2007-01-28 16:07:50 +00:00
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.\"
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.\" Copyright (c) 2007 Seccuris Inc.
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.\" All rights reserved.
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.\"
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2007-01-31 09:40:31 +00:00
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.\" This documentation was written by Robert N. M. Watson under contract to
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2007-01-28 16:07:50 +00:00
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.\" Seccuris Inc.
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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(s), this list of conditions and the following disclaimer as
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.\" the first lines of this file unmodified other than the possible
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.\" addition of one or more copyright notices.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice(s), 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 COPYRIGHT HOLDER(S) ``AS IS'' AND ANY
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.\" EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
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.\" WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
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.\" DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER(S) BE LIABLE FOR ANY
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.\" DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
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.\" (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
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.\" SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
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.\" CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH
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.\" DAMAGE.
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.\"
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.\" $FreeBSD$
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.\"
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.Dd January 28, 2007
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.Dt SF_BUF 9
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.Os
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.Sh NAME
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.Nm sf_buf
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.Nd manage temporary kernel address space mapping for memory pages
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.Sh SYNOPSIS
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.In sys/sf_buf.h
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.Ft struct sf_buf *
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.Fn sf_buf_alloc "struct vm_page *m" "int flags"
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.Ft void
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.Fn sf_buf_free "struct sf_buf *sf"
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.Ft vm_offset_t
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.Fn sf_buf_kva "struct sf_buf *sf"
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.Ft struct vm_page *
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.Fn sf_buf_page "struct sf_buf *sf"
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.Sh DESCRIPTION
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The
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.Nm
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interface, historically the
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.Xr sendfile 2
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buffer interface, allows kernel subsystems to manage temporary kernel address
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space mappings for physical memory pages.
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On systems with a direct memory map region (allowing all physical pages to be
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visible in the kernel address space at all times), the
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.Vt "struct sf_buf"
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will point to an address in the direct map region; on systems without a
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direct memory map region, the
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.Vt "struct sf_buf"
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will manage a temporary kernel address space mapping valid for the lifetime
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of the
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.Vt "struct sf_buf".
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.Pp
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Call
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.Fn sf_buf_alloc
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to allocate a
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.Vt "struct sf_buf"
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for a physical memory page.
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.Fn sf_buf_alloc
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is not responsible for arranging for the page to be present in physical
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memory; the caller should already have arranged for the page to be wired,
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i.e., by calling
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.Xr vm_page_wire 9 .
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Several flags may be passed to
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.Fn sf_buf_alloc :
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.Bl -tag -width SFB_CPUPRIVATE
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.It Dv SFB_CATCH
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Cause
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.Fn sf_buf_alloc
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to abort and return
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.Dv NULL
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if a signal is received waiting for a
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.Vt "struct sf_buf"
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to become available.
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.It Dv SFB_NOWAIT
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Cause
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.Fn sf_buf_alloc
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to return
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.Dv NULL
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rather than sleeping if a
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.Vt "struct sf_buf"
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is not immediately available.
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.It Dv SFB_CPUPRIVATE
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Cause
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.Fn sf_buf_alloc
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to only arrange that the temporary mapping be valid on the current CPU,
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avoiding unnecessary TLB shootdowns for mappings that will only be accessed
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on a single CPU at a time.
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The caller must ensure that accesses to the virtual address occur only on the
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CPU from which
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.Fn sf_buf_alloc
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was invoked, perhaps by using
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.Fn sched_pin .
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.El
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.Pp
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Call
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.Fn sf_buf_kva
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to return a kernel mapped address for the page.
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.Pp
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Call
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.Fn sf_buf_page
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to return a pointer to the page originally passed into
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.Fn sf_buf_alloc .
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.Pp
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Call
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.Fn sf_buf_free
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to release the
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.Vt "struct sf_buf"
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reference.
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The caller is responsible for releasing any wiring they have previously
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acquired on the physical page;
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.Fn sf_buf_free
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releases only the temporary kernel address space mapping, not the page
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itself.
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.Pp
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Uses of this interface include managing mappings of borrowed pages from user
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memory, such as in zero-copy socket I/O, or pages of memory from the buffer
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cache referenced by mbuf external storage for
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.Xr sendfile 2 .
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.Sh SEE ALSO
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.Xr sendfile 2 ,
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.Xr vm_page_wire 9
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.Sh AUTHORS
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The
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.Vt "struct sf_buf"
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API was designed and implemented by
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.An Alan L. Cox .
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This manual page was written by
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.An Robert N. M. Watson .
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