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

[Ilya Leoshkevich <iii@linux.ibm.com>]

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.