view Documentation/networking/tuntap.txt @ 897:329ea0ccb344

balloon: try harder to balloon up under memory pressure.

Currently if the balloon driver is unable to increase the guest's
reservation it assumes the failure was due to reaching its full
allocation, gives up on the ballooning operation and records the limit
it reached as the "hard limit". The driver will not try again until
the target is set again (even to the same value).

However it is possible that ballooning has in fact failed due to
memory pressure in the host and therefore it is desirable to keep
attempting to reach the target in case memory becomes available. The
most likely scenario is that some guests are ballooning down while
others are ballooning up and therefore there is temporary memory
pressure while things stabilise. You would not expect a well behaved
toolstack to ask a domain to balloon to more than its allocation nor
would you expect it to deliberately over-commit memory by setting
balloon targets which exceed the total host memory.

This patch drops the concept of a hard limit and causes the balloon
driver to retry increasing the reservation on a timer in the same
manner as when decreasing the reservation.

Also if we partially succeed in increasing the reservation
(i.e. receive less pages than we asked for) then we may as well keep
those pages rather than returning them to Xen.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
author Keir Fraser <keir.fraser@citrix.com>
date Fri Jun 05 14:01:20 2009 +0100 (2009-06-05)
parents 831230e53067
line source
1 Universal TUN/TAP device driver.
2 Copyright (C) 1999-2000 Maxim Krasnyansky <max_mk@yahoo.com>
4 Linux, Solaris drivers
5 Copyright (C) 1999-2000 Maxim Krasnyansky <max_mk@yahoo.com>
7 FreeBSD TAP driver
8 Copyright (c) 1999-2000 Maksim Yevmenkin <m_evmenkin@yahoo.com>
10 Revision of this document 2002 by Florian Thiel <florian.thiel@gmx.net>
12 1. Description
13 TUN/TAP provides packet reception and transmission for user space programs.
14 It can be seen as a simple Point-to-Point or Ethernet device, which,
15 instead of receiving packets from physical media, receives them from
16 user space program and instead of sending packets via physical media
17 writes them to the user space program.
19 In order to use the driver a program has to open /dev/net/tun and issue a
20 corresponding ioctl() to register a network device with the kernel. A network
21 device will appear as tunXX or tapXX, depending on the options chosen. When
22 the program closes the file descriptor, the network device and all
23 corresponding routes will disappear.
25 Depending on the type of device chosen the userspace program has to read/write
26 IP packets (with tun) or ethernet frames (with tap). Which one is being used
27 depends on the flags given with the ioctl().
29 The package from http://vtun.sourceforge.net/tun contains two simple examples
30 for how to use tun and tap devices. Both programs work like a bridge between
31 two network interfaces.
32 br_select.c - bridge based on select system call.
33 br_sigio.c - bridge based on async io and SIGIO signal.
34 However, the best example is VTun http://vtun.sourceforge.net :))
36 2. Configuration
37 Create device node:
38 mkdir /dev/net (if it doesn't exist already)
39 mknod /dev/net/tun c 10 200
41 Set permissions:
42 e.g. chmod 0666 /dev/net/tun
43 There's no harm in allowing the device to be accessible by non-root users,
44 since CAP_NET_ADMIN is required for creating network devices or for
45 connecting to network devices which aren't owned by the user in question.
46 If you want to create persistent devices and give ownership of them to
47 unprivileged users, then you need the /dev/net/tun device to be usable by
48 those users.
50 Driver module autoloading
52 Make sure that "Kernel module loader" - module auto-loading
53 support is enabled in your kernel. The kernel should load it on
54 first access.
56 Manual loading
57 insert the module by hand:
58 modprobe tun
60 If you do it the latter way, you have to load the module every time you
61 need it, if you do it the other way it will be automatically loaded when
62 /dev/net/tun is being opened.
64 3. Program interface
65 3.1 Network device allocation:
67 char *dev should be the name of the device with a format string (e.g.
68 "tun%d"), but (as far as I can see) this can be any valid network device name.
69 Note that the character pointer becomes overwritten with the real device name
70 (e.g. "tun0")
72 #include <linux/if.h>
73 #include <linux/if_tun.h>
75 int tun_alloc(char *dev)
76 {
77 struct ifreq ifr;
78 int fd, err;
80 if( (fd = open("/dev/net/tun", O_RDWR)) < 0 )
81 return tun_alloc_old(dev);
83 memset(&ifr, 0, sizeof(ifr));
85 /* Flags: IFF_TUN - TUN device (no Ethernet headers)
86 * IFF_TAP - TAP device
87 *
88 * IFF_NO_PI - Do not provide packet information
89 */
90 ifr.ifr_flags = IFF_TUN;
91 if( *dev )
92 strncpy(ifr.ifr_name, dev, IFNAMSIZ);
94 if( (err = ioctl(fd, TUNSETIFF, (void *) &ifr)) < 0 ){
95 close(fd);
96 return err;
97 }
98 strcpy(dev, ifr.ifr_name);
99 return fd;
100 }
102 3.2 Frame format:
103 If flag IFF_NO_PI is not set each frame format is:
104 Flags [2 bytes]
105 Proto [2 bytes]
106 Raw protocol(IP, IPv6, etc) frame.
108 Universal TUN/TAP device driver Frequently Asked Question.
110 1. What platforms are supported by TUN/TAP driver ?
111 Currently driver has been written for 3 Unices:
112 Linux kernels 2.2.x, 2.4.x
113 FreeBSD 3.x, 4.x, 5.x
114 Solaris 2.6, 7.0, 8.0
116 2. What is TUN/TAP driver used for?
117 As mentioned above, main purpose of TUN/TAP driver is tunneling.
118 It is used by VTun (http://vtun.sourceforge.net).
120 Another interesting application using TUN/TAP is pipsecd
121 (http://perso.enst.fr/~beyssac/pipsec/), an userspace IPSec
122 implementation that can use complete kernel routing (unlike FreeS/WAN).
124 3. How does Virtual network device actually work ?
125 Virtual network device can be viewed as a simple Point-to-Point or
126 Ethernet device, which instead of receiving packets from a physical
127 media, receives them from user space program and instead of sending
128 packets via physical media sends them to the user space program.
130 Let's say that you configured IPX on the tap0, then whenever
131 the kernel sends an IPX packet to tap0, it is passed to the application
132 (VTun for example). The application encrypts, compresses and sends it to
133 the other side over TCP or UDP. The application on the other side decompresses
134 and decrypts the data received and writes the packet to the TAP device,
135 the kernel handles the packet like it came from real physical device.
137 4. What is the difference between TUN driver and TAP driver?
138 TUN works with IP frames. TAP works with Ethernet frames.
140 This means that you have to read/write IP packets when you are using tun and
141 ethernet frames when using tap.
143 5. What is the difference between BPF and TUN/TAP driver?
144 BPF is an advanced packet filter. It can be attached to existing
145 network interface. It does not provide a virtual network interface.
146 A TUN/TAP driver does provide a virtual network interface and it is possible
147 to attach BPF to this interface.
149 6. Does TAP driver support kernel Ethernet bridging?
150 Yes. Linux and FreeBSD drivers support Ethernet bridging.