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From:   Huacai Chen <chenhuacai@kernel.org>
Date:   Wed, 31 Aug 2022 22:40:00 +0800
Message-ID: <CAAhV-H4N_XvmP9KA1M5crU44kHr33MZUVSsMY4Ugu5wQSv_LOQ@mail.gmail.com>
Subject: Re: [PATCH v7 0/5] LoongArch: Support toolchain with new relocation types
To:     Xi Ruoyao <xry111@xry111.site>
Cc:     Jinyang He <hejinyang@loongson.cn>,
        WANG Xuerui <kernel@xen0n.name>, loongarch@lists.linux.dev,
        LKML <linux-kernel@vger.kernel.org>,
        Youling Tang <tangyouling@loongson.cn>
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On Wed, Aug 31, 2022 at 4:09 PM Xi Ruoyao <xry111@xry111.site> wrote:
>
> On Wed, 2022-08-31 at 14:58 +0800, Jinyang He wrote:
> > That's right. Also I am wondering why new toolchain produce .got* in
> > kernel. It's unneeded. In the past, gcc create la.global and parsed
> > to la.pcrel by gas, and kernel works well. Now it seems we lost this
> > feature in gcc. I checked the x86 asm code just now. And some info
> > follows,
> >
> > LoongArch64, ./net/ipv4/udp_diag.s, *have reloc hint*
> >          pcalau12i       $r4,%got_pc_hi20(udplite_table)
> >          ld.d    $r4,$r4,%got_pc_lo12(udplite_table)
> >          b       udp_dump
> >
> > x86_64, ./net/ipv4/udp_diag.s
> >          movq    $udplite_table, %rdi
> >          jmp     udp_dump
> >
> > It seems related to -fno-PIE and -cmodel=kernel on x86_64.
> > Hope new gcc with this feature now.
>
> On x86_64 -mcmodel=kernel means "all code and data are located in [-
> 2GiB, 0) range.  We actually don't strictly require a "high" range as
> we're mostly a PIC-friendly architecture: note that we use a
> pcalau12i/addi.d pair for PIC addressing in [PC-2GiB, PC+2GiB, and a
> lu12i.w/addi.d pair for "non-PIC" addressing in [-2GiB, 2GiB), both are
> 2-insn sequence.
>
> If we can put the main kernel image and the modules in one 2GiB VA
> range, we can avoid GOT completely.  But it's not possible for now
> because main kernel image is loaded in XKPRANGE but the modules are in
> XKVRANGE.  So the best we can achieve before implementing
> CONFIG_RELOCATION is using GOT in modules, and avoid GOT in the main
> kernel image (with a new code model in GCC, which will benefit both the
> kernel and statically linked executables).
Emmm, can you implement this new code model in the near future?

Huacai
>
> --
> Xi Ruoyao <xry111@xry111.site>
> School of Aerospace Science and Technology, Xidian University
>