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+.. SPDX-License-Identifier: GPL-2.0
+
+#########
+UML HowTo
+#########
+
+:Author:  User Mode Linux Core Team
+:Last-updated: Friday Sep 04 14:50:55 BST 2020
+
+
+************
+Introduction
+************
+
+Welcome to User Mode Linux
+
+User Mode Linux is the first Open Source virtualization package (first
+release date 1991) and second virtualization package for an x86 PC.
+
+How is UML Different from a VM using Virtualization package X?
+==============================================================
+
+We have come to assume that virtualization also means some level of
+hardware emulation. In fact, it does not. As long as a virtualization
+package provides the OS with devices which the OS can recognize and
+has a driver for, the devices do not need to emulate real hardware.
+Most OSes today have built-in support for a number of "fake"
+devices used only under virtualization.
+User Mode Linux takes this concept to the ultimate extreme - there
+is not a single real device in sight. It is 100% artificial or if
+we use the correct term 100% paravirtual. All UML devices are abstract
+concepts which map onto something provided by the host - files, sockets,
+pipes, etc.
+
+The other major difference between UML and various virtualization
+packages is that there is a distinct difference between the way the UML
+kernel and the UML programs operate.
+The UML kernel is just a process running on Linux - same as any other
+program. It can be run by an unprivileged user and it does not require
+anything in terms of special CPU features.
+The UML userspace, however, is a bit different. The Linux kernel on the
+host machine assists UML in intercepting everything the program running
+on a UML instance is trying to do and making the UML kernel handle all
+of its requests.
+This is different from other virtualization packages which do not make any
+difference between the guest kernel and guest programs. This difference
+results in a number of advantages and disadvantages of UML over let's say
+QEMU which we will cover later in this document.
+
+
+Why Would I Want User Mode Linux?
+=================================
+
+
+1. If User Mode Linux kernel crashes, your host kernel is still fine. It
+   is not accelerated in any way (vhost, kvm, etc) and it is not trying to
+   access any devices directly.  It is, in fact, a process like any other.
+
+#. You can run a usermode kernel as a non-root user (you may need to
+   arrange appropriate permissions for some devices).
+
+#. You can run a very small VM with a minimal footprint for a specific
+   task (f.e. 32M or less).
+
+#. You can get extremely high performance for anything which is a "kernel
+   specific task" such as forwarding, firewalling, etc while still being
+   isolated from the host kernel.
+
+#. You can play with kernel concepts without breaking things.
+
+#. You are not bound by "emulating" hardware, so you can try weird and
+   wonderful concepts which are very difficult to support when emulating
+   real hardware such as time travel and making your system clock
+   dependent on what UML does (very useful for things like tests).
+
+#. It's fun.
+
+Why not to run UML
+==================
+
+1. The syscall interception technique used by UML makes it inherently
+   slower for any userspace applications. While it can do kernel tasks
+   on par with most other virtualization packages, its userspace is
+   **slow**. The root cause is that UML has a very high cost of creating
+   new processes and threads (something most Unix/Linux applications
+   take for granted).
+
+#. UML is strictly uniprocessor at present. If you want to run an
+   application which needs many CPUs to function, it is clearly the
+   wrong choice.
+
+***********************
+Building a UML instance
+***********************
+
+There is no UML installer in any distribution. While you can use off
+the shelf install media to install into a blank VM using a virtualization
+package, there is no UML equivalent. You have to use appropriate tools on
+your host to build a viable filesystem image.
+
+This is extremely easy on Debian - you can do it using debootstrap. It is
+also easy on OpenWRT - the build process can build UML images. All other
+distros - YMMV.
+
+Creating an image
+=================
+
+Create a sparse raw disk image:
+
+.. code-block:: shell
+
+    dd if=/dev/zero of=disk_image_name bs=1 count=1 seek=16G
+
+will create a 16G disk image. The OS will initially allocate only one
+block and will allocate more as they are written by UML. As of kernel
+version 4.19 UML fully supports TRIM (as usually used by flash drives).
+Using TRIM inside the UML image by specifying discard as a mount option
+or by running ``tune2fs -o discard /dev/ubdXX`` will request UML to
+return any unused blocks to the OS.
+
+Create a filesystem on the disk image and mount it:
+---------------------------------------------------
+
+.. code-block:: shell
+
+    mkfs.ext4 ./disk_image_name
+    mount ./disk_image_name /mnt
+
+This example uses ext4, any other filesystem such as ext3, btrfs, xfs,
+jfs, etc will work too.
+
+Create a minimal OS installation on the mounted filesystem
+----------------------------------------------------------
+
+.. code-block:: shell
+
+    debootstrap buster /mnt http://deb.debian.org/debian
+
+debootstrap does not set up the root password, fstab, hostname or
+anything related to networking. It is up to the user to do that.
+
+Set the root password
+---------------------
+
+The easiest way to do that is to chroot to the mounted image.
+
+.. code-block:: shell
+
+    chroot /mnt
+    passwd
+    exit
+
+Edit key system files
+=====================
+
+
+Edit fstab
+----------
+
+UML block devices are called ubds. The fstab created by debootstrap
+will be empty and it needs an entry for the root file system:
+
+.. code-block:: text
+
+    /dev/ubd0   ext4    discard,errors=remount-ro  0       1
+
+
+Edit hostname
+-------------
+
+It will be set to the same as the host on which you are creating the
+image. It is a good idea to change that to avoid "Oh, bummer, I
+rebooted the wrong machine".
+
+Edit network/interfaces on Debian or their equivalent
+-----------------------------------------------------
+
+UML supports two classes of network devices - the older uml_net ones
+which are scheduled for obsoletion. These are called ethX. It also
+suports the newer vector IO devices which are significantly faster
+and have support for some standard virtual network encapsulations like
+Ethernet over GRE and Ethernet over L2TPv3. These are called vec0.
+
+Depending on which one is in use, ``/etc/network/interfaces`` will
+need an entry like
+
+.. code-block:: text
+
+	auto eth0
+	iface eth0 inet dhcp
+
+or
+
+.. code-block:: text
+
+	auto vec0
+	iface eth0 inet dhcp
+
+
+We now have a UML image which is nearly ready to run, all we need is a
+UML kernel and modules for it.
+
+Most distributions have a UML package. Even if you intend to use your own
+kernel, testing the image with a stock one is always a good start. These
+packages come with a set of modules which should be copied to the target
+filesystem. The location is distribution dependent. For Debian these
+reside under /usr/lib/uml/modules. Copy recursively the content of this
+directory to the mounted UML filesystem:
+
+
+.. code-block:: shell
+
+    cp -rax /usr/lib/uml/modules /mnt/lib/modules
+
+If you have compiled your own kernel, you need to use the usual "install
+modules to a location" procedure by running
+
+.. code-block:: shell
+
+    make install MODULES_DIR=/mnt/lib/modules
+
+At this point the image is ready to be brought up.
+
+*************************
+Setting Up UML Networking
+*************************
+
+UML networking is designed to emulate an Ethernet connection. This
+connection may be either a point-to-point (similar to a connection
+between machines using a back-to-back cable) or a connection to a
+switch. UML supports a wide variety of means to build these
+connections to all of: local machine, remote machine(s), local and
+remote UML and other VM instances.
+
+.. csv-table:: Supported Transports
+   :header: "Transport", "Type", "Capabilities", "Speed (on 3.5GHz Ryzen)"
+   :widths: 20, 10, 30, 20
+
+    "tap", "vector", "checksum and tso offloads", "> 8Gbit"
+    "hybrid", "vector", "checksum and tso offloads, multipacket rx", "> 6GBit"
+    "raw", "vector", "checksum and tso offloads, multipacket rx, tx", "> 6GBit"
+    "Ethernet over gre", "vector", "multipacket rx, tx", "> 3Gbit"
+    "Ethernet over l2tpv3", "vector", "multipacket rx, tx", >" 3Gbit"
+    "bess", "vector", "multipacket rx, tx ", "> 3Gbit"
+    "fd", "vector", "dependent on fd type", "varies, up to >6Gbit"
+    "tuntap", "legacy", "none", "~ 500Mbit"
+    "daemon", "legacy", "none", "~ 450Mbit"
+    "socket", "legacy", "none", "~ 450Mbit"
+    "pcap", "legacy", "none", "~ 400Mbit, rx only"
+    "ethertap", "obsolete", "none", "uknown"
+    "vde", "obsolete", "none", "uknown"
+
+* All transports which have tso and checksum offloads can deliver speeds
+  approaching 10G on TCP streams.
+
+* All transports which have multi-packet rx and/or tx can deliver pps
+  rates of up to 1Mps or more.
+
+* All legacy transports are generally limited to ~600-700MBit and 0.05Mps
+
+* GRE and L2TPv3 allow connections to all of: local machine, remote
+  machines, remote network devices and remote UML instances.
+
+* Socket allows connections only between UML instances.
+
+* Daemon and bess require running a local switch. This switch may be
+  connected to the host as well.
+
+
+Network configuration privileges
+================================
+
+The majority of the supported networking modes need ``root`` privileges.
+For example, in the legacy tuntap networking mode, users were required
+to be part of the group associated with the tunnel device.
+
+For newer network drivers like the vector transports, ``root`` privilege
+is required to fire an ioctl to setup the tun interface and/or use
+raw sockets where needed.
+
+This can be achieved by granting the user a particular capability instead
+of running UML as root.  In case of vector transport, a user can add the
+capability ``CAP_NET_ADMIN`` or ``CAP_NET_RAW``, to the uml binary.
+Thenceforth, UML can be run with normal user privilges, along with
+full networking.
+
+For example:
+
+.. code-block:: shell
+
+    sudo setcap cap_net_raw,cap_net_admin+ep linux
+
+Configuring vector transports
+===============================
+
+All vector transports support a similar syntax:
+
+If X is the interface number as in vec0, vec1, vec2, etc, the general
+syntax for options is:
+
+.. code-block:: shell
+
+    vecX:transport="Transport Name",option=value,option=value,...,option=value
+
+Common options
+--------------
+
+These options are common for all transports:
+
+1. ``depth=int`` - sets the queue depth for vector IO. This is the
+   amount of packets UML will attempt to read or write in a single
+   system call. The default number is 64 and is generally sufficient
+   for most ~ 2-4G applications. Higher speeds may require larger values.
+
+#. ``mac=XX:XX:XX:XX:XX`` - sets the interface MAC value.
+
+#. ``gro=[0,1]`` - sets GRO on or off. The offload which is most
+   commonly enabled as a result of this option being on is TCP if it is
+   possible. Note, that it is usually enabled by default on local
+   machine interfaces (f.e. veth pairs), so it should be enabled in UML
+   for a lot of transports for networking to operate correctly.
+
+#. ``mtu=int`` - sets the interface mtu
+
+#. ``headroom=int`` - adjusts the default headroom (32 bytes) reserved
+   if a packet will need to be re-encapsulated into let's say VXLAN.
+
+#. ``vec=0`` - disable multipacket io and fall back to packet at a
+   time mode
+
+Shared Options
+--------------
+
+1. ``ifname=str`` Transports which bind to a local network interface
+   have a shared option - the name of the interface to bind to.
+
+#. ``src, dst, src_port, dst_port`` - all transports which use sockets
+   which have the notion of source and destination and/or source port
+   and destination port use these to specify them.
+
+#. ``v6=[0,1]`` to specify if a v6 connection is desired for all
+   transports which operate over IP. Additionally, for transports that
+   have some differences in the way they operate over v4 and v6 (f.e.
+   EoL2TPv3), sets the correct mode of operation. In the absense of this
+   option, the socket type is determined based on what do the src and dst
+   arguments resolve/parse to.
+
+tap transport
+-------------
+
+Example:
+
+.. code-block:: shell
+
+    vecX:transport=tap,ifname=tap0,depth=128,gro=1
+
+This will connect vec0 to tap0 on the host. Tap0 must already exist (f.e.
+created using tunctl) and UP.
+
+tap0 can be configured as a point-to-point interface and given an ip
+address so that UML can talk to the host. Alternatively, it is possible
+to connect UML to a tap interface which is a part of a bridge.
+
+While tap relies on the vector infrastructure, it is not a true vector
+transport at this point, because Linux does not support multi-packet
+IO on tap file descriptors for normal userspace apps like UML. This
+is a privilege which is offered only to something which can hook up
+to it at kernel level via specialized interfaces like vhost-net. A
+vhost-net like helper for UML is planned at some point in the future.
+
+Privileges required: tap transport requires either:
+
+1. tap interface to exist and be created persistent and owned by the
+   UML user using tunctl. Example ``tunctl -u uml-user -t tap0``
+
+#. UML binary to have ``CAP_NET_ADMIN`` privilege
+
+hybrid transport
+----------------
+
+Example:
+
+.. code-block:: shell
+
+    vecX:transport=hybrid,ifname=tap0,depth=128,gro=1
+
+This is an experimental/demo transport which couples tap for transmit
+and a raw socket for receive. The raw socket allows multi-packet
+receive resulting in significantly higher packet rates than normal tap
+
+Privileges required: hybrid requires ``CAP_NET_RAW`` capability by
+the UML user as well as the requirements for the tap transport.
+
+raw socket transport
+--------------------
+
+Example:
+
+.. code-block:: shell
+
+    vecX:transport=raw,ifname=veth0,depth=128,gro=1
+
+
+This transport uses vector IO on raw sockets. While you can bind to any
+interface including a physical one, the most common use it to bind to
+the "peer" side of a veth pair with the other side configured on the
+host.
+
+Example host configuration for Debian:
+
+**/etc/network/interfaces**
+
+.. code-block:: text
+
+    auto veth0
+    iface veth0 inet static
+	 address 192.168.4.1
+	 netmask 255.255.255.252
+	 roadcast 192.168.4.3
+	 pre-up ip link add veth0 type veth peer name p-veth0 && \
+           ifconfig p-veth0 up
+
+UML can now bind to p-veth0 like this:
+
+.. code-block:: text
+
+    vec0:transport=raw,ifname=p-veth0,depth=128,gro=1
+
+
+If the UML guest is configured with 192.168.4.2 and netmask 255.255.255.0
+it can talk to the host on 192.168.4.1
+
+The raw transport also provides some support for offloading some of the
+filtering to the host. The two options to control it are:
+
+1. bpffile = filename of raw bpf code to be loaded as a socket filter
+
+#. bpfflash = 0/1 allow loading of bpf from inside User Mode Linux.
+   This option allows the use of the ethtool load firmware command to
+   load bpf code.
+
+In either case the bpf code is loaded into the host kernel. While this is
+presently limited to legacy bpf syntax (not ebpf), it is still a security
+risk. It is not recommended to allow this unless the User Mode Linux
+instance is considered trusted.
+
+Privileges required: raw socket transport requires `CAP_NET_RAW`
+capability.
+
+gre socket transport
+--------------------
+
+Example:
+
+.. code-block:: text
+
+    vecX:transport=gre,src=src_host,dst=dst_host
+
+
+This will configure an Ethernet over ``GRE`` (aka ``GRETAP`` or
+``GREIRB``) tunnel which will connect the UML instance to a ``GRE``
+endpoint at host dst_host. ``GRE`` supports the following additional
+options:
+
+1. ``rx_key=int`` - gre 32 bit integer key for rx packets, if set,
+   ``txkey`` must be set too
+
+#. ``tx_key=int`` - gre 32 bit integer key for tx packets, if set
+   ``rx_key`` must be set too
+
+#. ``sequence=[0,1]`` - enable gre sequence
+
+#. ``pin_sequence=[0,1]`` - pretend that the sequence is always reset
+   on each packet (needed to interop with some really broken
+   implementations)
+
+#. ``v6=[0,1]`` - force v4 or v6 sockets respectively
+
+#. Gre checksum is not presently supported
+
+GRE has a number of caveats:
+
+1. You can use only one gre connection per ip address. There is no way to
+   multiplex connections as each GRE tunnel is terminated directly on
+   the UML instance.
+
+#. The key is not really a security feature. While it was intended as such
+   it's "security" is laughable. It is, however, a useful feature to
+   ensure that the tunnel is not misconfigured.
+
+An example configuration for a Linux host with a local address of
+192.168.128.1 to connect to a UML instance at 192.168.129.1
+
+**/etc/network/interfaces**
+
+.. code-block:: text
+
+    auto gt0
+    iface gt0 inet static
+     address 10.0.0.1
+     netmask 255.255.255.0
+     broadcast 10.0.0.255
+     mtu 1500
+     pre-up ip link add gt0 type gretap local 192.168.128.1 \
+            remote 192.168.129.1 || true
+     down ip link del gt0 || true
+
+Additionally, GRE has been tested versus a variety of network equipment.
+
+Privileges required: GRE requires ``CAP_NET_RAW``
+
+l2tpv3 socket transport
+-----------------------
+
+_Warning_. L2TPv3 has a "bug". It is the "bug" known as "has more
+options than GNU ls". While it has some advantages, there are usually
+easier (and less verbose) ways to connect a UML instance to something.
+For example, most devices which support L2TPv3 also support GRE.
+
+Example:
+
+.. code-block:: text
+
+    vec0:transport=l2tpv3,udp=1,src=$src_host,dst=$dst_host,srcport=$src_port,dstport=$dst_port,depth=128,rx_session=0xffffffff,tx_session=0xffff
+
+
+This will configure an Ethernet over L2TPv3 fixed tunnel which will
+connect the UML instance to a L2TPv3 endpoint at host $dst_host using
+the L2TPv3 UDP flavour and UDP destination port $dst_port.
+
+L2TPv3 always requires the following additional options:
+
+1. ``rx_session=int`` - l2tpv3 32 bit integer session for rx packets
+
+#. ``tx_session=int`` - l2tpv3 32 bit integer session for tx packets
+
+As the tunnel is fixed these are not negotiated and they are
+preconfigured on both ends.
+
+Additionally, L2TPv3 supports the following optional parameters
+
+#. ``rx_cookie=int`` - l2tpv3 32 bit integer cookie for rx packets - same
+   functionality as GRE key, more to prevent misconfiguration than provide
+   actual security
+
+#. ``tx_cookie=int`` - l2tpv3 32 bit integer cookie for tx packets
+
+#. ``cookie64=[0,1]`` - use 64 bit cookies instead of 32 bit.
+
+#. ``counter=[0,1]`` - enable l2tpv3 counter
+
+#. ``pin_counter=[0,1]`` - pretend that the counter is always reset on
+   each packet (needed to interop with some really broken implementations)
+
+#. ``v6=[0,1]`` - force v6 sockets
+
+#. ``udp=[0,1]`` - use raw sockets (0) or UDP (1) version of the protocol
+   L2TPv3 has a number of caveats:
+
+#. you can use only one connection per ip address in raw mode. There is
+   no way to multiplex connections as each L2TPv3 tunnel is terminated
+   directly on the UML instance. UDP mode can use different ports for
+   this purpose.
+
+Here is an example of how to configure a linux host to connect to UML
+via L2TPv3:
+
+
+**/etc/network/interfaces**
+
+.. code-block:: text
+
+    auto l2tp1
+    iface l2tp1 inet static
+     address 192.168.126.1
+     netmask 255.255.255.0
+     broadcast 192.168.126.255
+     mtu 1500
+     pre-up ip l2tp add tunnel remote 127.0.0.1 \
+            local 127.0.0.1 encap udp tunnel_id 2 \
+            peer_tunnel_id 2 udp_sport 1706 udp_dport 1707 && \
+            ip l2tp add session name l2tp1 tunnel_id 2 \
+            session_id 0xffffffff peer_session_id 0xffffffff
+     down ip l2tp del session tunnel_id 2 session_id 0xffffffff && \
+            ip l2tp del tunnel tunnel_id 2
+
+
+Privileges required: L2TPv3 requires ``CAP_NET_RAW`` for raw IP mode and
+no special privileges for the UDP mode.
+
+BESS socket transport
+---------------------
+
+BESS is a high performance modular network switch.
+
+https://github.com/NetSys/bess
+
+It has support for a simple sequential packet socket mode which in the
+more recent versions is using vector IO for high performance.
+
+Example:
+
+.. code-block:: text
+
+    vecX:transport=bess,src=unix_src,dst=unix_dst
+
+This will configure a BESS transport using the unix_src Unix domain
+socket address as source and unix_dst socket address as destination.
+
+For BESS configuration and how to allocate a BESS Unix domain socket port
+please see the BESS documentation.
+
+https://github.com/NetSys/bess/wiki/Built-In-Modules-and-Ports
+
+BESS transport does not require any special privileges.
+
+Configuring Legacy transports
+=============================
+
+Legacy transports are now considered obsolete. Please use the vector
+versions.
+
+***********
+Running UML
+***********
+
+This section assumes that either the user-mode-linux package from the
+distribution or a custom built kernel has been installed on the host.
+
+These add an executable called linux to the system. This is the UML
+kernel. It can be run just like any other executable.
+It will take most normal linux kernel arguments as command line
+arguments.  Additionally, it will need some UML specific arguments
+in order to do something useful.
+
+Arguments
+=========
+
+Mandatory Arguments:
+--------------------
+
+1. ``mem=int[K,M,G]`` - amount of memory. By default bytes. It will
+   also accept K, M or G qualifiers.
+
+#. ``ubdX[s,d,c,t]=`` virtual disk specification. This is not really
+   mandatory, but it is likely to be needed in nearly all cases so we can 
+   specify a root file system.
+   The simplest possible image specification is the name of the image
+   file for the filesystem (created using one of the methods described
+   in Generating an Image).
+
+   a. UBD devices support copy on write (COW). The changes are kept in
+      a separate file which can be discarded allowing a rollback to the
+      original pristine image.  If COW is desired, the UBD image is
+      specified as: ``cow_file,master_image``.
+      Example:``ubd0=Filesystem.cow,Filesystem.img``
+
+   #. UBD devices can be set to use synchronous IO. Any writes are
+      immediately flushed to disk. This is done by adding ``s`` after
+      the ``ubdX`` specification
+
+   #. UBD performs some euristics on devices specified as a single
+      filename to make sure that a COW file has not been specified as
+      the image. To turn them off, use the ``d`` flag after ``ubdX``
+
+   #. UBD supports TRIM - asking the Host OS to reclaim any unused
+      blocks in the image. To turn it off, specify the ``t`` flag after
+      ``ubdX``
+
+#. ``root=`` root device - most likely ``/dev/ubd0`` (this is a Linux
+   filesystem image)
+
+Important Optional Arguments
+----------------------------
+
+If UML is run as "linux" with no extra arguments, it will try to start an
+xterm for every console configured inside the image (up to 6 in most
+linux distributions). Each console is started inside an
+xterm. This makes it nice and easy to use UML on a host with a GUI. It is, however,
+the wrong approach if UML is to be used as a testing harness or run in
+a text-only environment.
+
+In order to change this behaviour we need to specify an alternative console
+and wire it to one of the supported "line" channels. For this we need to map a
+console to use something different from the default xterm.
+
+Example which will divert console number 1 to stdin/stdout:
+
+.. code-block:: text
+
+    con1=fd:0,fd:1
+
+UML supports a wide variety of serial line channels which are specified using the
+following syntax
+
+.. code-block:: text
+
+    conX=channel_type:options[,channel_type:options]
+
+
+If the channel specification contains two parts separated by comma, the first one
+is input, the second one output.
+
+1. The null channel - Discard all input or output. Example ``con=null`` will set all consoles to null by default.
+
+#. The fd channel - use file descriptor numbers for input/out. Example: ``con1=fd:0,fd:1.``
+
+#. The port channel - listen on tcp port number. Example: ``con1=port:4321``
+
+#. The pty and pts channels - use system pty/pts.
+
+#. The tty channel - bind to an existing system tty. Example: ``con1=/dev/tty8`` will make UML use the host 8th console (usually unused).
+
+#. The xterm channel - this is the default - bring up an xterm on this channel and
+   direct IO to it. Note, that in order for xterm to work, the host must have the
+   UML distribution package installed. This usually contains the port-helper and
+   other utilities needed for UML to communicate with the xterm. Alternatively,
+   these need to be complied and installed from source.  All options applicable
+   to consoles also apply to UML serial lines which are presented as ttyS inside UML.
+
+Starting UML
+============
+
+We can now run UML.
+
+.. code-block:: text
+
+    linux mem=2048M umid=TEST \
+	 ubd0=Filesystem.img \
+	 vec0:transport=tap,ifname=tap0,depth=128,gro=1 \
+	 root=/dev/ubda con=null con0=null,fd:2 con1=fd:0,fd:1
+
+This will run an instance with ``2048M RAM``, try to use the image file
+called ``Filesystem.img`` as root. It will connect to the host using tap0.
+All consoles except ``con1`` will be disabled and console 1 will
+use standard input/output making it appear in the same terminal it was started.
+
+Logging in
+============
+
+If you have not set up a password when generating the image, you will have to
+shut down the UML instance, mount the image, chroot into it and set it - as
+described in the Generating an Image section.  If the password is already set,
+you can just log in.
+
+The UML Management Console
+============================
+
+In addition to managing the image from "the inside" using normal sysadmin tools,
+it is possible
+to perform a number of low level operations using the UML management console.
+The UML management console is a low-level interface to the kernel on a running UML instance, somewhat like the i386 SysRq interface.  Since there is a full-blown
+operating system under UML, there is much greater flexibility possible
+than with the SysRq mechanism.
+
+There are a number of things you can do with the mconsole interface:
+
+* get the kernel version
+
+* add and remove devices
+
+* halt or reboot the machine
+
+* Send SysRq commands
+
+* Pause and resume the UML
+
+* Inspect processes running inside UML
+
+* Inspect UML internal /proc state
+
+You need the mconsole client (uml\_mconsole) which is a part of the UML
+tools package available in most Linux distritions.
+
+You also need ``CONFIG_MCONSOLE`` (under 'General Setup') enabled in the UML kernel.  When you boot UML, you'll see a line like:
+
+.. code-block:: text
+
+    mconsole initialized on /home/jdike/.uml/umlNJ32yL/mconsole
+
+If you specify a unique machine id one the UML command line, i.e.
+
+.. code-block:: text
+
+    umid=debian
+
+
+you'll see this
+
+.. code-block:: text
+
+    mconsole initialized on /home/jdike/.uml/debian/mconsole
+
+
+That file is the socket that uml_mconsole will use to communicate with
+UML.  Run it with either the umid or the full path as its argument:
+
+.. code-block:: text
+
+    host$ uml_mconsole debian
+
+or
+
+.. code-block:: text
+
+    host$ uml_mconsole /home/jdike/.uml/debian/mconsole
+
+
+You'll get a prompt, at which you can run one of these commands:
+
+* version
+
+* help
+
+* halt
+
+* reboot
+
+* config
+
+* remove
+
+* sysrq
+
+* help
+
+* cad
+
+* stop
+
+* go
+
+* proc
+
+* stack
+
+version
+-------
+
+This takes no arguments.  It prints the UML version.
+
+.. code-block:: text
+
+    (mconsole)  version
+
+    OK Linux OpenWrt 4.14.106 #0 Tue Mar 19 08:19:41 2019 x86_64
+
+
+There are a couple actual uses for this.  It's a simple no-op which
+can be used to check that a UML is running.  It's also a way of
+sending a device interrupt to the UML. UML mconsole is treated internally as
+a UML device.
+
+help
+----
+
+This takes no arguments. It prints a short help screen with the
+supported mconsole commands.
+
+
+halt and reboot
+---------------
+
+These take no arguments.  They shut the machine down immediately, with
+no syncing of disks and no clean shutdown of userspace.  So, they are
+pretty close to crashing the machine.
+
+.. code-block:: text
+
+    (mconsole)  halt
+
+    OK
+
+config
+------
+
+"config" adds a new device to the virtual machine. This is supported
+by most UML device drivers. It takes one argument, which is the
+device to add, with the same syntax as the kernel command line.
+
+.. code-block:: text
+
+    (mconsole) config ubd3=/home/jdike/incoming/roots/root*fs*debian22
+
+remove
+------
+
+"remove" deletes a device from the system.  Its argument is just the
+name of the device to be removed. The device must be idle in whatever
+sense the driver considers necessary.  In the case of the ubd driver,
+the removed block device must not be mounted, swapped on, or otherwise
+open, and in the case of the network driver, the device must be down.
+
+.. code-block:: text
+
+    (mconsole)  remove ubd3
+    (mconsole)  remove eth1
+
+sysrq
+-----
+
+This takes one argument, which is a single letter.  It calls the
+generic kernel's SysRq driver, which does whatever is called for by
+that argument.  See the SysRq documentation in
+Documentation/admin-guide/sysrq.rst in your favorite kernel tree to
+see what letters are valid and what they do.
+
+cad
+---
+
+This invokes the ``Ctl-Alt-Del`` action in the running image.  What exactly
+this ends up doing is up to init, systemd, etc.  Normally, it reboots the
+machine.
+
+stop
+----
+
+This puts the UML in a loop reading mconsole requests until a 'go'
+mconsole command is received. This is very useful as a
+debugging/snapshotting tool.
+
+go
+--
+
+This resumes a UML after being paused by a 'stop' command. Note that
+when the UML has resumed, TCP connections may have timed out and if
+the UML is paused for a long period of time, crond might go a little
+crazy, running all the jobs it didn't do earlier.
+
+proc
+----
+
+This takes one argument - the name of a file in /proc which is printed
+to the mconsole standard output
+
+stack
+-----
+
+This takes one argument - the pid number of a process. Its stack is
+printed to a standard output.
+
+*******************
+Advanced UML Topics
+*******************
+
+Sharing Filesystems between Virtual Machines
+============================================
+
+Don't attempt to share filesystems simply by booting two UMLs from the
+same file.  That's the same thing as booting two physical machines
+from a shared disk.  It will result in filesystem corruption.
+
+Using layered block devices
+---------------------------
+
+The way to share a filesystem between two virtual machines is to use
+the copy-on-write (COW) layering capability of the ubd block driver.
+Any changed blocks are stored
+in the private COW file, while reads come from either device - the
+private one if the requested block is valid in it, the shared one if
+not.  Using this scheme, the majority of data which is unchanged is
+shared between an arbitrary number of virtual machines, each of which
+has a much smaller file containing the changes that it has made.  With
+a large number of UMLs booting from a large root filesystem, this
+leads to a huge disk space saving.
+
+It will also help performance,
+since the host will be able to cache the shared data using a much
+smaller amount of memory, so UML disk requests will be served from the
+host's memory rather than its disks.
+There is a major caveat in doing this on multisocket NUMA machines.
+On such hardware, running many UML instances with a shared master image
+and COW changes may caise issues like NMIs from excess of inter-socket
+traffic.
+
+If you are running UML on high end hardware like this, make sure to
+bind UML to a set of logical cpus residing on the same socket using the
+``taskset`` command or have a look at the "tuning" section.
+
+To add a copy-on-write layer to an existing block device file, simply
+add the name of the COW file to the appropriate ubd switch:
+
+.. code-block:: text
+
+    ubd0=root_fs_cow,root_fs_debian_22
+
+where ``root_fs_cow`` is the private COW file and ``root_fs_debian_22`` is
+the existing shared filesystem.  The COW file need not exist.  If it
+doesn't, the driver will create and initialize it.
+
+Disk Usage
+----------
+
+UML has TRIM support which will release any unused space in its disk
+image files to the underlying OS. It is important to use either ls -ls
+or du to verify the actual file size.
+
+COW validity.
+-------------
+
+Any changes to the master image will invalidate all COW files. If this
+happens, UML will *NOT* automatically delete any of the COW files and
+will refuse to boot. In this case the only solution is to either
+restore the old image (including its last modified timestamp) or remove
+all COW files which will result in their recreation. Any changes in
+the COW files will be lost.
+
+Cows can moo - uml_moo : Merging a COW file with its backing file
+-----------------------------------------------------------------
+
+Depending on how you use UML and COW devices, it may be advisable to
+merge the changes in the COW file into the backing file every once in
+a while.
+
+The utility that does this is uml_moo.  Its usage is
+
+.. code-block:: text
+
+    uml_moo COW_file new_backing_file
+
+
+There's no need to specify the backing file since that information is
+already in the COW file header.  If you're paranoid, boot the new
+merged file, and if you're happy with it, move it over the old backing
+file.
+
+``uml_moo`` creates a new backing file by default as a safety measure.
+It also has a destructive merge option which will merge the COW file
+directly into its current backing file.  This is really only usable
+when the backing file only has one COW file associated with it.  If
+there are multiple COWs associated with a backing file, a -d merge of
+one of them will invalidate all of the others.  However, it is
+convenient if you're short of disk space, and it should also be
+noticeably faster than a non-destructive merge.
+
+``uml_moo`` is installed with the UML distribution packages and is
+available as a part of UML utilities.
+
+Host file access
+==================
+
+If you want to access files on the host machine from inside UML, you
+can treat it as a separate machine and either nfs mount directories
+from the host or copy files into the virtual machine with scp or rcp.
+However, since UML is running on the host, it can access those
+files just like any other process and make them available inside the
+virtual machine without needing to use the network.
+This is possible with the hostfs virtual filesystem.  With it, you
+can mount a host directory into the UML filesystem and access the
+files contained in it just as you would on the host.
+
+*SECURITY WARNING*
+
+Hostfs without any parameters to the UML Image will allow the image
+to mount any part of the host filesystem and write to it. Always
+confine hostfs to a specific "harmless" directory (f.e. ``/var/tmp``)
+if running UML. This is especially important if UML is being run as root.
+
+Using hostfs
+------------
+
+To begin with, make sure that hostfs is available inside the virtual
+machine with
+
+.. code-block:: text
+
+    UML# cat /proc/filesystems
+
+``hostfs`` should be listed.  If it's not, either rebuild the kernel
+with hostfs configured into it or make sure that hostfs is built as a
+module and available inside the virtual machine, and insmod it.
+
+
+Now all you need to do is run mount:
+
+.. code-block:: text
+
+    UML# mount none /mnt/host -t hostfs
+
+will mount the host's ``/`` on the virtual machine's ``/mnt/host``.
+If you don't want to mount the host root directory, then you can
+specify a subdirectory to mount with the -o switch to mount:
+
+.. code-block:: text
+
+    UML# mount none /mnt/home -t hostfs -o /home
+
+will mount the hosts's /home on the virtual machine's /mnt/home.
+
+hostfs as the root filesystem
+-----------------------------
+
+It's possible to boot from a directory hierarchy on the host using
+hostfs rather than using the standard filesystem in a file.
+To start, you need that hierarchy.  The easiest way is to loop mount
+an existing root_fs file:
+
+.. code-block:: text
+
+    host#  mount root_fs uml_root_dir -o loop
+
+
+You need to change the filesystem type of ``/`` in ``etc/fstab`` to be
+'hostfs', so that line looks like this:
+
+.. code-block:: text
+
+    /dev/ubd/0       /        hostfs      defaults          1   1
+
+Then you need to chown to yourself all the files in that directory
+that are owned by root.  This worked for me:
+
+.. code-block:: text
+
+    host#  find . -uid 0 -exec chown jdike {} \;
+
+Next, make sure that your UML kernel has hostfs compiled in, not as a
+module.  Then run UML with the boot device pointing at that directory:
+
+.. code-block:: text
+
+    ubd0=/path/to/uml/root/directory
+
+UML should then boot as it does normally.
+
+Hostfs Caveats
+--------------
+
+Hostfs does not support keeping track of host filesystem changes on the
+host (outside UML). As a result, if a file is changed without UML's
+knowledge, UML will not know about it and its own in-memory cache of
+the file may be corrupt. While it is possible to fix this, it is not
+something which is being worked on at present.
+
+Tuning UML
+============
+
+UML at present is strictly uniprocessor. It will, however spin up a
+number of threads to handle various functions.
+
+The UBD driver, SIGIO and the MMU emulation do that. If the system is
+idle, these threads will be migrated to other processors on a SMP host.
+This, unfortunately, will usually result in LOWER performance because of
+all of the cache/memory synchronization traffic between cores. As a
+result, UML will usually benefit from being pinned on a single CPU
+especially on a large system. This can result in performance differences
+of 5 times or higher on some benchmarks.
+
+Similarly, on large multi-node NUMA systems UML will benefit if all of
+its memory is allocated from the same NUMA node it will run on. The
+OS will *NOT* do that by default. In order to do that, the sysadmin
+needs to create a suitable tmpfs ramdisk bound to a particular node
+and use that as the source for UML RAM allocation by specifying it
+in the TMP or TEMP environment variables. UML will look at the values
+of ``TMPDIR``, ``TMP`` or ``TEMP`` for that. If that fails, it will
+look for shmfs mounted under ``/dev/shm``. If everything else fails use
+``/tmp/`` regardless of the filesystem type used for it.
+
+.. code-block:: shell
+
+    mount -t tmpfs -ompol=bind:X none /mnt/tmpfs-nodeX
+    TEMP=/mnt/tmpfs-nodeX taskset -cX linux options options options..
+
+*******************************************
+Contributing to UML and Developing with UML
+*******************************************
+
+UML is an excellent platform to develop new Linux kernel concepts -
+filesystems, devices, virtualization, etc. It provides unrivalled
+opportunities to create and test them without being constrained to
+emulating specific hardware.
+
+Example - want to try how linux will work with 4096 "proper" network
+devices?
+
+Not an issue with UML. At the same time, this is something which
+is difficult other virtualization packages - they are constrained by
+the number of devices allowed on the hardware bus they are trying
+to emulate (f.e. 16 on a PCI bus in qemu).
+
+If you have something to contribute such as a patch, a bugfix, a
+new feature, please send it to ``linux-um@lists.infradead.org``
+
+Please follow all standard Linux patch guidelines such as cc-ing
+relevant maintainers and run ``./sripts/check-patch.pl`` on your patch.
+
+Note - the list does not accept HTML or attachments, all emails must
+be formatted as plain text.
+
+Developing always goes hand in hand with debugging. First of all,
+you can always run UML under gdb and there will be a whole section
+later on on how to do that. That, however, is not the only way to
+debug a linux kernel. Quite often adding tracing statements and/or
+using UML specific approaches such as ptracing the UML kernel process
+are significantly more informative.
+
+Tracing UML
+=============
+
+When running UML consists of a main kernel thread and a number of
+helper threads. The ones of interest for tracing are NOT the ones
+that are already ptraced by UML as a part of its MMU emulation.
+
+These are usually the first three threads visible in a ps display.
+The one with the lowest PID number and using most CPU is usually the
+kernel thread. The other threads are the disk
+(ubd) device helper thread and the sigio helper thread.
+running ptrace on this thread usually results in the following picture:
+
+
+.. code-block:: text
+
+    host$ strace -p 16566
+    --- SIGIO {si*signo=SIGIO, si*code=POLL*IN, si*band=65} ---
+    epoll_wait(4, [{EPOLLIN, {u32=3721159424, u64=3721159424}}], 64, 0) = 1
+    epoll_wait(4, [], 64, 0)                = 0
+    rt_sigreturn({mask=[PIPE]})             = 16967
+    ptrace(PTRACE_GETREGS, 16967, NULL, 0xd5f34f38) = 0
+    ptrace(PTRACE*GETREGSET, 16967, NT*X86*XSTATE, [{iov*base=0xd5f35010, iov_len=832}]) = 0
+    ptrace(PTRACE*GETSIGINFO, 16967, NULL, {si*signo=SIGTRAP, si*code=0x85, si*pid=16967, si_uid=0}) = 0
+    ptrace(PTRACE_SETREGS, 16967, NULL, 0xd5f34f38) = 0
+    ptrace(PTRACE*SETREGSET, 16967, NT*X86*XSTATE, [{iov*base=0xd5f35010, iov_len=2696}]) = 0
+    ptrace(PTRACE_SYSEMU, 16967, NULL, 0)   = 0
+    --- SIGCHLD {si*signo=SIGCHLD, si*code=CLD*TRAPPED, si*pid=16967, si*uid=0, si*status=SIGTRAP, si*utime=65, si*stime=89} ---
+    wait4(16967, [{WIFSTOPPED(s) && WSTOPSIG(s) == SIGTRAP | 0x80}], WSTOPPED|__WALL, NULL) = 16967
+    ptrace(PTRACE_GETREGS, 16967, NULL, 0xd5f34f38) = 0
+    ptrace(PTRACE*GETREGSET, 16967, NT*X86*XSTATE, [{iov*base=0xd5f35010, iov_len=832}]) = 0
+    ptrace(PTRACE*GETSIGINFO, 16967, NULL, {si*signo=SIGTRAP, si*code=0x85, si*pid=16967, si_uid=0}) = 0
+    timer*settime(0, 0, {it*interval={tv*sec=0, tv*nsec=0}, it*value={tv*sec=0, tv_nsec=2830912}}, NULL) = 0
+    getpid()                                = 16566
+    clock*nanosleep(CLOCK*MONOTONIC, 0, {tv*sec=1, tv*nsec=0}, NULL) = ? ERESTART_RESTARTBLOCK (Interrupted by signal)
+    --- SIGALRM {si*signo=SIGALRM, si*code=SI*TIMER, si*timerid=0, si*overrun=0, si*value={int=1631716592, ptr=0x614204f0}} ---
+    rt_sigreturn({mask=[PIPE]})             = -1 EINTR (Interrupted system call)
+
+
+This is a typical picture from a mostly idle UML instance
+
+1. UML interrupt controller uses epoll - this is UML waiting for IO
+   interrupts:
+
+.. code-block:: text
+
+    epoll_wait(4, [{EPOLLIN, {u32=3721159424, u64=3721159424}}], 64, 0) = 1
+
+2. The sequence of ptrace calls is part of MMU emulation and runnin the
+   UML userspace
+
+#. ``timer_settime`` is part of the UML high res timer subsystem mapping
+   timer requests from inside UML onto the host high resultion timers.
+
+#. ``clock_nanosleep`` is UML going into idle (similar to the way a PC
+   will execute an ACPI idle).
+
+As you can see UML will generate quite a bit of output even in idle. The output can be very informative when observing IO. It shows
+the actual IO calls, their arguments and returns values.
+
+Kernel debugging
+================
+
+You can run UML under gdb now, though it will not necessarily agree to
+be started under it. If you are trying to track a runtime bug, it is
+much better to attach gdb to a running UML instance and let UML run.
+
+Assuming the same PID number as in the previous example, this would be:
+
+.. code-block:: text
+
+    gdb -p 16566
+
+This will STOP the UML instance, so you must enter `cont` at the GDB
+command line to request it to continue. It may be a good idea to make
+this into a gdb script and pass it to gdb as an argument.
+
+Developing Device Drivers
+=========================
+
+Nearly all UML drivers are monolithic. While it is possible to build a
+UML driver as a kernel module, that
+limits the possible functionality to in-kernel only and non-UML specific.
+The reason for this is that in order to really leverage UML, one needs
+to write a piece of userspace code which maps driver concepts onto
+actual userspace host calls.
+
+This forms the so called "user" portion of the driver. While it can
+reuse a lot of kernel concepts, it is generally just another piece of
+userspace code. This portion needs some matching "kernel" code which
+resides inside the UML image and which implements the Linux kernel part.
+
+*There are very few limitations on the way "kernel" and "user" interact*.
+
+UML does not have a strictly defined kernel to host API. It does not
+try to emulate a specific architecture or bus. UML's "kernel" and
+"user" can share memory, code and interact as needed to implement
+whatever design the software developer has in mind. The only
+limitations are purely technical. Due to a lot of functions and
+variables having the same names, the developer should be careful
+which includes and libraries they are trying to refer to.
+
+As a result a lot of userspace code consists of simple wrappers.
+F.e. ``os_close_file()`` is just a wrapper around ``close()``
+which ensures that the userspace function close does not clash
+with similarly named function(s) in the kernel part.
+
+
