On 9/9/22 5:26 PM, Andrii Nakryiko wrote:
On Fri, Sep 9, 2022 at 3:46 PM James Hilliard <james.hilliard1@xxxxxxxxx> wrote: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. Signed-off-by: James Hilliard <james.hilliard1@xxxxxxxxx> Signed-off-by: Daniel Borkmann <daniel@xxxxxxxxxxxxx> --- tools/lib/bpf/bpf_helpers.h | 10 +++++----- 1 file changed, 5 insertions(+), 5 deletions(-) diff --git a/tools/lib/bpf/bpf_helpers.h b/tools/lib/bpf/bpf_helpers.h index 7349b16b8e2f..30fc95e7cd76 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__) 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 @@ -148,8 +148,8 @@ 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" "r2 = %[map]\n\t"will this compile as is on GCC-BPF? I'm trying to understand what's the point. Once GCC supports this ASM syntax we can add similar check to __clang_major__, instead of allowing it for all GCC versions? We must have done __clang_major__ check for a reason, old Clangs probably had some problems compiling this. Maybe Daniel remembers?
Yes, clang >= 8 is needed to ensure 'slot' is propagated as 'const' instead of variables. clang 6 added support for inline asm but clang 8
is needed for above 'const' propagation into asm code.
-- 2.34.1
