linux-kernel - Re: [RFC PATCH v3 0/3] Introduce BPF map tracing capability

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Date:   Thu, 4 Nov 2021 09:14:49 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Yonghong Song <yhs@...com>
Cc:     Joe Burton <jevburton.kernel@...il.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Network Development <netdev@...r.kernel.org>,
        bpf <bpf@...r.kernel.org>, Petar Penkov <ppenkov@...gle.com>,
        Stanislav Fomichev <sdf@...gle.com>,
        Joe Burton <jevburton@...gle.com>
Subject: Re: [RFC PATCH v3 0/3] Introduce BPF map tracing capability

On Wed, Nov 3, 2021 at 9:23 PM Yonghong Song <yhs@...com> wrote:
>
> asm("") indeed helped preserve the call.
>
> [$ ~/tmp2] cat t.c
> int __attribute__((noinline)) foo() { asm(""); return 1; }
> int bar() { return foo() + foo(); }
> [$ ~/tmp2] clang -O2 -c t.c
> [$ ~/tmp2] llvm-objdump -d t.o
>
> t.o:    file format elf64-x86-64
>
> Disassembly of section .text:
>
> 0000000000000000 <foo>:
>         0: b8 01 00 00 00                movl    $1, %eax
>         5: c3                            retq
>         6: 66 2e 0f 1f 84 00 00 00 00 00 nopw    %cs:(%rax,%rax)
>
> 0000000000000010 <bar>:
>        10: 50                            pushq   %rax
>        11: e8 00 00 00 00                callq   0x16 <bar+0x6>
>        16: e8 00 00 00 00                callq   0x1b <bar+0xb>
>        1b: b8 02 00 00 00                movl    $2, %eax
>        20: 59                            popq    %rcx
>        21: c3                            retq
> [$ ~/tmp2]
>
> Note with asm(""), foo() is called twice, but the compiler optimization
> knows foo()'s return value is 1 so it did calculation at compiler time,
> assign the 2 to %eax and returns.

Missed %eax=2 part...
That means that asm("") is not enough.
Maybe something like:
int __attribute__((noinline)) foo()
{
  int ret = 0;
  asm volatile("" : "=r"(var) : "0"(var));
  return ret;
}