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

[Youling Tang]

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@xxxxxxxxxxx> 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