linux-kernel - Re: [PATCH v4 0/4] LoongArch: Support new relocation types

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Date:   Mon, 1 Aug 2022 10:34:07 +0800
From:   Huacai Chen <chenhuacai@...nel.org>
To:     Youling Tang <tangyouling@...ngson.cn>
Cc:     Xi Ruoyao <xry111@...111.site>, loongarch@...ts.linux.dev,
        LKML <linux-kernel@...r.kernel.org>,
        WANG Xuerui <kernel@...0n.name>,
        Jinyang He <hejinyang@...ngson.cn>,
        Lulu Cheng <chenglulu@...ngson.cn>
Subject: Re: [PATCH v4 0/4] LoongArch: Support new relocation types

Hi, all,

On Mon, Aug 1, 2022 at 10:16 AM Youling Tang <tangyouling@...ngson.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
> >>
> >> 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
>