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.\" by Samuel J. Greear <sjg@thesjg.com>
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.Dd March 17, 2010
.Dt LWBUF 9
.Os
.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" "struct lwbuf *"
.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
.Nm lwbuf
kernel functions are used for maintaining a lightweight reference to and
accessing an arbitrary
.Vt vm_page_t .
.Pp
.Fn lwbuf_alloc
returns a pointer to a lightweight buffer representing
.Fa m .
The
.Fa lwb
argument is an lwbuf structure, used to avoid allocation of an lwbuf
on the x86_64 architecture.
.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
.Vt vm_page_t
or
.Vt vm_offset_t
of the lightweight buffer
.Fa lwb .
.Pp
.Fn lwbuf_set_global
ensures that a
.Vt 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
.Nm lwbuf
is CPU-dependent.
On x86_64 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 .