Messages in this thread | | | Subject | Re: [PATCH v4 0/4] LoongArch: Support new relocation types | From | Xi Ruoyao <> | Date | Mon, 01 Aug 2022 17:55:26 +0800 |
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On Mon, 2022-08-01 at 10:34 +0800, Huacai Chen wrote: > Hi, all, > > On Mon, Aug 1, 2022 at 10:16 AM Youling Tang <tangyouling@loongson.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?
I don't like the hack too. I only developed it as an intellectual game.
We need to consider multiple combinations:
(1) Old GCC + old Binutils. We need -mla-local-with-abs for KBUILD_CFLAGS_MODULE.
(2) Old GCC + new Binutils. We need -mla-local-with-abs for KBUILD_CFLAGS_MODULE, *and* adding the support for R_LARCH_ABS{_HI20,_LO12,64_LO20,64_HI12} in the kernel module loader.
(3) New GCC + old Binutils. As new GCC should support our new attribute (I now intend to send V2 patch to gcc-patches using "movable" as the attribute name), no special action is needed.
Basically, we need:
(1) Handle R_LARCH_ABS{_HI20,_LO12,64_LO20,64_HI12} in the kernel module loader. (2) Add -Wa,-mla-local-with-abs into KBUILD_CFLAGS_MODULE if GCC version is <= 12. -- Xi Ruoyao <xry111@xry111.site> School of Aerospace Science and Technology, Xidian University
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