Floating-point API

Kernel code is normally prohibited from using floating-point (FP) registers or instructions, including the C float and double data types. This rule reduces system call overhead, because the kernel does not need to save and restore the userspace floating-point register state.

However, occasionally drivers or library functions may need to include FP code. This is supported by isolating the functions containing FP code to a separate translation unit (a separate source file), and saving/restoring the FP register state around calls to those functions. This creates “critical sections” of floating-point usage.

The reason for this isolation is to prevent the compiler from generating code touching the FP registers outside these critical sections. Compilers sometimes use FP registers to optimize inlined memcpy or variable assignment, as floating-point registers may be wider than general-purpose registers.

Usability of floating-point code within the kernel is architecture-specific. Additionally, because a single kernel may be configured to support platforms both with and without a floating-point unit, FPU availability must be checked both at build time and at run time.

Several architectures implement the generic kernel floating-point API from linux/fpu.h, as described below. Some other architectures implement their own unique APIs, which are documented separately.

Build-time API

Floating-point code may be built if the option ARCH_HAS_KERNEL_FPU_SUPPORT is enabled. For C code, such code must be placed in a separate file, and that file must have its compilation flags adjusted using the following pattern:

CFLAGS_foo.o += $(CC_FLAGS_FPU)
CFLAGS_REMOVE_foo.o += $(CC_FLAGS_NO_FPU)

Architectures are expected to define one or both of these variables in their top-level Makefile as needed. For example:

CC_FLAGS_FPU := -mhard-float

or:

CC_FLAGS_NO_FPU := -msoft-float

Normal kernel code is assumed to use the equivalent of CC_FLAGS_NO_FPU.

Runtime API

The runtime API is provided in linux/fpu.h. This header cannot be included from files implementing FP code (those with their compilation flags adjusted as above). Instead, it must be included when defining the FP critical sections.

bool kernel_fpu_available(void)

This function reports if floating-point code can be used on this CPU or platform. The value returned by this function is not expected to change at runtime, so it only needs to be called once, not before every critical section.

void kernel_fpu_begin(void)

void kernel_fpu_end(void)

These functions create a floating-point critical section. It is only valid to call kernel_fpu_begin() after a previous call to kernel_fpu_available() returned true. These functions are only guaranteed to be callable from (preemptible or non-preemptible) process context.

Preemption may be disabled inside critical sections, so their size should be minimized. They are not required to be reentrant. If the caller expects to nest critical sections, it must implement its own reference counting.