the same value as minimum_version_id if they do not provide a
> load_state_old handler. Since the load_state_old functionality is
> required only for a handful of devices that need to retain migration
> compatibility with a pre-vmstate implementation, this means the bulk
> of devices have pointless boilerplate. Relax the definition so that
> minimum_version_id_old is ignored if there is no load_state_old handler.
> 
> Note that under the old scheme we would segfault if the vmstate
> specified a minimum_version_id_old that was less than minimum_version_id
> but did not provide a load_state_old function, and the incoming state
> specified a version number between minimum_version_id_old and
> minimum_version_id. Under the new scheme this will just result in
> our failing the migration.
> 
> Signed-off-by: Peter Maydell <peter.maydell@linaro.org>


Reviewed-by: Michael S. Tsirkin <mst@redhat.com>

> ---
> There's obviously scope for subsequent cleanup to delete all the
> now-redundant setting of minimum_version_id_old in vmstates; in this
> patch I only changed the occurrences in the documentation.
> 
>  docs/migration.txt | 12 +++++-------
>  vmstate.c          |  9 +++++----
>  2 files changed, 10 insertions(+), 11 deletions(-)
> 
> diff --git a/docs/migration.txt b/docs/migration.txt
> index 0e0a1d4..fe1f2bb 100644
> --- a/docs/migration.txt
> +++ b/docs/migration.txt
> @@ -139,7 +139,6 @@ static const VMStateDescription vmstate_kbd = {
>      .name = "pckbd",
>      .version_id = 3,
>      .minimum_version_id = 3,
> -    .minimum_version_id_old = 3,
>      .fields      = (VMStateField []) {
>          VMSTATE_UINT8(write_cmd, KBDState),
>          VMSTATE_UINT8(status, KBDState),
> @@ -168,12 +167,13 @@ You can see that there are several version fields:
>  - minimum_version_id: the minimum version_id that VMState is able to understand
>    for that device.
>  - minimum_version_id_old: For devices that were not able to port to vmstate, we can
> -  assign a function that knows how to read this old state.
> +  assign a function that knows how to read this old state. This field is
> +  ignored if there is no load_state_old handler.
>  
>  So, VMState is able to read versions from minimum_version_id to
> -version_id.  And the function load_state_old() is able to load state
> -from minimum_version_id_old to minimum_version_id.  This function is
> -deprecated and will be removed when no more users are left.
> +version_id.  And the function load_state_old() (if present) is able to
> +load state from minimum_version_id_old to minimum_version_id.  This
> +function is deprecated and will be removed when no more users are left.
>  
>  ===  Massaging functions ===
>  
> @@ -255,7 +255,6 @@ const VMStateDescription vmstate_ide_drive_pio_state = {
>      .name = "ide_drive/pio_state",
>      .version_id = 1,
>      .minimum_version_id = 1,
> -    .minimum_version_id_old = 1,
>      .pre_save = ide_drive_pio_pre_save,
>      .post_load = ide_drive_pio_post_load,
>      .fields      = (VMStateField []) {
> @@ -275,7 +274,6 @@ const VMStateDescription vmstate_ide_drive = {
>      .name = "ide_drive",
>      .version_id = 3,
>      .minimum_version_id = 0,
> -    .minimum_version_id_old = 0,
>      .post_load = ide_drive_post_load,
>      .fields      = (VMStateField []) {
>          .... several fields ....
> diff --git a/vmstate.c b/vmstate.c
> index b689f2f..bfa34cc 100644
> --- a/vmstate.c
> +++ b/vmstate.c
> @@ -19,11 +19,12 @@ int vmstate_load_state(QEMUFile *f, const VMStateDescription *vmsd,
>      if (version_id > vmsd->version_id) {
>          return -EINVAL;
>      }
> -    if (version_id < vmsd->minimum_version_id_old) {
> -        return -EINVAL;
> -    }
>      if  (version_id < vmsd->minimum_version_id) {
> -        return vmsd->load_state_old(f, opaque, version_id);
> +        if (vmsd->load_state_old &&
> +            version_id >= vmsd->minimum_version_id_old) {
> +            return vmsd->load_state_old(f, opaque, version_id);
> +        }
> +        return -EINVAL;
>      }
>      if (vmsd->pre_load) {
>          int ret = vmsd->pre_load(opaque);
> -- 
> 1.9.0