Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
MIME-Version: 1.0
References: <20211102021432.2807760-1-jevburton.kernel@gmail.com>
 <20211103001245.muyte7exph23tmco@ast-mbp.dhcp.thefacebook.com>
 <fcec81dd-3bb9-7dcf-139d-847538b6ad20@fb.com> <CAN22DihwJ7YDFSPk+8CCs0RcSWvZOpNV=D1u+42XabztS6hcKQ@mail.gmail.com>
 <CAADnVQJ_ger=Tjn=9SuUTES6Tt5k_G0M+6T_ELzFtw_cSVs83A@mail.gmail.com> <55c95c15-ccad-bb31-be87-ad17db7cb02a@fb.com>
In-Reply-To: <55c95c15-ccad-bb31-be87-ad17db7cb02a@fb.com>
From:   Alexei Starovoitov <alexei.starovoitov@gmail.com>
Date:   Thu, 4 Nov 2021 09:14:49 -0700
Message-ID: <CAADnVQK6kMbX379dy5XOo1s7DricX1z9WZ04PhUD6DoEPO+jsg@mail.gmail.com>
Subject: Re: [RFC PATCH v3 0/3] Introduce BPF map tracing capability
To:     Yonghong Song <yhs@fb.com>
Cc:     Joe Burton <jevburton.kernel@gmail.com>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Andrii Nakryiko <andrii@kernel.org>,
        Martin KaFai Lau <kafai@fb.com>,
        Song Liu <songliubraving@fb.com>,
        John Fastabend <john.fastabend@gmail.com>,
        KP Singh <kpsingh@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Network Development <netdev@vger.kernel.org>,
        bpf <bpf@vger.kernel.org>, Petar Penkov <ppenkov@google.com>,
        Stanislav Fomichev <sdf@google.com>,
        Joe Burton <jevburton@google.com>
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

On Wed, Nov 3, 2021 at 9:23 PM Yonghong Song <yhs@fb.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;
}