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Subject: Re: [PATCH 1/3] LoongArch: tools: Add relocs tool support
To:     Xi Ruoyao <xry111@xry111.site>, Jinyang He <hejinyang@loongson.cn>,
        Huacai Chen <chenhuacai@kernel.org>
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Cc:     linux-kernel@vger.kernel.org, loongarch@lists.linux.dev,
        Xuerui Wang <kernel@xen0n.name>,
        Jiaxun Yang <jiaxun.yang@flygoat.com>
From:   Youling Tang <tangyouling@loongson.cn>
Message-ID: <29cd929b-185b-1c4b-f200-08f2a724b59d@loongson.cn>
Date:   Tue, 6 Sep 2022 10:16:32 +0800
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Hi, Ruoyao & Jinyang

On 09/05/2022 10:52 AM, Youling Tang wrote:
> Hi, Ruoyao
>
> On 09/04/2022 12:53 AM, Xi Ruoyao wrote:
>> On Sun, 2022-09-04 at 00:23 +0800, Jinyang He wrote:
>>> On 2022/9/3 18:49, Xi Ruoyao wrote:
>>>
>>>> On Sat, 2022-09-03 at 09:57 +0800, Youling Tang wrote:
>>>>>>> Unlike (pre-r6) MIPS, LoongArch has a complete support for PIC, and
>>>>>>> currently LoongArch toolchain always produces PIC (except, if
>>>>>>> -Wa,-mla-
>>>>>>> {local,global}-with-abs or la.abs macros are used explicitly).
>>>>>>>
>>>>>>> So would it be easier to review and correct the uses of "la.abs"
>>>>>>> in the
>>>>>>> code, and make the main kernel image a real PIE?  Then we can
>>>>>>> load it
>>>>>>> everywhere w/o any need to do relocation at load time.
>>>>> At the beginning I also wanted to make the main kernel image a real
>>>>> PIE
>>>>> and tried it, some of the "la.abs" can be modified, but I encountered
>>>>> difficulties in modifying the  exception handling code part, the
>>>>> kernel
>>>>> will not  boot after modification :(, I will continue to work hard
>>>>> try.
>>
>>>> I just tried the same thing and get the same result :(.  Will spend
>>>> several hours reading the LoongArch manual about exception...
>>
>> The reason is the handler code is not executed in linker address, but
>> copied elsewhere.  Then PC-relative offset is broken.  I managed to work
>> around it by creating a trampoline and jump into the handler, instead of
>> copy the handler code.  Then I could remove most "la.abs" occurrence
>> (except two in kernel entry point, which seem deliberately used):
>>
>> - https://github.com/xry111/linux/commit/56a433f
>> - https://github.com/xry111/linux/commit/48203e6
>>
>
> Thank you very much.
>
> After applying the above two patches and the following modifications,
> the relocation can be successful after removing the
> apply_r_loongarch_la_rel (for la.abs relocation) implementation. I
> tested it in the qemu environment.
>
> --- a/arch/loongarch/kernel/head.S
> +++ b/arch/loongarch/kernel/head.S
> @@ -113,9 +113,11 @@ SYM_CODE_START(smpboot_entry)
>         li.d            t0, CSR_DMW1_INIT       # CA, PLV0
>         csrwr           t0, LOONGARCH_CSR_DMWIN1
>
> -       la.abs          t0, 0f
> -       jr              t0
> -0:
> +       li.d            t0, CACHE_BASE
> +       pcaddi          t1, 0
> +       or              t0, t0, t1
> +       jirl            zero, t0, 0xc
>
> Youling.
>
>> Using the trampoline in handler table will definitely lead to sub-
>> optimal performance. I just use it as a proof-of-concept. Later we may
>> use some assembler trick to generate hard-coded handler table with
>> correct PC-relative offsets.
>>
>>> The following ideas are based on experience, without validation. Patches
>>> show that three types of relocation are needed to be done.
>>> 1, GOT is generated by toolchain, so  I think eliminating them by
>>> toolchain is better.
>>
>> https://gcc.gnu.org/pipermail/gcc-patches/2022-September/600797.html
>>
>> I stop to read the mail here because it's 00:52 AM now :).
>>
>>> 2, Ex_table is generated but striped relocation info. We can plays pcrel
>>> way to resolve this problem. One of ways like follows, (pseudo-code)

Switch to relative exception tables:

https://github.com/tangyouling/linux/commit/6525b8da
https://github.com/tangyouling/linux/commit/b6ac0827

Will switch to the relative exception tables after applying the above
two patches. So there is no need to relocate the exception table
(remove relocate_exception_table).

Now we can remove the relocation of la.abs , got and ex_table, but
still need to relocate LARCH_64. Is there anything else that needs to
be modified to eliminate this relocation?

Thanks,
Youling.

>>
>> /* snip */
>>
>>
>