linux-kernel - Re: [RFC] ftrace: Add support to keep some functions out of ftrace

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Message-ID: <20220818165024.433f56fd@gandalf.local.home>
Date:   Thu, 18 Aug 2022 16:50:24 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Jiri Olsa <olsajiri@...il.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Ingo Molnar <mingo@...hat.com>, bpf <bpf@...r.kernel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>,
        Stanislav Fomichev <sdf@...gle.com>,
        Hao Luo <haoluo@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>
Subject: Re: [RFC] ftrace: Add support to keep some functions out of ftrace

On Thu, 18 Aug 2022 22:27:07 +0200
Jiri Olsa <olsajiri@...il.com> wrote:

> ok, so the problem with __attribute__((patchable_function_entry(5))) is that
> it puts function address into __patchable_function_entries section, which is
> one of ftrace locations source:
> 
>   #define MCOUNT_REC()    . = ALIGN(8);     \
>     __start_mcount_loc = .;                 \
>     KEEP(*(__mcount_loc))                   \
>     KEEP(*(__patchable_function_entries))   \
>     __stop_mcount_loc = .;                  \
>    ...
> 
> 
> it looks like __patchable_function_entries is used for other than x86 archs,
> so we perhaps we could have x86 specific MCOUNT_REC macro just with
> __mcount_loc section?

So something like this:

#ifdef CONFIG_X86
# define NON_MCOUNT_PATCHABLE KEEP(*(__patchable_function_entries))
# define MCOUNT_PATCHABLE
#else
# define NON_MCOUNT_PATCHABLE
# define MCOUNT_PATCHABLE  KEEP(*(__patchable_function_entries))
#endif

  #define MCOUNT_REC()    . = ALIGN(8);     \
    __start_mcount_loc = .;                 \
    KEEP(*(__mcount_loc))                   \
    MCOUNT_PATCHABLE			    \
    __stop_mcount_loc = .;                  \
    NON_MCOUNT_PATCHABLE		    \
   ...

??

-- Steve