Re: [PATCH v4 --for 4.6 COLOPre 14/25] tools/libxl: introduce libxl__domain_restore_device_model to load qemu state

[Ian Campbell <ian.campbell@citrix.com>]

On Wed, 2015-07-15 at 15:45 +0800, Yang Hongyang wrote:
> In normal migration, the qemu state was passed to qemu as a parameter.
> With COLO, Secondary vm is running. So we will do the following steps
> at every checkpoint:
> 1. suspend both primay vm and secondary vm

"primary"

> 2. sync the state
> 3. resume both primary vm and secondary vm
> Primary will send qemu's state in step2, and
> Secondary's qemu should read it and restore the state before it
> is resumed. We can not pass the state to qemu as a parameter because
> Secondary QEMU already started at this point, so we introduce
> libxl__domain_restore_device_model() to do it.
> This API should be called before resuming secondary vm.
> 
> Signed-off-by: Yang Hongyang <yanghy@cn.fujitsu.com>
> Signed-off-by: Wen Congyang <wency@cn.fujitsu.com>
> Cc: Anthony Perard <anthony.perard@citrix.com>
> ---
>  tools/libxl/libxl_dom_save.c | 29 +++++++++++++++++++++++++++++
>  tools/libxl/libxl_internal.h |  3 +++
>  tools/libxl/libxl_qmp.c      | 10 ++++++++++
>  3 files changed, 42 insertions(+)
> 
> diff --git a/tools/libxl/libxl_dom_save.c b/tools/libxl/libxl_dom_save.c
> index f89f5d4..0926b71 100644
> --- a/tools/libxl/libxl_dom_save.c
> +++ b/tools/libxl/libxl_dom_save.c
> @@ -675,6 +675,35 @@ out:
>      return ret;
>  }
>  
> +int libxl__domain_restore_device_model(libxl__gc *gc, uint32_t domid)
> +{
> +    char *state_file;
> +    int rc;
> +
> +    switch (libxl__device_model_version_running(gc, domid)) {
> +    case LIBXL_DEVICE_MODEL_VERSION_QEMU_XEN_TRADITIONAL:
> +        /* not supported now */
> +        rc = ERROR_INVAL;
> +        break;
> +    case LIBXL_DEVICE_MODEL_VERSION_QEMU_XEN:
> +        /*
> +         * This function may be called too many times for the same gc,
> +         * so we use NOGC, and free the memory before return to avoid
> +         * OOM.
> +         */

It occurs to me that domid shouldn't change for the duration of a COLO
run, right? 

Thus I think the path can be allocated once at start of day and not per
iteration, and can be stored in suspend_state (or similar) and passed in
here. Hence no complexity like a nested ao is needed.

> +        state_file = libxl__sprintf(NOGC,
> +                                    XC_DEVICE_MODEL_RESTORE_FILE".%d",
> +                                    domid);
> +        rc = libxl__qmp_restore(gc, domid, state_file);
> +        free(state_file);
> +        break;
> +    default:
> +        rc = ERROR_INVAL;
> +    }
> +
> +    return rc;
> +}
> +
>  /*
>   * Local variables:
>   * mode: C
> diff --git a/tools/libxl/libxl_internal.h b/tools/libxl/libxl_internal.h
> index 0eb5f41..fb777c1 100644
> --- a/tools/libxl/libxl_internal.h
> +++ b/tools/libxl/libxl_internal.h
> @@ -1074,6 +1074,7 @@ _hidden int libxl__domain_rename(libxl__gc *gc, uint32_t domid,
>  
>  _hidden int libxl__toolstack_restore(uint32_t domid, const uint8_t *buf,
>                                       uint32_t size, void *data);
> +_hidden int libxl__domain_restore_device_model(libxl__gc *gc, uint32_t domid);
>  _hidden int libxl__domain_resume_device_model(libxl__gc *gc, uint32_t domid);
>  
>  _hidden const char *libxl__userdata_path(libxl__gc *gc, uint32_t domid,
> @@ -1702,6 +1703,8 @@ _hidden int libxl__qmp_stop(libxl__gc *gc, int domid);
>  _hidden int libxl__qmp_resume(libxl__gc *gc, int domid);
>  /* Save current QEMU state into fd. */
>  _hidden int libxl__qmp_save(libxl__gc *gc, int domid, const char *filename);
> +/* Load current QEMU state from fd. */
> +_hidden int libxl__qmp_restore(libxl__gc *gc, int domid, const char *filename);
>  /* Set dirty bitmap logging status */
>  _hidden int libxl__qmp_set_global_dirty_log(libxl__gc *gc, int domid, bool enable);
>  _hidden int libxl__qmp_insert_cdrom(libxl__gc *gc, int domid, const libxl_device_disk *disk);
> diff --git a/tools/libxl/libxl_qmp.c b/tools/libxl/libxl_qmp.c
> index 6484f5e..080cb9f 100644
> --- a/tools/libxl/libxl_qmp.c
> +++ b/tools/libxl/libxl_qmp.c
> @@ -904,6 +904,16 @@ int libxl__qmp_save(libxl__gc *gc, int domid, const char *filename)
>                             NULL, NULL);
>  }
>  
> +int libxl__qmp_restore(libxl__gc *gc, int domid, const char *state_file)
> +{
> +    libxl__json_object *args = NULL;
> +
> +    qmp_parameters_add_string(gc, &args, "filename", state_file);
> +
> +    return qmp_run_command(gc, domid, "xen-load-devices-state", args,
> +                           NULL, NULL);
> +}
> +
>  static int qmp_change(libxl__gc *gc, libxl__qmp_handler *qmp,
>                        char *device, char *target, char *arg)
>  {