On Tue, Apr 25, 2023 at 11:29 AM Yonghong Song <yhs@xxxxxxxx> wrote:
>
>
>
> On 4/25/23 11:22 AM, Stanislav Fomichev wrote:
> > On Tue, Apr 25, 2023 at 11:10 AM Yonghong Song <yhs@xxxxxxxx> wrote:
> >>
> >>
> >>
> >> On 4/25/23 10:04 AM, Stanislav Fomichev wrote:
> >>> On Mon, Apr 24, 2023 at 6:56 PM Yonghong Song <yhs@xxxxxxxx> wrote:
> >>>>
> >>>>
> >>>>
> >>>> On 4/24/23 5:01 PM, Stanislav Fomichev wrote:
> >>>>> From: Peng Wei <pengweiprc@xxxxxxxxxx>
> >>>>>
> >>>>> Compiling C++ BPF programs with existing bpf_helper_defs.h is not
> >>
> >> Just curious, why you want to compile BPF programs with C++?
> >> The patch looks good to me. But it would be great to know
> >> some reasoning since a lot of stuff, e.g., some CORE related
> >> intrinsics, not available for C++.
> >
> > Can you share more? What's not available? Any pointers to the docs maybe?
>
> Sorry, it is an attribute, instead of instrinsics.
>
> The attribute preserve_access_index/btf_type_tag/btf_decl_tag are all C
> only.
Interesting, thanks! I don't think we use btf_type_tag/btf_decl_tag in
the program we want to try c++, but losing preserve_access_index might
be unfortunate :-( But we'll see..
Btw, any reason these are explicitly opted out from c++? Doesn't seem
like there is anything c-specific in them?
The c++ we are talking about here is mostly "c with classes +
templates"; no polymorphism / inheritance.
> In llvm-project/clang/include/clang/Basic/Attr.td:
>
> def BPFPreserveAccessIndex : InheritableAttr,
> TargetSpecificAttr<TargetBPF> {
> let Spellings = [Clang<"preserve_access_index">];
> let Subjects = SubjectList<[Record], ErrorDiag>;
> let Documentation = [BPFPreserveAccessIndexDocs];
> let LangOpts = [COnly];
> }
>
> def BTFDeclTag : InheritableAttr {
> let Spellings = [Clang<"btf_decl_tag">];
> let Args = [StringArgument<"BTFDeclTag">];
> let Subjects = SubjectList<[Var, Function, Record, Field, TypedefName],
> ErrorDiag>;
> let Documentation = [BTFDeclTagDocs];
> let LangOpts = [COnly];
> }
>
> def BTFTypeTag : TypeAttr {
> let Spellings = [Clang<"btf_type_tag">];
> let Args = [StringArgument<"BTFTypeTag">];
> let Documentation = [BTFTypeTagDocs];
> let LangOpts = [COnly];
> }
>
>
>
> >
> > People here want to try to use c++ to see if templating helps with v4
> > vs v6 handling.
> > We have a bunch of copy-paste around this place and would like to see
> > whether c++ could make it a bit more readable.
> >
> >>>>> possible due to stricter C++ type conversions. C++ complains
> >>>>> about (void *) type conversions:
> >>>>>
> >>>>> bpf_helper_defs.h:57:67: error: invalid conversion from ‘void*’ to ‘void* (*)(void*, const void*)’ [-fpermissive]
> >>>>> 57 | static void *(*bpf_map_lookup_elem)(void *map, const void *key) = (void *) 1;
> >>>>> | ^~~~~~~~~~
> >>>>> | |
> >>>>> | void*
> >>>>>
> >>>>> Extend bpf_doc.py to use proper function type instead of void.
> >>>>
> >>>> Could you specify what exactly the compilation command triggering the
> >>>> above error?
> >>>
> >>> The following does it for me:
> >>> clang++ --include linux/types.h ./tools/lib/bpf/bpf_helper_defs.h
> >>
> >> Thanks. It would be good if you add the above compilation command
> >> in the commit message.
> >
> > Sure, will add.
> >
> >>>
> >>>
> >>>>>
> >>>>> Before:
> >>>>> static void *(*bpf_map_lookup_elem)(void *map, const void *key) = (void *) 1;
> >>>>>
> >>>>> After:
> >>>>> static void *(*bpf_map_lookup_elem)(void *map, const void *key) = (void *(*)(void *map, const void *key)) 1;
> >>>>>
> >>>>> Signed-off-by: Peng Wei <pengweiprc@xxxxxxxxxx>
> >>>>> Signed-off-by: Stanislav Fomichev <sdf@xxxxxxxxxx>
> >>>>> ---
> >>>>> scripts/bpf_doc.py | 7 ++++++-
> >>>>> 1 file changed, 6 insertions(+), 1 deletion(-)
> >>>>>
> >>>>> diff --git a/scripts/bpf_doc.py b/scripts/bpf_doc.py
> >>>>> index eaae2ce78381..fa21137a90e7 100755
> >>>>> --- a/scripts/bpf_doc.py
> >>>>> +++ b/scripts/bpf_doc.py
> >>>>> @@ -827,6 +827,9 @@ COMMANDS
> >>>>> print(' *{}{}'.format(' \t' if line else '', line))
> >>>>>
> >>>>> print(' */')
> >>>>> + fptr_type = '%s%s(*)(' % (
> >>>>> + self.map_type(proto['ret_type']),
> >>>>> + ((' ' + proto['ret_star']) if proto['ret_star'] else ''))
> >>>>> print('static %s %s(*%s)(' % (self.map_type(proto['ret_type']),
> >>>>> proto['ret_star'], proto['name']), end='')
> >>>>> comma = ''
> >>>>> @@ -845,8 +848,10 @@ COMMANDS
> >>>>> one_arg += '{}'.format(n)
> >>>>> comma = ', '
> >>>>> print(one_arg, end='')
> >>>>> + fptr_type += one_arg
> >>>>>
> >>>>> - print(') = (void *) %d;' % helper.enum_val)
> >>>>> + fptr_type += ')'
> >>>>> + print(') = (%s) %d;' % (fptr_type, helper.enum_val))
> >>>>> print('')
> >>>>>
> >>>>> ###############################################################################
