Re: [Qemu-devel] [PATCH v2 for-2.11] migration, xen: Fix block image lock issue on live migration

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

* Anthony PERARD (anthony.perard@citrix.com) wrote:
> When doing a live migration of a Xen guest with libxl, the images for
> block devices are locked by the original QEMU process, and this prevent
> the QEMU at the destination to take the lock and the migration fail.
> 
> From QEMU point of view, once the RAM of a domain is migrated, there is
> two QMP commands, "stop" then "xen-save-devices-state", at which point a
> new QEMU is spawned at the destination.
> 
> Release locks in "xen-save-devices-state" so the destination can takes
> them, if it's a live migration.
> 
> This patch add the "live" parameter to "xen-save-devices-state" which
> default to true so older version of libxenlight can work with newer
> version of QEMU.
> 
> Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>

Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

> ---
> Changes in V2:
> - add the live parameter
> 
> CC: Kevin Wolf <kwolf@redhat.com>
> 
> also CCing libxl maintainers:
> CC: Ian Jackson <ian.jackson@eu.citrix.com>
> CC: Wei Liu <wei.liu2@citrix.com>
> ---
>  migration/savevm.c  | 23 ++++++++++++++++++++++-
>  qapi/migration.json |  6 +++++-
>  2 files changed, 27 insertions(+), 2 deletions(-)
> 
> diff --git a/migration/savevm.c b/migration/savevm.c
> index 4a88228614..7bc4e23e65 100644
> --- a/migration/savevm.c
> +++ b/migration/savevm.c
> @@ -2242,13 +2242,20 @@ int save_snapshot(const char *name, Error **errp)
>      return ret;
>  }
>  
> -void qmp_xen_save_devices_state(const char *filename, Error **errp)
> +void qmp_xen_save_devices_state(const char *filename, bool has_live, bool live,
> +                                Error **errp)
>  {
>      QEMUFile *f;
>      QIOChannelFile *ioc;
>      int saved_vm_running;
>      int ret;
>  
> +    if (!has_live) {
> +        /* live default to true so old version of Xen tool stack can have a
> +         * successfull live migration */
> +        live = true;
> +    }
> +
>      saved_vm_running = runstate_is_running();
>      vm_stop(RUN_STATE_SAVE_VM);
>      global_state_store_running();
> @@ -2263,6 +2270,20 @@ void qmp_xen_save_devices_state(const char *filename, Error **errp)
>      qemu_fclose(f);
>      if (ret < 0) {
>          error_setg(errp, QERR_IO_ERROR);
> +    } else {
> +        /* libxl calls the QMP command "stop" before calling
> +         * "xen-save-devices-state" and in case of migration failure, libxl
> +         * would call "cont".
> +         * So call bdrv_inactivate_all (release locks) here to let the other
> +         * side of the migration take controle of the images.
> +         */
> +        if (live && !saved_vm_running) {
> +            ret = bdrv_inactivate_all();
> +            if (ret) {
> +                error_setg(errp, "%s: bdrv_inactivate_all() failed (%d)",
> +                           __func__, ret);
> +            }
> +        }
>      }
>  
>   the_end:
> diff --git a/qapi/migration.json b/qapi/migration.json
> index bbc4671ded..03f57c9616 100644
> --- a/qapi/migration.json
> +++ b/qapi/migration.json
> @@ -1075,6 +1075,9 @@
>  # data. See xen-save-devices-state.txt for a description of the binary
>  # format.
>  #
> +# @live: Optional argument to ask QEMU to treat this command as part of a live
> +# migration. Default to true. (since 2.11)
> +#
>  # Returns: Nothing on success
>  #
>  # Since: 1.1
> @@ -1086,7 +1089,8 @@
>  # <- { "return": {} }
>  #
>  ##
> -{ 'command': 'xen-save-devices-state', 'data': {'filename': 'str'} }
> +{ 'command': 'xen-save-devices-state',
> +  'data': {'filename': 'str', '*live':'bool' } }
>  
>  ##
>  # @xen-set-replication:
> -- 
> Anthony PERARD
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK