table of contents
FPU_KERN(9) | Kernel Developer's Manual | FPU_KERN(9) |
NAME¶
fpu_kern
—
SYNOPSIS¶
struct fpu_kern_ctx *fpu_kern_alloc_ctx
(u_int
flags);
void
fpu_kern_free_ctx
(struct
fpu_kern_ctx *ctx);
int
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¶
Thefpu_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:
FPU_KERN_NOWAIT
- Do not wait for the available memory if the request could not be satisfied without sleep.
- 0
- No special handling is required.
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:
- td
- Currently must be curthread.
- ctx
- The context save area previously allocated by
fpu_kern_alloc_ctx
() and not currently in use by another call tofpu_kern_enter
(). - flags
- This argument currently accepts the following flags:
FPU_KERN_NORMAL
- Indicates that the caller intends to access the full FPU state. Must be specified currently.
FPU_KERN_KTHR
- Indicates that no saving of the current FPU state should be performed,
if the thread called fpu_kern_thread(9) function.
This is intended to minimize code duplication in callers which could
be used from both kernel thread and syscall contexts. The
fpu_kern_leave
() function correctly handles such contexts.
The function does not sleep or block. It could cause the
Device Not Available
exception during execution, and
on the first FPU access after the function returns, as well as after each
context switch (see Intel Software Developer Manual for the reference).
Currently, no errors are defined which can be returned by
fpu_kern_enter
() to the caller.
The fpu_kern_leave
() function ends the
region started by fpu_kern_enter
(). The uses of FPU
in the kernel after the call to fpu_kern_leave
() are
erroneous until the next call to fpu_kern_enter
() is
performed. 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 other invocation of
fpu_kern_enter
(). There are no errors defined for
the function, it 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¶
Thefpu_kern
is currently implemented only for the i386
and amd64 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¶
Thefpu_kern
facitily and this manual page were written
by Konstantin Belousov
<kib@FreeBSD.org>.
October 23, 2014 | Linux 4.9.0-9-amd64 |