Hi James. > The bpf_tail_call_static function is currently not defined unless > using clang >= 8. > > To support bpf_tail_call_static on GCC we can check if __clang__ is > not defined to enable bpf_tail_call_static. > > We also need to check for the GCC style __BPF__ in addition to __bpf__ > for this to work as GCC does not define __bpf__. No need for that complication. I just pushed a patch to GCC that makes it define __bpf__ as a target macro, in addition to __BPF__, like LLVM does. https://gcc.gnu.org/pipermail/gcc-patches/2022-August/600534.html > We need to use GCC assembly syntax when the compiler does not define > __clang__ as LLVM inline assembly is not fully compatible with GCC. > > Signed-off-by: James Hilliard <james.hilliard1@xxxxxxxxx> > --- > tools/lib/bpf/bpf_helpers.h | 19 +++++++++++++------ > 1 file changed, 13 insertions(+), 6 deletions(-) > > diff --git a/tools/lib/bpf/bpf_helpers.h b/tools/lib/bpf/bpf_helpers.h > index 7349b16b8e2f..a0650b840cda 100644 > --- a/tools/lib/bpf/bpf_helpers.h > +++ b/tools/lib/bpf/bpf_helpers.h > @@ -131,7 +131,7 @@ > /* > * Helper function to perform a tail call with a constant/immediate map slot. > */ > -#if __clang_major__ >= 8 && defined(__bpf__) > +#if (!defined(__clang__) || __clang_major__ >= 8) && (defined(__bpf__) || defined(__BPF__)) > static __always_inline void > bpf_tail_call_static(void *ctx, const void *map, const __u32 slot) > { > @@ -139,8 +139,8 @@ bpf_tail_call_static(void *ctx, const void *map, const __u32 slot) > __bpf_unreachable(); > > /* > - * Provide a hard guarantee that LLVM won't optimize setting r2 (map > - * pointer) and r3 (constant map index) from _different paths_ ending > + * Provide a hard guarantee that the compiler won't optimize setting r2 > + * (map pointer) and r3 (constant map index) from _different paths_ ending > * up at the _same_ call insn as otherwise we won't be able to use the > * jmpq/nopl retpoline-free patching by the x86-64 JIT in the kernel > * given they mismatch. See also d2e4c1e6c294 ("bpf: Constant map key > @@ -148,12 +148,19 @@ bpf_tail_call_static(void *ctx, const void *map, const __u32 slot) > * > * Note on clobber list: we need to stay in-line with BPF calling > * convention, so even if we don't end up using r0, r4, r5, we need > - * to mark them as clobber so that LLVM doesn't end up using them > - * before / after the call. > + * to mark them as clobber so that the compiler doesn't end up using > + * them before / after the call. > */ > - asm volatile("r1 = %[ctx]\n\t" > + asm volatile( > +#ifdef __clang__ > + "r1 = %[ctx]\n\t" > "r2 = %[map]\n\t" > "r3 = %[slot]\n\t" > +#else > + "mov %%r1,%[ctx]\n\t" > + "mov %%r2,%[map]\n\t" > + "mov %%r3,%[slot]\n\t" > +#endif > "call 12" > :: [ctx]"r"(ctx), [map]"r"(map), [slot]"i"(slot) > : "r0", "r1", "r2", "r3", "r4", "r5");
