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.
+     */
+    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);
+} FWCfgDataGeneratorClass;
+
 typedef struct fw_cfg_file FWCfgFile;
 
 #define FW_CFG_ORDER_OVERRIDE_VGA    70
@@ -263,6 +295,26 @@ void fw_cfg_add_file_callback(FWCfgState *s, const char *filename,
 void *fw_cfg_modify_file(FWCfgState *s, const char *filename, void *data,
                          size_t len);
 
+/**
+ * fw_cfg_add_from_generator:
+ * @s: fw_cfg device being modified
+ * @filename: name of new fw_cfg file item
+ * @gen_id: name of object implementing FW_CFG_DATA_GENERATOR interface
+ * @errp: pointer to a NULL initialized error object
+ *
+ * Add a new NAMED fw_cfg item with the content generated from the
+ * @gen_id object. The data generated by the @gen_id object is copied
+ * into the data structure of the fw_cfg device.
+ * The next available (unused) selector key starting at FW_CFG_FILE_FIRST
+ * will be used; also, a new entry will be added to the file directory
+ * structure residing at key value FW_CFG_FILE_DIR, containing the item name,
+ * data size, and assigned selector key value.
+ *
+ * Returns: the size of the generated item data on success, or 0 on errors.
+ */
+size_t fw_cfg_add_from_generator(FWCfgState *s, const char *filename,
+                                 const char *gen_id, Error **errp);
+
 FWCfgState *fw_cfg_init_io_dma(uint32_t iobase, uint32_t dma_iobase,
                                 AddressSpace *dma_as);
 FWCfgState *fw_cfg_init_io(uint32_t iobase);
diff --git a/hw/nvram/fw_cfg.c b/hw/nvram/fw_cfg.c
index 8dd50c2c72..84578e83aa 100644
--- a/hw/nvram/fw_cfg.c
+++ b/hw/nvram/fw_cfg.c
@@ -1032,6 +1032,36 @@ void *fw_cfg_modify_file(FWCfgState *s, const char *filename,
     return NULL;
 }
 
+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);
+    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);
+
+    return size;
+}
+
 static void fw_cfg_machine_reset(void *opaque)
 {
     MachineClass *mc = MACHINE_GET_CLASS(qdev_get_machine());
@@ -1333,12 +1363,18 @@ static const TypeInfo fw_cfg_mem_info = {
     .class_init    = fw_cfg_mem_class_init,
 };
 
+static const TypeInfo fw_cfg_data_generator_interface_info = {
+    .name = TYPE_FW_CFG_DATA_GENERATOR_INTERFACE,
+    .parent = TYPE_INTERFACE,
+    .class_size = sizeof(FWCfgDataGeneratorClass),
+};
 
 static void fw_cfg_register_types(void)
 {
     type_register_static(&fw_cfg_info);
     type_register_static(&fw_cfg_io_info);
     type_register_static(&fw_cfg_mem_info);
+    type_register_static(&fw_cfg_data_generator_interface_info);
 }
 
 type_init(fw_cfg_register_types)
-- 
2.21.3

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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On 6/16/20 5:31 PM, Daniel P. Berrangé wrote:
> 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));

g_byte_array_steal() has been added in GLib 2.64,
QEMU supports up to 2.48.

I guess I have to use g_byte_array_free_to_bytes()
and g_memdup(g_bytes_get_data()) to achieve something
similar. I'll try.

> 
> 
> 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
> 

On Mon, Jun 22, 2020 at 03:54:58PM +0200, Philippe Mathieu-Daudé wrote:
> On 6/16/20 5:31 PM, Daniel P. Berrangé wrote:
> > 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));
> 
> g_byte_array_steal() has been added in GLib 2.64,
> QEMU supports up to 2.48.
> 
> I guess I have to use g_byte_array_free_to_bytes()
> and g_memdup(g_bytes_get_data()) to achieve something
> similar. I'll try.

Or can possibly use the simpler g_byte_array_free and avoid the g_autoptr


Regards,
Daniel
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On 6/16/20 8:31 AM, Daniel P. Berrangé wrote:
>    fw_cfg_add_file(s, filename, g_byte_array_steal(data, NULL),
>                    (guint)g_byte_array_get_size(data));

FWIW, you can't both read the size and steal the data in the argument list like
this -- the evaluation order is unspecified.


r~