Re: [PATCH v9 1/5] hw/nvram/fw_cfg: Add the FW_CFG_DATA_GENERATOR interface

["Daniel P. Berrangé" <berrange@redhat.com>]

On Mon, Jun 15, 2020 at 12:34:53PM +0200, Philippe Mathieu-Daudé wrote:
> The FW_CFG_DATA_GENERATOR allows any object to produce
> blob of data consumable by the fw_cfg device.
> 
> Reviewed-by: Laszlo Ersek <lersek@redhat.com>
> Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
> ---
>  docs/specs/fw_cfg.txt     |  9 ++++++-
>  include/hw/nvram/fw_cfg.h | 52 +++++++++++++++++++++++++++++++++++++++
>  hw/nvram/fw_cfg.c         | 36 +++++++++++++++++++++++++++
>  3 files changed, 96 insertions(+), 1 deletion(-)
> 
> diff --git a/docs/specs/fw_cfg.txt b/docs/specs/fw_cfg.txt
> index 8f1ebc66fa..bc16daa38a 100644
> --- a/docs/specs/fw_cfg.txt
> +++ b/docs/specs/fw_cfg.txt
> @@ -219,7 +219,7 @@ To check the result, read the "control" field:
>  
>  = Externally Provided Items =
>  
> -As of v2.4, "file" fw_cfg items (i.e., items with selector keys above
> +Since v2.4, "file" fw_cfg items (i.e., items with selector keys above
>  FW_CFG_FILE_FIRST, and with a corresponding entry in the fw_cfg file
>  directory structure) may be inserted via the QEMU command line, using
>  the following syntax:
> @@ -230,6 +230,13 @@ Or
>  
>      -fw_cfg [name=]<item_name>,string=<string>
>  
> +Since v5.1, QEMU allows some objects to generate fw_cfg-specific content,
> +the content is then associated with a "file" item using the 'gen_id' option
> +in the command line, using the following syntax:
> +
> +    -object <generator-type>,id=<generated_id>,[generator-specific-options] \
> +    -fw_cfg [name=]<item_name>,gen_id=<generated_id>
> +
>  See QEMU man page for more documentation.
>  
>  Using item_name with plain ASCII characters only is recommended.
> diff --git a/include/hw/nvram/fw_cfg.h b/include/hw/nvram/fw_cfg.h
> index 25d9307018..ca69666847 100644
> --- a/include/hw/nvram/fw_cfg.h
> +++ b/include/hw/nvram/fw_cfg.h
> @@ -9,11 +9,43 @@
>  #define TYPE_FW_CFG     "fw_cfg"
>  #define TYPE_FW_CFG_IO  "fw_cfg_io"
>  #define TYPE_FW_CFG_MEM "fw_cfg_mem"
> +#define TYPE_FW_CFG_DATA_GENERATOR_INTERFACE "fw_cfg-data-generator"
>  
>  #define FW_CFG(obj)     OBJECT_CHECK(FWCfgState,    (obj), TYPE_FW_CFG)
>  #define FW_CFG_IO(obj)  OBJECT_CHECK(FWCfgIoState,  (obj), TYPE_FW_CFG_IO)
>  #define FW_CFG_MEM(obj) OBJECT_CHECK(FWCfgMemState, (obj), TYPE_FW_CFG_MEM)
>  
> +#define FW_CFG_DATA_GENERATOR_CLASS(class) \
> +    OBJECT_CLASS_CHECK(FWCfgDataGeneratorClass, (class), \
> +                       TYPE_FW_CFG_DATA_GENERATOR_INTERFACE)
> +#define FW_CFG_DATA_GENERATOR_GET_CLASS(obj) \
> +    OBJECT_GET_CLASS(FWCfgDataGeneratorClass, (obj), \
> +                     TYPE_FW_CFG_DATA_GENERATOR_INTERFACE)
> +
> +typedef struct FWCfgDataGeneratorClass {
> +    /*< private >*/
> +    InterfaceClass parent_class;
> +    /*< public >*/
> +
> +    /**
> +     * get_data:
> +     * @obj: the object implementing this interface
> +     *
> +     * Returns: pointer to start of the generated item data
> +     *
> +     * The returned pointer is a QObject weak reference, @obj owns
> +     * the reference and may free it at any time in the future.

This description is a bit odd. We're just returning a plain byte
array pointer, not a QObject, nor a reference, not will it be
free'd at any time.

> +     */
> +    const void *(*get_data)(Object *obj);
> +    /**
> +     * get_length:
> +     * @obj: the object implementing this interface
> +     *
> +     * Returns: the size of the generated item data in bytes
> +     */
> +    size_t (*get_length)(Object *obj);

I'd be inclined to have a single method that returns a GByteArray,
instead of separate methods for data & length.

That gives you a sized byte array, with a well define lifetime,
which is what the caller really wants here. ie

    /**
      * get_data:
      * @obj: the object implementing this interface
      *
      * Returns: reference to a byte array containing the data.
      * The caller should release the reference when no longer
      * required.
      */
     GByteArray *(*get_data)(Object *obj);

> +} FWCfgDataGeneratorClass;
> +

....


> +size_t fw_cfg_add_from_generator(FWCfgState *s, const char *filename,
> +                                 const char *gen_id, Error **errp)
> +{
> +    FWCfgDataGeneratorClass *klass;
> +    Object *obj;
> +    size_t size;
> +
> +    obj = object_resolve_path_component(object_get_objects_root(), gen_id);
> +    if (!obj) {
> +        error_setg(errp, "Cannot find object ID '%s'", gen_id);
> +        return 0;
> +    }
> +    if (!object_dynamic_cast(obj, TYPE_FW_CFG_DATA_GENERATOR_INTERFACE)) {
> +        error_setg(errp, "Object ID '%s' is not a '%s' subclass",
> +                   gen_id, TYPE_FW_CFG_DATA_GENERATOR_INTERFACE);
> +        return 0;
> +    }
> +    klass = FW_CFG_DATA_GENERATOR_GET_CLASS(obj);

...then the following:

> +    size = klass->get_length(obj);
> +    if (size == 0) {
> +        error_setg(errp, "Object ID '%s' failed to generate fw_cfg data",
> +                   gen_id);
> +        return 0;
> +    }
> +    fw_cfg_add_file(s, filename, g_memdup(klass->get_data(obj), (guint)size),
> +                    size);

Can be replaced with:

   g_autoptr(GByteArray) data = klass->get_data(obj);

   fw_cfg_add_file(s, filename, g_byte_array_steal(data, NULL),
                   (guint)g_byte_array_get_size(data));


If there's a real possibility of failure, then an 'Error **errp' should
be added  to the 'get_data' method, so this code doesn't have to invent
a error message with no useful info on the real failure.

> +
> +    return size;
> +}
> +

Regards,
Daniel
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