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Subject: Re: [PATCH v4 0/4] LoongArch: Support new relocation types
To:     Huacai Chen <chenhuacai@kernel.org>, Xi Ruoyao <xry111@xry111.site>
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Cc:     loongarch@lists.linux.dev, LKML <linux-kernel@vger.kernel.org>,
        WANG Xuerui <kernel@xen0n.name>,
        Jinyang He <hejinyang@loongson.cn>,
        Lulu Cheng <chenglulu@loongson.cn>
From:   Youling Tang <tangyouling@loongson.cn>
Message-ID: <bbadd26e-956a-6842-4858-b138ce55fe84@loongson.cn>
Date:   Mon, 1 Aug 2022 12:31:42 +0800
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Hi, all

On 08/01/2022 10:34 AM, Huacai Chen wrote:
> Hi, all,
>
> On Mon, Aug 1, 2022 at 10:16 AM Youling Tang <tangyouling@loongson.cn> wrote:
>>
>> Hi, Ruoyao
>>
>> On 07/30/2022 10:52 AM, Xi Ruoyao wrote:
>>> On Sat, 2022-07-30 at 10:24 +0800, Xi Ruoyao wrote:
>>>> On Sat, 2022-07-30 at 01:55 +0800, Xi Ruoyao wrote:
>>>>> On Fri, 2022-07-29 at 20:19 +0800, Youling Tang wrote:
>>>>>
>>>>>> On 07/29/2022 07:45 PM, Xi Ruoyao wrote:
>>>>>>> Hmm... The problem is the "addresses" of per-cpu symbols are
>>>>>>> faked: they
>>>>>>> are actually offsets from $r21.  So we can't just load such an
>>>>>>> offset
>>>>>>> with PCALA addressing.
>>>>>>>
>>>>>>> It looks like we'll need to introduce an attribute for GCC to
>>>>>>> make
>>>>>>> an
>>>>>>> variable "must be addressed via GOT", and add the attribute into
>>>>>>> PER_CPU_ATTRIBUTES.
>>>>>
>>>>>> Yes, we need a GCC attribute to specify the per-cpu variable.
>>>>>
>>>>> GCC patch adding "addr_global" attribute for LoongArch:
>>>>> https://gcc.gnu.org/pipermail/gcc-patches/2022-July/599064.html
>>>>>
>>>>> An experiment to use it:
>>>>> https://github.com/xry111/linux/commit/c1d5d70
>>>>
>>>> Correction: https://github.com/xry111/linux/commit/c1d5d708

Using the new toolchain (with the "addr_global" attribute) to build the
kernel can successfully load the nf_tables.ko module after applying the
"c1d5d708" commit.

Thanks,
Youling
>>>>
>>>> It seems 7-bit SHA is not enough for kernel repo.
>>>
>>> If addr_global is rejected or not implemented (for example, building the
>>> kernel with GCC 12), *I expect* the following hack to work (I've not
>>> tested it because I'm AFK now).  Using visibility in kernel seems
>>> strange, but I think it may make some sense because the modules are some
>>> sort of similar to an ELF shared object being dlopen()'ed, and our way
>>> to inject per-CPU symbols is analog to ELF interposition.
>>>
>>> arch/loongarch/include/asm/percpu.h:
>>>
>>>    #if !__has_attribute(__addr_global__) && defined(MODULE)
>>>    /* Magically remove "static" for per-CPU variables.  */
>>>    # define ARCH_NEEDS_WEAK_PER_CPU
>>>    /* Force GOT-relocation for per-CPU variables.  */
>>>    # define PER_CPU_ATTRIBUTES __attribute__((__visibility__("default")))
>>>    #endif
>>>
>>> arch/loongarch/Makefile:
>>>
>>>    # Hack for per-CPU variables, see PER_CPU_ATTRIBUTES in
>>>    # include/asm/percpu.h
>>>    if (call gcc-does-not-support-addr-global)
>>>      KBUILD_CFLAGS_MODULE += -fPIC -fvisibility=hidden
>>>    endif
>>>
>> Using the old toolchain (GCC 12) can successfully load the nf_tables.ko
>> module after applying the above patch.
> I don't like such a hack..., can we consider using old relocation
> types when building by old toolchains?
>
> Huacai
>>
>> Thanks,
>> Youling
>>