On Tue, Mar 8, 2022 at 9:20 AM Alan Jowett <Alan.Jowett@xxxxxxxxxxxxx> wrote:
>
> BPF helper function names are common across different platforms, but the
> id assigned to helper functions may vary. This change provides an option
> to generate eBPF ELF files with relocations for helper functions which
> permits resolving helper function names to ids at load time instead of
> at compile time.
>
> Add a level of indirection to bpf_doc.py (and generated bpf_helper_defs.h)
> to permit compile time generation of bpf-helper function declarations to
> facilitating late binding of bpf-helpers to helper id for cases where
> different platforms use different helper ids, but the same helper
> functions.
>
> Example use case would be:
> "#define BPF_HELPER(return_type, name, args, id) \
> extern return_type name args"
>
> To generate:
> extern void * bpf_map_lookup_elem (void *map, const void *key);
>
> Instead of:
> static void *(*bpf_map_lookup_elem) (void *map, const void *key) \
> = (void*) 1;
>
> This would result in the bpf-helpers having external linkage and permit
> late binding of BPF programs to helper function ids.
>
Ugh...
BPF_HELPER(void*, bpf_map_lookup_elem, (void *map, const void *key), 1);
Looks pretty awful :(
But I also have the question about why different platforms should have
different IDs for the same subset of BPF helpers? Wouldn't it be
better to preserve IDs across platforms to simplify cross-platform BPF
object files?
We can probably also define some range for platform-specific BPF
helpers starting from some high ID?
> Signed-off-by: Alan Jowett <alanjo@xxxxxxxxxxxxx>
> ---
> scripts/bpf_helpers_doc.py | 8 ++++++--
> 1 file changed, 6 insertions(+), 2 deletions(-)
>
> diff --git a/scripts/bpf_helpers_doc.py b/scripts/bpf_helpers_doc.py
> index 867ada23281c..442b5e87687e 100755
> --- a/scripts/bpf_helpers_doc.py
> +++ b/scripts/bpf_helpers_doc.py
> @@ -519,6 +519,10 @@ class PrinterHelpers(Printer):
> for fwd in self.type_fwds:
> print('%s;' % fwd)
> print('')
> + print('#ifndef BPF_HELPER')
> + print('#define BPF_HELPER(return_type, name, args, id) static return_type(*name) args = (void*) id')
> + print('#endif')
> + print('')
>
> def print_footer(self):
> footer = ''
> @@ -558,7 +562,7 @@ class PrinterHelpers(Printer):
> print(' *{}{}'.format(' \t' if line else '', line))
>
> print(' */')
> - print('static %s %s(*%s)(' % (self.map_type(proto['ret_type']),
> + print('BPF_HELPER(%s%s, %s, (' % (self.map_type(proto['ret_type']),
> proto['ret_star'], proto['name']), end='')
> comma = ''
> for i, a in enumerate(proto['args']):
> @@ -577,7 +581,7 @@ class PrinterHelpers(Printer):
> comma = ', '
> print(one_arg, end='')
>
> - print(') = (void *) %d;' % len(self.seen_helpers))
> + print('), %d);' % len(self.seen_helpers))
> print('')
>
> ###############################################################################
> --
> 2.25.1
