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From: "Stefan O'Rear" <sorear2@gmail.com>
Date: Sun, 14 Jan 2018 23:24:53 -0800
Message-ID: <CADJ6UvOZz=WZ18wYVhZcH0u58u_snDxmgbwsfTOS2ski-4O7tg@mail.gmail.com>
Subject: Re: [patches] [PATCH v2 2/6] riscv/ftrace: Add dynamic function
 tracer support
To: patches@groups.riscv.org
Cc: Palmer Dabbelt <palmer@sifive.com>, Albert Ou <albert@sifive.com>,
        Christoph Hellwig <hch@lst.de>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ingo Molnar <mingo@redhat.com>,
        Masahiro Yamada <yamada.masahiro@socionext.com>,
        Kamil Rytarowski <n54@gmx.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        linux-kernel@vger.kernel.org, Alan Kao <alankao@andestech.com>,
        Greentime Hu <greentime@andestech.com>
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On Sun, Jan 14, 2018 at 10:47 PM, Alan Kao <nonerkao@gmail.com> wrote:
> +       /*
> +        * For the dynamic ftrace to work, here we should reserve at least
> +        * 8 bytes for a functional auipc-jalr pair. Pseudo inst nop may be
> +        * interpreted as different length in different models, so we manually
> +        * *expand* two 4-byte nops here.
> +        *
> +        * Calling ftrace_update_ftrace_func would overwrite the nops below.
> +        * Check ftrace_modify_all_code for details.
> +        */
> +       addi    x0, x0, 0
> +       addi    x0, x0, 0

This relies on behavior of the assembler which is undocumented and, if
my reading of the specification is correct, a bug.

The documented way to assemble an sequence of 2 4-byte NOPs regardless
of subtarget is as follows:

.option push
.option norvc
    nop
    nop
.option pop

I have filed https://github.com/riscv/riscv-binutils-gdb/issues/135 to
get clarity on the assembler behavior; the explicit approach may be
preferable even if the assembler behavior turns out to be correct.

-s