ROUGH DRAFT: seeded by LXC page (outline looks more comprehensive than current QEMU
QEMU (Quick EMUlator) is a generic, open source hardware emulator and virtualization suite. Often it is used in conjunction with acceleration in the form of a Type-I hypervisor such as KVM (Kernel-based Virtual Machine) or Xen. If no accelerator is used, QEMU will run entirely in user-space using its built in binary translator TCG (Tiny Code Generator). Using QEMU without an accelerator is relatively inefficient and slow.
This article typically uses KVM as the accelerator of choice due to its GPL licensing and availability. Without KVM nearly all commands described here will still work (unless KVM specific).
para-virtualization (emulated microcode)
Full virtualization (direct machine code execution)
QEMU/KVM is a full virtualization.
Limitations of QEMU
POSIX file capabilities
This section details the QEMU setup of the Linux kernel for a host that is using Gentoo OS on either a x86_64 or an amd64 CPU hardware platform.
Consult distro-specific for their setup of a QEMU host, if the host is not installed with Gentoo Linux.
BIOS and UEFI firmware
In order to utilize KVM either Vt-x (vmx) or AMD-V (svm) must be supported by the processor. Vt-x or AMD-V are Intel and AMD's respective technologies for permitting multiple operating systems to concurrently execute operations on the processors.
To inspect hardware for virtualization support issue the following command:
grep --color -E "vmx|svm" /proc/cpuinfo
For a period manufacturers were shipping with virtualization turned off by default in the system BIOS. Note that changing this feature in the BIOS may actually require full removal of power from the system to take effect. If restarting the system does not work try shutting down, unplugging the system and pressing the power button in an unplugged state to discharge any residual energy from the power supply unit (PSU). Reapply power to the system to verify success.
If KVM support is available there should be a "kvm" device listed at /dev/kvm. This will take effect after the system has booted to a KVM enabled kernel.
Described below are the basic requirements for KVM kernel configuration for the host OS. A more complete and up-to-date list can be found at the KVM Tuning Kernel page.
Kernel options required for QEMU
Different guest (virtualized) OS may require additional kernel options. These are covered in the corresponding #Usage section pages.
General setup ---> Timers subsystem ---> <*> High Resolution Timer Support
[*] Virtualization ---> <*> Kernel-based Virtual Machine (KVM) support
This includes support for ARM64 processors.
Physical CPU Processor Support - Host
For the QEMU host, choose one of the two CPU architectures found on its hardware platform:
[*] Virtualization ---> <*> KVM for Intel processors support
[*] Virtualization ---> <*> KVM for AMD processors support
If both "KVM for Intel processors support" and "KVM for AMD processors support" are set as built into the kernel (
*) an error message will appear from kprint from early boot. Since the system has only one type processor (Intel or AMD) enabling one or both options as modules (
M) will make the error message disappear.
Virtual CPU Processor Support - Guest(s)
For a list of supporting CPUs that the guest platform(s) on this host should support, consult the [QEMU use flags] and insert desire target(s) into the USE= string in the portage configuration /etc/portage/make.conf file.
[*] Networking support Networking Options ---> [*] Network packet filtering framework (Netfilter) ---> [*] Advanced netfilter configuration Core Netfilter Configuration ---> <*> "conntrack" connection tracking match support <*> CHECKSUM target support IPv6: Netfilter Configuration ---> <*> ip6tables NAT support <*> Ethernet Bridge tables (ebtables) support ---> <*> ebt: nat table support <*> ebt: mark filter support [*] QoS and/or fair queueing ---> <*> Hierarchical Token Bucket (HTB) <*> Stochastic Fairness Queueing (SFQ) <*> Ingress/classifier-action Qdisc <*> Netfilter mark (FW) <*> Universal 32bit comparisons w/ hashing (U32) [*] Actions <*> Traffic Policing
Handling Kernel Config at CLI
To set the various kernel configuration settings from the command lines, the linux/scripts/kconfig/merge_config.sh shall be used here:
Mandatory kernel configuration options to set:
CONFIG_VIRTUALIZATION=y CONFIG_KVM=y CONFIG_KVM_INTEL=y CONFIG_KVM_AMD=y
Useful kernel configuration options to use:
CONFIG_VHOST_NET=y CONFIG_HIGH_RES_TIMER=y CONFIG_HPET=y CONFIG_COMPACTION=y CONFIG_MIGRATION=y CONFIG_KSM=y CONFIG_SYSFS=y CONFIG_PROCFS=y CONFIG_HUGEPAGE=y CONFIG_CGROUPS=y
Accelerated networking, required for
vhost-net USE flag (recommend):
Device Drivers ---> [*] VHOST drivers ---> <*> Host kernel accelerator for virtio net
[*] Virtualization ---> <*> Host kernel accelerator for virtio net
Device Drivers ---> [*] Network device support ---> [*] Network core driver support <*> Universal TUN/TAP device driver support
Needed for 802.1d Ethernet bridging:
[*] Networking support ---> Networking options ---> <*> The IPv6 protocol <*> 802.1d Ethernet Bridging
Intel VT-g (integrated graphics adapter virtualization)
Mediated device passthrough for Intel GPUs (Broadwell and newer) .
Device Drivers ---> <*> VFIO Non-Privileged userspace driver framework <*> Mediated device driver framework Graphics Support ---> <*> Intel 8xx/9xx/G3x/G4x/HD Graphics [*] Enable Intel GVT-g graphics virtualization host support <*> Enable KVM/VFIO support for Intel GVT-g
QEMU userspace utilities
Mounted cgroup filesystem
If a QEMU front-end is to be used (instead of /usr/bin/virsh), disregard the rest of this Network configuration section and consult the QEMU front-ends wiki page for a network configuration to be maintained by this desired QEMU front-end.
Simple network configuration
Host configuration for VLANs inside the bridge which are connected to container's virtual Ethernet pair device
Host configuration with NAT networking (nftables)
Host configuration with NAT networking (iptables)
Guest configuration for a virtual Ethernet pair device connected by bridge
Adjusting guest config of the container after using template script
Some packages are aware of the
qemu USE flag.
Review the possible USE flags for QEMU:
USE flags for app-emulation/qemu QEMU + Kernel-based Virtual Machine userland tools
||Adds support for braille displays using brltty|
||Enables support for Linux's Async IO|
||Enable alsa output for sound emulation|
||Enable eBPF support for RSS implementation.|
||Use the bzlib compression library|
||Use Linux capabilities library to control privilege|
||Enable disassembly support with dev-libs/capstone|
||Support ISOs / -cdrom directives via HTTP or HTTPS.|
||Enable extra debug codepaths, like asserts and extra output. If you want to get meaningful backtraces see https://wiki.gentoo.org/wiki/Project:Quality_Assurance/Backtraces|
||Add extra documentation (API, Javadoc, etc). It is recommended to enable per package instead of globally|
||Enables firmware device tree support|
||Use Linux file capabilities to control privilege rather than set*id (this is orthogonal to USE=caps which uses capabilities at runtime e.g. libcap)|
||Enables FUSE block device export|
||Enables GlusterFS cluster fileystem via sys-cluster/glusterfs|
||Enable TLS support for the VNC console server. For 1.4 and newer this also enables WebSocket support. For 2.0 through 2.3 also enables disk quorum support.|
||Add support for x11-libs/gtk+ (The GIMP Toolkit)|
||Enable Infiniband RDMA transport support|
||Enable efficient I/O via sys-libs/liburing.|
||Enable direct iSCSI support via net-libs/libiscsi instead of indirectly via the Linux block layer that sys-block/open-iscsi does.|
||Add support for the JACK Audio Connection Kit|
||Enable jemalloc allocator support|
||Enable jpeg image support for the VNC console server|
||Enable support for lzo compression|
||Enable multipath persistent reservation passthrough via sys-fs/multipath-tools.|
||Enable the ncurses-based console|
||Enable NFS support|
||Add Native Language Support (using gettextGNU locale utilities)|
||Enable NUMA support|
||Add support for OpenGL (3D graphics)|
||Add support for OSS (Open Sound System)|
||Add support for PAM (Pluggable Authentication Modules)DANGEROUS to arbitrarily flip|
||Pin the versions of BIOS firmware to the version included in the upstream release. This is needed to sanely support migration/suspend/resume/snapshotting/etc... of instances. When the blobs are different, random corruption/bugs/crashes/etc... may be observed.|
||Enable qemu plugin API via shared library loading.|
||Enable png image support for the VNC console server|
||Enable pulseaudio output for sound emulation|
||Add optional support/bindings for the Python language|
||Enable rados block device backend support, see https://docs.ceph.com/en/mimic/rbd/qemu-rbd/|
||Add support for the Simple Authentication and Security Layer|
||Enable the SDL-based console|
||SDL Image support for icons|
||Enable seccomp (secure computing mode) to perform system call filtering at runtime to increase security of programs|
||!!internal use only!! Security Enhanced Linux support, this must be set by the selinux profile or breakage will occur|
||Enable TCP/IP in hypervisor via net-libs/libslirp|
||Enable smartcard support|
||Enable support for Snappy compression (as implemented in app-arch/snappy)|
||Enable Spice protocol support via app-emulation/spice|
||Enable SSH based block device support via net-libs/libssh2|
||Build the User and Software MMU (system) targets as well as tools as static binaries|
||Build the User targets as static binaries|
||Enable SystemTAP/DTrace tracing|
||Enable dependencies and/or preparations necessary to run tests (usually controlled by FEATURES=test but can be toggled independently)|
||Enable virtual/udev integration (device discovery, power and storage device support, etc)|
||Enable USB passthrough via dev-libs/libusb|
||Use sys-apps/usbredir to redirect USB devices to another machine over TCP|
||Enable VDE-based networking|
||Enable accelerated networking using vhost-net, see https://www.linux-kvm.org/page/VhostNet|
||Enable shared file system access using the FUSE protocol carried over virtio.|
||Enable experimental Virgil 3d (virtual software GPU)|
||Enable VirtFS via virtio-9p-pci / fsdev. See https://wiki.qemu.org/Documentation/9psetup|
||Enable VNC (remote desktop viewer) support|
||Enable terminal support (x11-libs/vte) in the GTK+ interface|
||Add support for getting and setting POSIX extended attributes, through sys-apps/attr. Requisite for the virtfs backend.|
||Enables support for Xen backends|
||Enable support for ZSTD compression|
More than one USE flag (
spice) can be enabled for graphical output. If graphics are desired it is generally recommended to enable more than one graphical USE flag.
If virt-manager is going to be used, be sure to enable the
spiceUSE flags on the qemu package for correct operation.
Additional ebuild configuration frobs are provided as the USE_EXPAND variables QEMU_USER_TARGETS and QEMU_SOFTMMU_TARGETS. See app-emulation/qemu for a list of all the available targets (there are a heck of a lot of them; most of them are very obscure and may be ignored; leaving these variables at their default values will disable almost everything which is probably just fine for most users).
For each target specified, a qemu executable will be built. A
softmmu target is the standard qemu use-case of emulating an entire system (like VirtualBox or VMWare, but with optional support for emulating CPU hardware along with peripherals).
user targets execute user-mode code only; the (somewhat shockingly ambitious) purpose of these targets is to "magically" allow importing user-space linux ELF binaries from a different architecture into the native system (that is, they are like multilib, without the awkward need for a software stack or CPU capable of running it).
In order to enable QEMU_USER_TARGETS and QEMU_SOFTMMU_TARGETS we can edit the variables globally in /etc/portage/make.conf, i.e.:
QEMU_SOFTMMU_TARGETS="arm x86_64 sparc" QEMU_USER_TARGETS="x86_64"
Or, the /etc/portage/package.use file(s) can be modified. Two equivalent syntaxes are available: traditional USE flag syntax, i.e.:
app-emulation/qemu qemu_softmmu_targets_arm qemu_softmmu_targets_x86_64 qemu_softmmu_targets_sparc app-emulation/qemu qemu_user_targets_x86_64
Another alternative is to use the newer sexy USE_EXPAND-specific syntax:
app-emulation/qemu QEMU_SOFTMMU_TARGETS: arm x86_64 sparc QEMU_USER_TARGETS: x86_64
Install QEMU host
After reviewing and adding any desired USE flags, emerge app-emulation/qemu:
emerge --ask app-emulation/qemu
We could take this entire 'Guest setup' section and put it under a new QEMU/guest wiki group but a trend has already been established as QEMU/Linux guest, QEMU/Windows guest, ...
As a demonstration of completeness of configuration settings for the Linux kernel, the kernel configuration file will be initialized to tinyconfig which is the smallest buildable Linux kernel (but not necessarily the bootable or functional one).
Backup your /usr/src/linux/.config, if needed
Now blow away the .config with the smallest default setting possible, execute:
cd /usr/src/linux make tinyconfig