Rusty's Remarkably Unreliable Guide to Lguest
2 - or, A Young Coder's Illustrated Hypervisor
5 Lguest is designed to be a minimal hypervisor for the Linux kernel, for
6 Linux developers and users to experiment with virtualization with the
7 minimum of complexity. Nonetheless, it should have sufficient
8 features to make it useful for specific tasks, and, of course, you are
9 encouraged to fork and enhance it.
13 - Kernel module which runs in a normal kernel.
14 - Simple I/O model for communication.
15 - Simple program to create new guests.
16 - Logo contains cute puppies:
21 - No ABI: being tied to a specific kernel anyway, you can change anything.
22 - Many opportunities for improvement or feature implementation.
26 - Lguest runs the same kernel as guest and host. You can configure
27 them differently, but usually it's easiest not to.
29 You will need to configure your kernel with the following options:
31 CONFIG_HIGHMEM64G=n ("High Memory Support" "64GB")[1]
32 CONFIG_TUN=y/m ("Universal TUN/TAP device driver support")
33 CONFIG_EXPERIMENTAL=y ("Prompt for development and/or incomplete code/drivers")
34 CONFIG_PARAVIRT=y ("Paravirtualization support (EXPERIMENTAL)")
35 CONFIG_LGUEST=y/m ("Linux hypervisor example code")
38 CONFIG_HZ=100 ("Timer frequency")[2]
40 - A tool called "lguest" is available in this directory: type "make"
41 to build it. If you didn't build your kernel in-tree, use "make
44 - Create or find a root disk image. There are several useful ones
45 around, such as the xm-test tiny root image at
48 For more serious work, I usually use a distribution ISO image and
49 install it under qemu, then make multiple copies:
51 dd if=/dev/zero of=rootfile bs=1M count=2048
52 qemu -cdrom image.iso -hda rootfile -net user -net nic -boot d
54 - "modprobe lg" if you built it as a module.
56 - Run an lguest as root:
58 Documentation/lguest/lguest 64m vmlinux --tunnet=192.168.19.1 --block=rootfile root=/dev/lgba
61 64m: the amount of memory to use.
63 vmlinux: the kernel image found in the top of your build directory. You
64 can also use a standard bzImage.
66 --tunnet=192.168.19.1: configures a "tap" device for networking with this
69 --block=rootfile: a file or block device which becomes /dev/lgba
72 root=/dev/lgba: this (and anything else on the command line) are
73 kernel boot parameters.
75 - Configuring networking. I usually have the host masquerade, using
76 "iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE" and "echo 1 >
77 /proc/sys/net/ipv4/ip_forward". In this example, I would configure
78 eth0 inside the guest at 192.168.19.2.
80 Another method is to bridge the tap device to an external interface
81 using --tunnet=bridge:
, and perhaps run dhcp on the guest
82 to obtain an IP address. The bridge needs to be configured first:
83 this option simply adds the tap interface to it.
85 A simple example on my system:
93 Then use --tunnet=bridge:lg0 when launching the guest.
96 on how to get bridging working.
98 - You can also create an inter-guest network using
99 "--sharenet=
": any two guests using the same file are on
100 the same network. This file is created if it does not exist.
104 Lguest uses a simplified DMA model plus shared memory for I/O. Guests
105 can communicate with each other if they share underlying memory
106 (usually by the lguest program mmaping the same file), but they can
107 use any non-shared memory to communicate with the lguest process.
109 Guests can register DMA buffers at any key (must be a valid physical
110 address) using the LHCALL_BIND_DMA(key, dmabufs, num<<8|irq)
111 hypercall. "dmabufs" is the physical address of an array of "num"
112 "struct lguest_dma": each contains a used_len, and an array of
113 physical addresses and lengths. When a transfer occurs, the
114 "used_len" field of one of the buffers which has used_len 0 will be
115 set to the length transferred and the irq will fire.
117 Using an irq value of 0 unbinds the dma buffers.
119 To send DMA, the LHCALL_SEND_DMA(key, dma_physaddr) hypercall is used,
120 and the bytes used is written to the used_len field. This can be 0 if
121 noone else has bound a DMA buffer to that key or some other error.
122 DMA buffers bound by the same guest are ignored.
125 Rusty Russell rusty@rustcorp.com.au.
127 [1] These are on various places on the TODO list, waiting for you to
128 get annoyed enough at the limitation to fix it.
129 [2] Lguest is not yet tickless when idle. See [1].
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