.\" Copyright (c) 2010 by The DragonFly Project and Samuel J. Greear.
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.\" This code is derived from software contributed to The DragonFly Project
.\" by Samuel J. Greear <sjg@thesjg.com>
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.Dd March 17, 2010
.Os
.Dt lwbuf 9
.Sh NAME
.Nm lwbuf_alloc ,
.Nm lwbuf_free ,
.Nm lwbuf_page ,
.Nm lwbuf_kva ,
.Nm lwbuf_set_global
.Nd "Lightweight buffers"
.Sh SYNOPSIS
.In cpu/lwbuf.h
.Ft "struct lwbuf *"
.Fn lwbuf_alloc "vm_page_t m"
.Ft void
.Fn lwbuf_free "struct lwbuf *lwb"
.Ft vm_page_t
.Fn lwbuf_page "struct lwbuf *lwb"
.Ft vm_offset_t
.Fn lwbuf_kva "struct lwbuf *lwb"
.Ft void
.Fn lwbuf_set_global "struct lwbuf *lwb"
.Sh DESCRIPTION
The
.Fa lwbuf
kernel functions are used for maintaining a lightweight reference to and
accessing an arbitrary
.Fa vm_page_t .
.Pp
.Fn lwbuf_alloc
returns a pointer to a lightweight buffer representing
.Fa m .
.Pp
.Fn lwbuf_free
frees all resources associated with the lightweight buffer
.Fa lwb .
.Pp
.Fn lwbuf_page
and
.Fn lwbuf_kva
return the associated
.Fa vm_page_t
or
.Fa vm_offset_t
of the lightweight buffer
.Fa lwb .
.Pp
.Fn lwbuf_set_global
ensures that a vm_offset_t previously obtained through
.Fa lwbuf_kva
will be valid on all processors without subsequent calls to
.Fa lwbuf_kva .
It should not be used.
.Sh IMPLEMENTATION NOTES
The implementation of
.Fa lwbuf
is cpu-dependent. On i386, pages taken from
per-processor pools of kernel virtual address space (KVA) are used to map
arbitrary
.Fa vm_page_t
objects. On x86_64 such tricks are unnecessary, the
kernel maintains a direct map of KVA covering all physical memory.
.Pp
Lightweight buffers are thread and cross-processor safe with a number of
limitations. Allocated buffers are not internally cached or reference counted.
Any consumer of lightweight buffers may elect to share allocated buffers
or allow them to be used in other threads or on other processors, but care
must be taken. Buffers must be externally refcounted or in some other manner
freed only after last use.
.Sh HISTORY
A
.Nm lwbuf
implementation first appeared in
.Dx 2.5 .
.Sh Authors
The
.Nm lwbuf
implementation and this manpage were written by
.An Samuel J. Greear .