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

Message

Vincent Chen Feb. 19, 2020, 7:28 a.m. UTC

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].

Becuase 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.

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(-)