cb80ddd115
The flag specifies that the block which uses FPU must be executed in critical section, i.e. take no context switches, and does not need an FPU save area during the execution. It is intended to be applied around fast and short code pathes where save area allocation is impossible or undesirable, due to context or due to the relative cost of calculation vs. allocation. Sponsored by: The FreeBSD Foundation MFC after: 2 weeks
211 lines
6.7 KiB
Groff
211 lines
6.7 KiB
Groff
.\" Copyright (c) 2014
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.\" Konstantin Belousov <kib@FreeBSD.org>. All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
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.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
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.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
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.\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
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.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
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.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
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.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
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.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
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.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
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.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
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.\"
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.\" $FreeBSD$
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.\"
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.Dd October 23, 2014
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.Dt FPU_KERN 9
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.Os
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.Sh NAME
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.Nm fpu_kern
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.Nd "facility to use the FPU in the kernel"
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.Sh SYNOPSIS
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.Ft struct fpu_kern_ctx *
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.Fn fpu_kern_alloc_ctx "u_int flags"
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.Ft void
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.Fn fpu_kern_free_ctx "struct fpu_kern_ctx *ctx"
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.Ft int
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.Fn fpu_kern_enter "struct thread *td" "struct fpu_kern_ctx *ctx" "u_int flags"
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.Ft int
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.Fn fpu_kern_leave "struct thread *td" "struct fpu_kern_ctx *ctx"
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.Ft int
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.Fn fpu_kern_thread "u_int flags"
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.Ft int
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.Fn is_fpu_kern_thread "u_int flags"
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.Sh DESCRIPTION
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The
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.Nm
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family of functions allows the use of FPU hardware in kernel code.
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Modern FPUs are not limited to providing hardware implementation for
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floating point arithmetic; they offer advanced accelerators for cryptography
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and other computational-intensive algorithms.
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These facilities share registers with the FPU hardware.
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.Pp
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Typical kernel code does not need access to the FPU.
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Saving a large register file on each entry to the kernel would waste
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time.
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When kernel code uses the FPU, the current FPU state must be saved to
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avoid corrupting the user-mode state, and vice versa.
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.Pp
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The management of the save and restore is automatic.
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The processor catches accesses to the FPU registers
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when the non-current context tries to access them.
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Explicit calls are required for the allocation of the save area and
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the notification of the start and end of the code using the FPU.
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.Pp
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The
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.Fn fpu_kern_alloc_ctx
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function allocates the memory used by
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.Nm
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to track the use of the FPU hardware state and the related software state.
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The
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.Fn fpu_kern_alloc_ctx
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function requires the
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.Fa flags
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argument, which currently accepts the following flags:
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.Bl -tag -width ".Dv FPU_KERN_NOWAIT" -offset indent
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.It Dv FPU_KERN_NOWAIT
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Do not wait for the available memory if the request could not be satisfied
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without sleep.
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.It 0
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No special handling is required.
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.El
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.Pp
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The function returns the allocated context area, or
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.Va NULL
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if the allocation failed.
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.Pp
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The
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.Fn fpu_kern_free_ctx
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function frees the context previously allocated by
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.Fn fpu_kern_alloc_ctx .
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.Pp
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The
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.Fn fpu_kern_enter
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function designates the start of the region of kernel code where the
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use of the FPU is allowed.
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Its arguments are:
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.Bl -tag -width ".Fa ctx" -offset indent
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.It Fa td
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Currently must be
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.Va curthread .
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.It Fa ctx
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The context save area previously allocated by
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.Fn fpu_kern_alloc_ctx
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and not currently in use by another call to
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.Fn fpu_kern_enter .
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.It Fa flags
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This argument currently accepts the following flags:
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.Bl -tag -width ".Dv FPU_KERN_NORMAL" -offset indent
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.It Dv FPU_KERN_NORMAL
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Indicates that the caller intends to access the full FPU state.
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Must be specified currently.
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.It Dv FPU_KERN_KTHR
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Indicates that no saving of the current FPU state should be performed,
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if the thread called
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.Xr fpu_kern_thread 9
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function.
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This is intended to minimize code duplication in callers which
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could be used from both kernel thread and syscall contexts.
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The
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.Fn fpu_kern_leave
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function correctly handles such contexts.
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.It Dv FPU_KERN_NOCTX
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Avoid nesting save area.
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If the flag is specified, the
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.Fa ctx
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must be passed as
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.Va NULL .
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The flag should only be used for really short code blocks
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which can be executed in a critical section.
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It avoids the need to allocate the FPU context by the cost
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of increased system latency.
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.El
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.El
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.Pp
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The function does not sleep or block.
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It could cause the
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.Nm Device Not Available
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exception during execution, and on the first FPU access after the
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function returns, as well as after each context switch
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(see Intel Software Developer Manual for the reference).
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Currently, no errors are defined which can be returned by
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.Fn fpu_kern_enter
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to the caller.
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.Pp
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The
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.Fn fpu_kern_leave
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function ends the region started by
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.Fn fpu_kern_enter .
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The uses of FPU in the kernel after the call to
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.Fn fpu_kern_leave
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are erroneous until the next call to
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.Fn fpu_kern_enter
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is performed.
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The function takes the
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.Fa td
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thread argument, which currently must be
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.Va curthread ,
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and the
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.Fa ctx
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context pointer, previously passed to
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.Fn fpu_kern_enter .
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After the function returns, the context may be freed or reused
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by other invocation of
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.Fn fpu_kern_enter .
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There are no errors defined for the function, it always returns 0.
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.Pp
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The
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.Fn fpu_kern_thread
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function enables an optimization for threads which never leave to
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the usermode.
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The current thread will reuse the usermode save area for the kernel FPU state
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instead of requiring an explicitly allocated context.
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There are no flags defined for the function, and no error states
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that the function returns.
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Once this function has been called, neither
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.Fn fpu_kern_enter
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nor
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.Fn fpu_kern_leave
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is required to be called and the fpu is available for use in the calling thread.
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.Pp
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The
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.Fn is_fpu_kern_thread
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function returns the boolean indicating whether the current thread
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entered the mode enabled by
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.Fn fpu_kern_thread .
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There is currently no flags defined for the function, the return
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value is true if the current thread have the permanent FPU save area,
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and false otherwise.
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.Sh NOTES
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The
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.Nm
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is currently implemented only for the i386 and amd64 architectures.
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.Pp
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There is no way to handle floating point exceptions raised from
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kernel mode.
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.Pp
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The unused
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.Fa flags
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arguments
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to the
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.Nm
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functions are to be extended to allow specification of the
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set of the FPU hardware state used by the code region.
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This would allow optimizations of saving and restoring the state.
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.Sh AUTHORS
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The
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.Nm
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facitily and this manual page were written by
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.An Konstantin Belousov Aq Mt kib@FreeBSD.org .
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