Re: [PATCH v2 bpf-next 6/6] bpf: Document BTF_KIND_FLOAT in btf.rst

[Yonghong Song <yhs@fb.com>]

On 2/19/21 5:00 AM, Ilya Leoshkevich wrote:
> On Thu, 2021-02-18 at 21:41 -0800, Yonghong Song wrote:
>>
>>
>> On 2/18/21 6:25 PM, Ilya Leoshkevich wrote:
>>> Also document the expansion of the kind bitfield.
>>>
>>> Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
>>> ---
>>>    Documentation/bpf/btf.rst | 17 +++++++++++++++--
>>>    1 file changed, 15 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/Documentation/bpf/btf.rst b/Documentation/bpf/btf.rst
>>> index 44dc789de2b4..4f25c992d442 100644
>>> --- a/Documentation/bpf/btf.rst
>>> +++ b/Documentation/bpf/btf.rst
>>> @@ -84,6 +84,7 @@ sequentially and type id is assigned to each
>>> recognized type starting from id
>>>        #define BTF_KIND_FUNC_PROTO     13      /* Function
>>> Proto       */
>>>        #define BTF_KIND_VAR            14      /* Variable     */
>>>        #define BTF_KIND_DATASEC        15      /* Section      */
>>> +    #define BTF_KIND_FLOAT          16      /* Floating
>>> point       */
>>>    
>>>    Note that the type section encodes debug info, not just pure
>>> types.
>>>    ``BTF_KIND_FUNC`` is not a type, and it represents a defined
>>> subprogram.
>>> @@ -95,8 +96,8 @@ Each type contains the following common data::
>>>            /* "info" bits arrangement
>>>             * bits  0-15: vlen (e.g. # of struct's members)
>>>             * bits 16-23: unused
>>> -         * bits 24-27: kind (e.g. int, ptr, array...etc)
>>> -         * bits 28-30: unused
>>> +         * bits 24-28: kind (e.g. int, ptr, array...etc)
>>> +         * bits 29-30: unused
>>>             * bit     31: kind_flag, currently used by
>>>             *             struct, union and fwd
>>>             */
>>> @@ -452,6 +453,18 @@ map definition.
>>>      * ``offset``: the in-section offset of the variable
>>>      * ``size``: the size of the variable in bytes
>>>    
>>> +2.2.16 BTF_KIND_FLOAT
>>> +~~~~~~~~~~~~~~~~~~~~~
>>> +
>>> +``struct btf_type`` encoding requirement:
>>> + * ``name_off``: any valid offset
>>> + * ``info.kind_flag``: 0
>>> + * ``info.kind``: BTF_KIND_FLOAT
>>> + * ``info.vlen``: 0
>>> + * ``size``: the size of the float type in bytes.
>>
>> I would be good to specify the allowed size in bytes 2, multiple of
>> 4.
>> currently we do not have a maximum value, maybe 128. have a float
>> type
>> something like 2^10 seems strange.
> 
> I tried to write this all down and realized it's simpler to enumerate
> the allowed values: 2, 4, 8, 12 and 16. I don't think there are 32-byte
> floats on any of the architectures supported by the kernel.

This make senses. My above 128 means 128bits (sorry!), which is 16 
bytes, align with what you suggested.

>