| Message ID | 1582253275-28181-1-git-send-email-vincent.chen@sifive.com (mailing list archive) |
|---|---|
| Headers | show |
| Series | solve static percpu symbol issue in module and refine code model of module | expand |
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!
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(-)