On Fri, Sep 04, 2020 at 03:11:15PM +0100, anton.ivanov@xxxxxxxxxxxxxxxxxx wrote: > From: Anton Ivanov <anton.ivanov@xxxxxxxxxxxxxxxxxx> > > 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@xxxxxxxxxxxxxxxxxx> > --- > .../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@xxxxxxxxxxxxxxxxxxx`` > + > +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.
