Re: [PATCH 0/2] solve static percpu symbol issue in module and refine code model of module

[Palmer Dabbelt <palmer@dabbelt.com>]

On Thu, 20 Feb 2020 18:47:53 PST (-0800), vincent.chen@sifive.com wrote:
> The compiler uses the PIC-relative method to access static variables
> instead of GOT when the code model is PIC. Therefore, the limitation of
> the access range from the instruction to the symbol address is +-2GB.
> Under this circumstance, the kernel cannot load a kernel module if this
> module has static per-CPU symbols declared by DEFINE_PER_CPU(). The reason
> is that kernel relocates the .data..percpu section of the kernel module to
> the end of kernel's .data..percpu. Hence, the distance between the per-CPU
> symbols and the instruction will exceed the 2GB limits. To solve this
> problem, the kernel should place the loaded module in the memory area
> [&_end-2G, VMALLOC_END].
>
> Because the loaded module locates in the region [＆_end-2G，VMALLOC_END]
> at runtime, the distance from the module start to the end of the kernel
> image does not exceed 2GB. Hence, the second patch changes the code model
> of the kernel module from PIC to medany to improve the performance of data
> access.
>
> Changes from v1->v2
> 1. Unify the definition of VMALLOC_MODULE_START
> 2. Modify the indent
>
> Vincent Chen (2):
>   riscv: avoid the PIC offset of static percpu data in module beyond 2G
>     limits
>   riscv: Replace PIC with medany to improve data accessing in module
>
>  arch/riscv/Makefile        |  6 ++++--
>  arch/riscv/kernel/module.c | 18 ++++++++++++++++++
>  2 files changed, 22 insertions(+), 2 deletions(-)

Looking at this again, I think this is actually a good candidate for fixes.
Unless there's any opposition I'll target it for rc5.  It's on fixes.

Thanks!