[PATCH] docs: add a new User Mode Linux HowTo
Mike Rapoport
rppt at kernel.org
Sun Sep 6 03:56:50 EDT 2020
On Fri, Sep 04, 2020 at 03:11:15PM +0100, anton.ivanov at cambridgegreys.com wrote:
> From: Anton Ivanov <anton.ivanov at cambridgegreys.com>
>
> The new howto migrates the portions of the old howto which
> are still relevant to a new document, updates them to linux 5.x
> and adds documentation for vector transports and other new
> features.
>
> Signed-off-by: Anton Ivanov <anton.ivanov at cambridgegreys.com>
> ---
> .../virt/uml/user_mode_linux_howto_v2.rst | 1304 +++++++++++++++++
I think it would make sense to rename the existing user_mode_linux.rst
to something like user_mode_linux_v2.4.rst as there were almost no
contents changes since then.
> 1 file changed, 1304 insertions(+)
> create mode 100644 Documentation/virt/uml/user_mode_linux_howto_v2.rst
>
> diff --git a/Documentation/virt/uml/user_mode_linux_howto_v2.rst b/Documentation/virt/uml/user_mode_linux_howto_v2.rst
> new file mode 100644
> index 000000000000..3bcdd3aaebb5
> --- /dev/null
> +++ b/Documentation/virt/uml/user_mode_linux_howto_v2.rst
> @@ -0,0 +1,1304 @@
> +.. SPDX-License-Identifier: GPL-2.0
> +
> +#########
> +UML HowTo
> +#########
> +
> +:Author: User Mode Linux Core Team
> +:Last-updated: Friday Sep 04 14:50:55 BST 2020
You may want to add
.. contents:: :local:
for TOC
> +
> +
> +************
> +Introduction
> +************
> +
> +Welcome to User Mode Linux
> +
> +User Mode Linux is the first Open Source virtualization package (first
Maybe s/package/platform/g ?
> +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
Do we really need to use '1.' as the list index? I think sphinx will
manage with '#' here as well. Besides, do we really want a numbered list
or bullet list will suffice?
This comment applies to other lists below.
> + 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).
Can you spell out 'for example' or use e.g. please?
> +#. 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
> +=====================
> +
> +
Empty line is not needed
> +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
typo: supports
> +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
The csv-table is not readable in text mode.
> + :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.
I think 2-4G applications is not very clear. Presuming this is about
network bandwidth, I'd rephrase as "... sufficient for most $aapplications that
need throughput of ~ 2-4Gbit".
> +
> +#. ``mac=XX:XX:XX:XX:XX`` - sets the interface MAC value.
^ address?
> +
> +#. ``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
Took me a while to (hopefully) understand the intention. Maybe
When possible this option enables TCP offload in the most cases
> + 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
^ MTU ?
> +
> +#. ``headroom=int`` - adjusts the default headroom (32 bytes) reserved
> + if a packet will need to be re-encapsulated into let's say VXLAN.
^ for instance ?
> +
> +#. ``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.
^ connected to a brigde ?
> +
> +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
^ applications
> +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
broadcast ?
> + 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
^ GRE, and below where this corresponds to the abbreviation rather
than parameter name.
> +--------------------
> +
> +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
^ for interoperability
> + implementations)
> +
> +#. ``v6=[0,1]`` - force v4 or v6 sockets respectively
^ IPv4 or IPv6
> +
> +#. 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.
For GRE it was plain dst_host, and here it is $dst_host. I think
$dst_host is nicer, but it's really up to you, just please make it
consistent.
> +
> +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)
^ for interoperability
> +
> +#. ``v6=[0,1]`` - force v6 sockets
IPv6
> +
> +#. ``udp=[0,1]`` - use raw sockets (0) or UDP (1) version of the protocol
Missing empty line.
> + 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:
> +
Extra empty line :)
> +
> +**/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
> +
ditto
> +
> +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 link here would be nice
> +
> + 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 line wrapping here does not match the reset of the text.
> +
> +#. The fd channel - use file descriptor numbers for input/out. Example: ``con1=fd:0,fd:1.``
Ditto
> +
> +#. 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).
Ditto
> +
> +#. 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
Line wrapping
> +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
Emtpy lines here and below are not needed
> +
> +* 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:
Line wrapping
> +
> +.. 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
Emtpy lines here and below are not needed
> +
> +* halt
> +
> +* reboot
> +
> +* config
> +
> +* remove
> +
> +* sysrq
> +
> +* help
> +
> +* cad
> +
> +* stop
> +
> +* go
> +
> +* proc
> +
> +* stack
> +
> +version
> +-------
> +
> +This takes no arguments. It prints the UML version.
^ command ?
> +
> +.. code-block:: text
> +
> + (mconsole) version
> +
> + OK Linux OpenWrt 4.14.106 #0 Tue Mar 19 08:19:41 2019 x86_64
> +
Extra empty line
> +
> +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.
> +
Extra empty line
> +
> +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
The line above seems to short, do you mind to rewrap this paragraph?
> +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,
Maybe "Sharing of the filesystem data 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.
I think rcp can be dropped ;-)
> +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
^ with ?
> +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 at 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.
^ checkpatch.pl
I think a reference to sending-patches would be nice here
> +
> +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
Line wrapping
> +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.
> +
> +
> --
> 2.20.1
>
--
Sincerely yours,
Mike.
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