NAME

fpu_kern — facility to use the FPU in the kernel

SYNOPSIS

struct fpu_kern_ctx *
fpu_kern_alloc_ctx(u_int flags);

void
fpu_kern_free_ctx(struct fpu_kern_ctx *ctx);

void
fpu_kern_enter(struct thread *td, struct fpu_kern_ctx *ctx, u_int flags);

int
fpu_kern_leave(struct thread *td, struct fpu_kern_ctx *ctx);

int
fpu_kern_thread(u_int flags);

int
is_fpu_kern_thread(u_int flags);

DESCRIPTION

The fpu_kern family of functions allows the use of FPU hardware in kernel code. Modern FPUs are not limited to providing hardware implementation for floating point arithmetic; they offer advanced accelerators for cryptography and other computational-intensive algorithms. These facilities share registers with the FPU hardware.

Typical kernel code does not need access to the FPU. Saving a large register file on each entry to the kernel would waste time. When kernel code uses the FPU, the current FPU state must be saved to avoid corrupting the user-mode state, and vice versa.

The management of the save and restore is automatic. The processor catches accesses to the FPU registers when the non-current context tries to access them. Explicit calls are required for the allocation of the save area and the notification of the start and end of the code using the FPU.

The fpu_kern_alloc_ctx() function allocates the memory used by fpu_kern to track the use of the FPU hardware state and the related software state. The fpu_kern_alloc_ctx() function requires the flags argument, which currently accepts the following flags:

The function returns the allocated context area, or NULL if the allocation failed.

The fpu_kern_free_ctx() function frees the context previously allocated by fpu_kern_alloc_ctx().

The fpu_kern_enter() function designates the start of the region of kernel code where the use of the FPU is allowed. Its arguments are:

The function does not sleep or block. It could cause an FPU trap during execution, and on the first FPU access after the function returns, as well as after each context switch. On i386 and amd64 this will be the Device Not Available exception (see Intel Software Developer Manual for the reference).

The fpu_kern_leave() function ends the region started by fpu_kern_enter(). It is erroneous to use the FPU in the kernel before fpu_kern_enter() or after fpu_kern_leave(). The function takes the td thread argument, which currently must be curthread, and the ctx context pointer, previously passed to fpu_kern_enter(). After the function returns, the context may be freed or reused by another invocation of fpu_kern_enter(). The function always returns 0.

The fpu_kern_thread() function enables an optimization for threads which never leave to the usermode. The current thread will reuse the usermode save area for the kernel FPU state instead of requiring an explicitly allocated context. There are no flags defined for the function, and no error states that the function returns. Once this function has been called, neither fpu_kern_enter() nor fpu_kern_leave() is required to be called and the fpu is available for use in the calling thread.

The is_fpu_kern_thread() function returns the boolean indicating whether the current thread entered the mode enabled by fpu_kern_thread(). There is currently no flags defined for the function, the return value is true if the current thread have the permanent FPU save area, and false otherwise.

NOTES

The fpu_kern is currently implemented only for the i386, amd64, and arm64 architectures.

There is no way to handle floating point exceptions raised from kernel mode.

The unused flags arguments to the fpu_kern functions are to be extended to allow specification of the set of the FPU hardware state used by the code region. This would allow optimizations of saving and restoring the state.

AUTHORS

The fpu_kern facitily and this manual page were written by Konstantin Belousov <kib@FreeBSD.org>. The arm64 support was added by
Andrew Turner <andrew@FreeBSD.org>.

BUGS

fpu_kern_leave() should probably have type void (like fpu_kern_enter()).