Re: [PATCH v6 1/5] hw/nvram/fw_cfg: Add the FW_CFG_DATA_GENERATOR interface

[Laszlo Ersek]

On 05/19/20 20:20, Philippe Mathieu-Daudé wrote:
> The FW_CFG_DATA_GENERATOR allow any object to product

(1) I suggest:

s/allow/allows/
s/product/produce/

> blob of data consumable by the fw_cfg device.
> 
> Signed-off-by: Philippe Mathieu-Daudé <address@hidden>
> ---
>  include/hw/nvram/fw_cfg.h | 49 +++++++++++++++++++++++++++++++++++++++
>  hw/nvram/fw_cfg.c         | 30 ++++++++++++++++++++++++
>  2 files changed, 79 insertions(+)
> 
> diff --git a/include/hw/nvram/fw_cfg.h b/include/hw/nvram/fw_cfg.h
> index 25d9307018..74b4790fae 100644
> --- a/include/hw/nvram/fw_cfg.h
> +++ b/include/hw/nvram/fw_cfg.h
> @@ -9,11 +9,40 @@
>  #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
> +     */
> +    const void *(*get_data)(Object *obj);

I'm not familiar with QOM, so please excuse any dumb questions.

"const" suggests the blob returned remains owned by "obj"; that answers
the question whether the caller should attempt to free the blob. (The
answer is "no".)

(2) However, will this perhaps expose other functions, currently taking
non-const-qualified pointers, to which we'd like to pass the blob
returned by the above member function?

Because, then we'd have to cast away "const", and I find that much
uglier than removing the "const" from *here*, and adding a more verbose
comment as replacement.

Yes, this is clearly speculation -- IOW just a question. If all the
functions we're going to pass the return value to are fine with
pointer-to-const, then this interface should be OK.

(Obviously when I say "cast away const", I think of functions that do
not actually modify the object pointed-to by the non-const-qualified
pointer.)

> +    /**
> +     * 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 +292,26 @@ 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
> + * @generator_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
> + * @generator_id object. The data referenced by the starting pointer is 
> copied

(3) s/referenced by the starting pointer/generated by the @generator_id
object/

> + * 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, -1 otherwise.

(4) I don't like ssize_t for a return value like this.

First, get_length() returns size_t, which may not be representable in an
ssize_t.

(Actually, it's worse than that; POSIX says, "the type ssize_t shall be
capable of storing values at least in the range [-1, {SSIZE_MAX}]" --
and if I run "getconf SSIZE_MAX", I get 32767. Indeed, _POSIX_SSIZE_MAX,
which is the minimum for any implementation's SSIZE_MAX, is 32767.)

Second, is a zero-sized blob useful in fw_cfg (from a generator)?

If it is not useful, then this function should return size_t, and use
retval=0 for signaling an error.

If a zero-sized blob is useful, then the function should return a bool
(in addition to producing "errp"), and output the blob size as a
separate parameter.

> + */
> +ssize_t fw_cfg_add_from_generator(FWCfgState *s, const char *filename,
> +                                  const char *generator_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..e18cb074df 100644
> --- a/hw/nvram/fw_cfg.c
> +++ b/hw/nvram/fw_cfg.c
> @@ -1032,6 +1032,30 @@ void *fw_cfg_modify_file(FWCfgState *s, const char 
> *filename,
>      return NULL;
>  }
>  
> +ssize_t fw_cfg_add_from_generator(FWCfgState *s, const char *filename,
> +                                  const char *generator_id, Error **errp)
> +{
> +    FWCfgDataGeneratorClass *k;
> +    Object *o;

(5) Not sure about QEMU coding standards, but the above single-char
variable names (especially "o") terrify me. Please use "klass" and "obj".

Do ignore my request if these variable names are just fine in QEMU.

> +    size_t sz;
> +
> +    o = object_resolve_path_component(object_get_objects_root(), 
> generator_id);
> +    if (!o) {
> +        error_setg(errp, "Cannot find object ID %s", generator_id);
> +        return -1;
> +    }
> +    if (!object_dynamic_cast(o, TYPE_FW_CFG_DATA_GENERATOR_INTERFACE)) {
> +        error_setg(errp, "Object '%s' is not a fw_cfg-data-generator 
> subclass",
> +                         generator_id);

(6) We should probably not open code
TYPE_FW_CFG_DATA_GENERATOR_INTERFACE as "fw_cfg-data-generator" even in
the error message.

(7) If this branch is taken, would that arguably merit an assertion
failure? I mean, can the dynamic cast fail without QEMU having a related
bug somewhere? (Maybe this is going to be answered in the rest of the
series.) Because I see those OBJECT_CHECK macros near the top of
"fw_cfg.h", and those boil down to object_dynamic_cast_assert().

> +        return -1;
> +    }
> +    k = FW_CFG_DATA_GENERATOR_GET_CLASS(o);
> +    sz = k->get_length(o);
> +    fw_cfg_add_file(s, filename, g_memdup(k->get_data(o), sz), sz);

(g_memdup() takes a "guint" for "byte_size". Whether that matches
"size_t" is anyone's guess. I guess it can't be helped.)

> +
> +    return sz;

Right, this is the size_t --> ssize_t conversion that makes me
uncomfortable.

I'm OK if you ignore all of my comments, these are simply the thoughts
that crossed my mind.

Thanks
Laszlo

> +}
> +
>  static void fw_cfg_machine_reset(void *opaque)
>  {
>      MachineClass *mc = MACHINE_GET_CLASS(qdev_get_machine());
> @@ -1333,12 +1357,18 @@ 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)
>