Re: [Qemu-devel] [PATCH v6 27/28] migration/qmp: add command migrate-pause

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Peter Xu (peterx@redhat.com) wrote:
> It pauses an ongoing migration.  Currently it only supports postcopy.
> Note that this command will work on either side of the migration.
> Basically when we trigger this on one side, it'll interrupt the other
> side as well since the other side will get notified on the disconnect
> event.
> 
> However, it's still possible that the other side is not notified, for
> example, when the network is totally broken, or due to some firewall
> configuration changes.  In that case, we will also need to run the same
> command on the other side so both sides will go into the paused state.
> 
> Signed-off-by: Peter Xu <peterx@redhat.com>
> ---
>  migration/migration.c | 27 +++++++++++++++++++++++++++
>  qapi/migration.json   | 16 ++++++++++++++++
>  2 files changed, 43 insertions(+)
> 
> diff --git a/migration/migration.c b/migration/migration.c
> index bb57ed9ade..139abec0c3 100644
> --- a/migration/migration.c
> +++ b/migration/migration.c
> @@ -1448,6 +1448,33 @@ void qmp_migrate_recover(const char *uri, Error **errp)
>      qemu_start_incoming_migration(uri, errp);
>  }
>  
> +void qmp_migrate_pause(Error **errp)
> +{
> +    MigrationState *ms = migrate_get_current();
> +    MigrationIncomingState *mis = migration_incoming_get_current();
> +    int ret;
> +
> +    if (ms->state == MIGRATION_STATUS_POSTCOPY_ACTIVE) {
> +        /* Source side, during postcopy */
> +        ret = qemu_file_shutdown(ms->to_dst_file);

This doesn't feel thread safe; although I'm not sure how to make it so.
If the migration finishes just after we check the state but before the
shutdown we end up using a bogus QEMUFile*
Making all the places that close a QEMUFile* set hte pointer Null before
they do the close doesn't help because you still race with that.

(The race is small, but still)

Dave

> +        if (ret) {
> +            error_setg(errp, "Failed to pause source migration");
> +        }
> +        return;
> +    }
> +
> +    if (mis->state == MIGRATION_STATUS_POSTCOPY_ACTIVE) {
> +        ret = qemu_file_shutdown(mis->from_src_file);
> +        if (ret) {
> +            error_setg(errp, "Failed to pause destination migration");
> +        }
> +        return;
> +    }
> +
> +    error_setg(errp, "migrate-pause is currently only supported "
> +               "during postcopy-active state");
> +}
> +
>  bool migration_is_blocked(Error **errp)
>  {
>      if (qemu_savevm_state_blocked(errp)) {
> diff --git a/qapi/migration.json b/qapi/migration.json
> index dfbcb02d4c..3d9cfeb8f1 100644
> --- a/qapi/migration.json
> +++ b/qapi/migration.json
> @@ -1192,3 +1192,19 @@
>  ##
>  { 'command': 'migrate-recover', 'data': { 'uri': 'str' },
>    'allow-oob': true }
> +
> +##
> +# @migrate-pause:
> +#
> +# Pause a migration.  Currently it only supports postcopy.
> +#
> +# Returns: nothing.
> +#
> +# Example:
> +#
> +# -> { "execute": "migrate-pause" }
> +# <- { "return": {} }
> +#
> +# Since: 2.12
> +##
> +{ 'command': 'migrate-pause', 'allow-oob': true }
> -- 
> 2.14.3
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK