17.5.6. Floating point in kernel modules
It is generally hard / impossible to use floating point operations in the kernel. TODO understand details.
A quick (x86-only for now because lazy) example is shown at: kernel_modules/float.c
Usage:
insmod float.ko myfloat=1 enable_fpu=1
We have to call: kernel_fpu_begin() before starting FPU operations, and kernel_fpu_end() when we are done. This particular example however did not blow up without it at lkmc 7f917af66b17373505f6c21d75af9331d624b3a9 + 1:
insmod float.ko myfloat=1 enable_fpu=0
The v5.1 documentation under arch/x86/include/asm/fpu/api.h reads:
* Use kernel_fpu_begin/end() if you intend to use FPU in kernel context. It * disables preemption so be careful if you intend to use it for long periods * of time.
The example sets in the kernel_modules/Makefile:
CFLAGS_REMOVE_float.o += -mno-sse -mno-sse2
to avoid:
error: SSE register return with SSE disabled
We found those flags with ./build-modules --verbose.
Bibliography: