linux-kernel - Re: [PATCH] libbpf: add GCC support for bpf_tail_call

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Date:   Mon, 29 Aug 2022 22:19:52 +0200
From:   "Jose E. Marchesi" <jose.marchesi@...cle.com>
To:     James Hilliard <james.hilliard1@...il.com>
Cc:     bpf@...r.kernel.org, Andrii Nakryiko <andrii@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Martin KaFai Lau <martin.lau@...ux.dev>,
        Song Liu <song@...nel.org>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        Stanislav Fomichev <sdf@...gle.com>,
        Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Tom Rix <trix@...hat.com>, linux-kernel@...r.kernel.org,
        llvm@...ts.linux.dev
Subject: Re: [PATCH] libbpf: add GCC support for bpf_tail_call_static


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@...il.com>
> ---
>  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");