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

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Date:   Sat, 30 Jul 2022 10:52:54 +0800
From:   Xi Ruoyao <xry111@...111.site>
To:     Youling Tang <tangyouling@...ngson.cn>, loongarch@...ts.linux.dev
Cc:     linux-kernel@...r.kernel.org, WANG Xuerui <kernel@...0n.name>,
        Huacai Chen <chenhuacai@...nel.org>,
        Jinyang He <hejinyang@...ngson.cn>,
        Lulu Cheng <chenglulu@...ngson.cn>
Subject: Re: [PATCH v4 0/4] LoongArch: Support new relocation types

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

-- 
Xi Ruoyao <xry111@...111.site>
School of Aerospace Science and Technology, Xidian University