1.\" 2.\" Copyright (c) 2003 Networks Associates Technology, Inc. 3.\" All rights reserved. 4.\" 5.\" This software was developed for the FreeBSD Project by Jake Burkholder, 6.\" Safeport Network Services, and Network Associates Laboratories, the 7.\" Security Research Division of Network Associates, Inc. under 8.\" DARPA/SPAWAR contract N66001-01-C-8035 ("CBOSS"), as part of the DARPA 9.\" CHATS research program. 10.\" 11.\" Redistribution and use in source and binary forms, with or without 12.\" modification, are permitted provided that the following conditions 13.\" are met: 14.\" 1. Redistributions of source code must retain the above copyright 15.\" notice, this list of conditions and the following disclaimer. 16.\" 2. Redistributions in binary form must reproduce the above copyright 17.\" notice, this list of conditions and the following disclaimer in the 18.\" documentation and/or other materials provided with the distribution. 19.\" 20.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND 21.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 22.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 23.\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE 24.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 25.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 26.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 27.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 28.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 29.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 30.\" SUCH DAMAGE. 31.\" 32.Dd April 8, 2003 33.Dt PAE 4 i386 34.Os 35.Sh NAME 36.Nm PAE 37.Nd Physical Address Extensions 38.Sh SYNOPSIS 39.Cd "options PAE" 40.Sh DESCRIPTION 41The 42.Dv PAE 43option provides support for the physical address extensions capability 44of the 45.Tn Intel 46.Tn Pentium Pro 47and above CPUs, 48and allows for up to 64 gigabytes of memory to be used in systems capable 49of supporting it. 50With the 51.Dv PAE 52option, memory above 4 gigabytes is simply added to the general page pool. 53The system makes no distinction between memory above or below 4 gigabytes, 54and no specific facility is provided for a process or the kernel to access 55more memory than they would otherwise be able to access, through a sliding 56window or otherwise. 57.Sh SEE ALSO 58.Xr smp 4 , 59.Xr tuning 7 , 60.Xr config 8 , 61.Xr bus_dma 9 62.Sh HISTORY 63The 64.Dv PAE 65option first appeared in 66.Fx 4.9 67and 68.Fx 5.1 . 69.Sh AUTHORS 70.An Jake Burkholder Aq Mt jake@FreeBSD.org 71.Sh BUGS 72Since KLD modules are not compiled with the same options headers that 73the kernel is compiled with, 74they must not be loaded into a kernel compiled with the 75.Dv PAE 76option. 77.Pp 78Many devices or their device drivers are not capable of direct memory access 79to physical addresses above 4 gigabytes. 80In order to make use of direct memory access IO in a system with more than 814 gigabytes of memory when the 82.Dv PAE 83option is used, 84these drivers must use a facility for remapping or substituting physical 85memory which is not accessible to the device. 86One such facility is provided by the 87.Nm busdma 88interface. 89Device drivers which do not account for such devices will not work reliably 90in a system with more than 4 gigabytes of memory when the 91.Dv PAE 92option is used, 93and may cause data corruption. 94The 95.Pa PAE 96kernel configuration file includes the 97.Dv PAE 98option, and explicitly excludes all device drivers which are known to not work 99or have not been tested in a system with the 100.Dv PAE 101option and more than 4 gigabytes of memory. 102.Pp 103Many parameters which determine how memory is used in the kernel are based on 104the amount of physical memory. 105The formulas used to determine the values of these parameters for specific 106memory configurations may not take into account the fact there may be more 107than 4 gigabytes of memory, and may not scale well to these memory 108configurations. 109In particular, 110it may be necessary to increase the amount of virtual address space available 111to the kernel, 112or to reduce the amount of a specific resource that is heavily used, 113in order to avoid running out of virtual address space. 114The 115.Dv KVA_PAGES 116option may be used to increase the kernel virtual address space, 117and the 118.Va kern.maxvnodes 119.Xr sysctl 8 120may be used to decrease the number of vnodes allowed, 121an example of a resource that the kernel is likely to overallocate in 122large memory configurations. 123For optimal performance and stability it may be necessary to consult the 124.Xr tuning 7 125manual page, and make adjustments to the parameters documented there. 126