[src/uebayasi-xip]: src/share/man/man4 Brief introduction of xip(4).

[uebayasi <uebayasi%NetBSD.org@localhost>]

details:   https://anonhg.NetBSD.org/src/rev/d168645b3be9
branches:  uebayasi-xip
changeset: 751835:d168645b3be9
user:      uebayasi <uebayasi%NetBSD.org@localhost>
date:      Fri Nov 05 08:55:26 2010 +0000

description:
Brief introduction of xip(4).

diffstat:

 share/man/man4/xip.4 |  94 ++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 files changed, 94 insertions(+), 0 deletions(-)

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+.\"     $NetBSD: xip.4,v 1.1.2.1 2010/11/05 08:55:26 uebayasi Exp $
+.\"
+.\" Copyright (c) 2010 The NetBSD Foundation, Inc.
+.\" All rights reserved.
+.\"
+.\" Redistribution and use in source and binary forms, with or without
+.\" modification, are permitted provided that the following conditions
+.\" are met:
+.\" 1. Redistributions of source code must retain the above copyright
+.\"    notice, this list of conditions and the following disclaimer.
+.\" 2. Redistributions in binary form must reproduce the above copyright
+.\"    notice, this list of conditions and the following disclaimer in the
+.\"    documentation and/or other materials provided with the distribution.
+.\"
+.\" THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS
+.\" ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
+.\" TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
+.\" PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS
+.\" BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+.\" CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+.\" SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+.\" INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+.\" CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+.\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+.\" POSSIBILITY OF SUCH DAMAGE.
+.\"
+.Dd November 4, 2010
+.Dt XIP 4
+.Os
+.
+.Sh NAME
+.Nm XIP
+.Nd eXecute-In-Place
+.
+.Sh SYNOPSIS
+.Cd options XIP
+.
+.Sh DESCRIPTION
+The
+.Nm
+option enables a functionality for the kernel to execute programs
+directly from memory-addressable device pages.
+Normally the kernel allocates memory pages (page cache) and copies
+file contents onto them, because most filesystem storage devices
+are accessed by I/O.
+There are devices whole data can be memory-mapped.
+.Pp
+By using
+.Nm ,
+the kernel maps those file contents directly into user process's
+address space.
+This brings us to save memory used for page caches, and shorten
+program load time compared to slow I/O storage devices.
+.Pp
+Needless to say, not all block devices can be capable of XIP.
+Only memory-mappable block devices, i.e., NOR FlashROM, and RAM
+based memory disk.
+To support XIP, block device drivers need to allocate a VM device
+physical segment beforehand, and return it at the request of
+DIOCGPHYSSEG ioctl, called from VFS.
+VM device physical segment is allocated by calling the
+.Xr bus_space_physload_device
+function in
+.Xr bus_space 9
+API.
+.
+.Sh SEE ALSO
+.Xr flash 4 ,
+.Xr xmd 4 ,
+.Xr mount 8 ,
+.Xr bus_space 9 ,
+.Xr disk 9 ,
+.Xr uvm 9
+.
+.Sh CAVEATS
+.Pp
+Don't confuse memory-mappable NOR FlashROM and FlashROM based
+storage device.  To access the latter, CPU uses I/O instead of
+MMIO.
+.
+.Sh HISTORY
+.Pp
+.Nm
+first appeared in
+.Nx 6.0 .
+.
+.Sh AUTHORS
+The
+.Nm
+support was added by Masao Uebayashi.
+.
+.Sh BUGS
+.Pp
+Write operation is not supported.