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From:   Huacai Chen <chenhuacai@kernel.org>
Date:   Sat, 30 Jul 2022 14:14:59 +0800
Message-ID: <CAAhV-H7WxRp4u1iBs47LN1Sj3rDBdbLv1u7EpSt0Bt4QuhFSXQ@mail.gmail.com>
Subject: Re: [PATCH v4 0/4] LoongArch: Support new relocation types
To:     Xi Ruoyao <xry111@xry111.site>
Cc:     Youling Tang <tangyouling@loongson.cn>, 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>
Content-Type: text/plain; charset="UTF-8"
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Hi, Ruoyao,

On Sat, Jul 30, 2022 at 10:53 AM Xi Ruoyao <xry111@xry111.site> 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
> >
> > 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.
Sadly, I don't know what visibility is, does it have something to do
with __visible in include/linux/compiler_attributes.h?

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