On 12/07/2022 05:40, Andrii Nakryiko wrote: > CC Quentin as well > > On Mon, Jul 11, 2022 at 5:11 PM James Hilliard > <james.hilliard1@xxxxxxxxx> wrote: >> >> On Mon, Jul 11, 2022 at 5:36 PM Yonghong Song <yhs@xxxxxx> wrote: >>> >>> >>> >>> On 7/6/22 10:28 AM, James Hilliard wrote: >>>> The current bpf_helper_defs.h helpers are llvm specific and don't work >>>> correctly with gcc. >>>> >>>> GCC appears to required kernel helper funcs to have the following >>>> attribute set: __attribute__((kernel_helper(NUM))) >>>> >>>> Generate gcc compatible headers based on the format in bpf-helpers.h. >>>> >>>> This adds conditional blocks for GCC while leaving clang codepaths >>>> unchanged, for example: >>>> #if __GNUC__ && !__clang__ >>>> void *bpf_map_lookup_elem(void *map, const void *key) __attribute__((kernel_helper(1))); >>>> #else >>>> static void *(*bpf_map_lookup_elem)(void *map, const void *key) = (void *) 1; >>>> #endif >>> >>> It does look like that gcc kernel_helper attribute is better than >>> '(void *) 1' style. The original clang uses '(void *) 1' style is >>> just for simplicity. >> >> Isn't the original style going to be needed for backwards compatibility with >> older clang versions for a while? > > I'm curious, is there any added benefit to having this special > kernel_helper attribute vs what we did in Clang for a long time? Did > GCC do it just to be different and require workarounds like this or > there was some technical benefit to this? > > This duplication of definitions with #if for each one looks really > awful, IMO. I'd rather have a macro invocation like below (or > something along those lines) for each helper: > > BPF_HELPER_DEF(2, void *, bpf_map_update_elem, void *map, const void > *key, const void *value, __u64 flags); > > And then define BPF_HELPER_DEF() once based on whether it's Clang or GCC. Hi, for what it's worth I agree with Andrii, I would rather avoid the #if/else/endif and dual definition for each helper in the header, using a macro should keep it more readable indeed. The existing one (BPF_HELPER(return_type, name, args, id)) can likely be adapted. Also I note that contrarily to clang's helpers, you don't declare GCC's as "static" (although I'm not sure of the effect of declaring them static in this case). Thanks, Quentin
