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  1. Bubblemelon created this gist Jul 26, 2018.
    150 changes: 150 additions & 0 deletions qemu-kvm_detailed.md
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    # [Using QEMU KVM](#using-qemu-kvm)

    - [Installation](#installation)
    - [Qemu Overview](#qemu-overview)
    - [The Qemu VM Instance](#qemu-vm-instance)
    - [Details on the `qemu-kvm` options used](#qemu-kvm-cmd-details)
    - [Qemu Resources](#qemu-resources)
    - [Qemu Binaries Explained](#qemu-bin-explain)
    - [Logging into a QEMU VM](#login-to-qemu-kvm)
    - [Managing a QEMU VM](#managing-a-qemu-vm)
    - [The QEMU Console](#the-qemu-console)
    - [Shutdown and Starting up the Same Instance](#shutdown-Startup)
    - [Saving the State of a VM](#save-state)
    ---
    ## [Installation](#installation)

    KVM requires a CPU with virtualization extensions, found on most consumer CPUs. These extensions are called Intel VT or AMD-V. Command to check for CPU support:

    ```bash
    egrep '^flags.*(vmx|svm)' /proc/cpuinfo
    ```

    **Optional**:

    Install [Virtualization packages](https://docs.fedoraproject.org/quick-docs/en-US/getting-started-with-virtualization.html); this includes Libvirt tools.
    ```bash
    sudo dnf install @virtualization
    ```

    ## [Qemu Overview](#qemu-overview)

    Virtual machines created with `qemu-kvm` are not visible for libvirt-based tools, meaning if `virt-manager` was launched, the VM will not show up on this interface.

    Virtualization stack simplified:
    ```
    ------- Vagrant
    ----- Libvirt e.g. virsh + virt-manager
    ---- Qemu e.g. qemu-kvm
    --- KVM i.e. Kernel Based Virtual Machine
    ```
    This is simply because Qemu cannot communicate up the virtualization stack. [OpenSuse KVM Cookbook is a great resource](https://www.suse.com/documentation/opensuse121/pdfdoc/book_kvm/book_kvm.pdf). Communication can only go down the stack.

    ## [The Qemu VM Instance](#qemu-vm-instance)

    Run this command to get the a qemu-kvm virtual machine for Red Hat CoreOS instance running:

    ```
    $ qemu-kvm -m 2048M -accel kvm -smp cores=4 -fw_cfg \
    name=opt/com.coreos/config,file=key.ign -drive file=rhcos.qcow2.qemu \
    -net nic,model=virtio -net user,hostfwd=tcp::2222-:22
    ```
    <br/>

    ### [Details on the `qemu-kvm` options used](#qemu-kvm-cmd-details):

    `-m` Sets guest startup RAM size to megs megabytes. Default is 128 MiB. Optionally, a suffix of "M" or "G" can be used to signify a value in megabytes or gigabytes respectively. `2048M` is equivalent to `2G`.

    `-accel` Enables a `kvm` accelerator. `tcg` is the default.

    `-smp` Specifies symmetric multiprocessing of four cores.

    `-fw_cfg` Creates a firmware configuration entry named `opt/com.coreos/config` with the contents from `key.ign`. See this [qemu fw_cfg doc](https://github.com/qemu/qemu/blob/master/docs/specs/fw_cfg.txt).

    `-drive` Creates a new drive and is a shortcut to defining `-blockdev` and `device` options. `-drive file=` defines the image `rhcos.qcow2.qemu` which is in the qcow2 format to use with this drive. Run `qemu-img info rhcos.qcow2.qemu` for more information on this file.

    `-net nic,...` Defines a paravirtualized network adapter via `model=virtio`. Specify `-net nic,model=help` for a list of available devices for the target vm.

    `-net user,` Redirects TCP or UDP connections from host to guest, in this case with TCP `hostfwd=tcp::2222-:22`.

    `-M` The machine type e.g Standard PC, ISA-only PC, or Intel-Mac was not specified in this command, so it will default to `pc-0.12`. Use `qemu-kvm -M ?` to display a list of valid parameters.

    <br/>

    ## [Qemu Resources](#qemu-resources)

    For more information on the general command structure to start a qemu vm, see page 94 titled [Basic Installation with qemu-kvm](https://www.suse.com/documentation/opensuse121/pdfdoc/book_kvm/book_kvm.pdf)

    Additionally, run `man qemu-system-x86_64` or `man qemu-kvm` and see [QEMU Quickstart](https://qemu.weilnetz.de/doc/qemu-doc.html#pcsys_005fquickstart) for a more detailed explaination of each of the qemu options/flags.

    ### [Qemu Binaries Explained](#qemu-bin-explain)

    On the terminal, type `qemu-` and hit tab. You may notice that there's a number of qemu commands.

    ```
    qemu-aarch64 qemu-io qemu-mipsel qemu-ppc64le qemu-system-alpha qemu-system-mips64 qemu-system-s390x qemu-system-xtensaeb
    qemu-alpha qemu-keymap qemu-mipsn32 qemu-pr-helper qemu-system-arm qemu-system-mips64el qemu-system-sh4 qemu-x86_64
    qemu-arm qemu-kvm qemu-mipsn32el qemu-s390x qemu-system-cris qemu-system-mipsel qemu-system-sh4eb
    qemu-armeb qemu-m68k qemu-nbd qemu-sh4 qemu-system-i386 qemu-system-moxie qemu-system-sparc
    qemu-cris qemu-microblaze qemu-nios2 qemu-sh4eb qemu-system-lm32 qemu-system-nios2 qemu-system-sparc64
    qemu-ga qemu-microblazeel qemu-or1k qemu-sparc qemu-system-m68k qemu-system-or1k qemu-system-tricore
    qemu-hppa qemu-mips qemu-ppc qemu-sparc32plus qemu-system-microblaze qemu-system-ppc qemu-system-unicore32
    qemu-i386 qemu-mips64 qemu-ppc64 qemu-sparc64 qemu-system-microblazeel qemu-system-ppc64 qemu-system-x86_64
    qemu-img qemu-mips64el qemu-ppc64abi32 qemu-system-aarch64 qemu-system-mips qemu-system-ppcemb qemu-system-xtensa
    ```
    > A full QEMU installation includes several binaries, each of which emulates a different CPU architecture.
    > The binary filenames take the form `qemu-system-arch`, where arch is an architecture code, such as ppc for PowerPC or x86_64 for x86-64.
    > An exception is x86 systems, which are emulated via the qemu binary.
    >
    > Binaries with names of the form `qemu-arch` provide user-mode emulation for their respective architectures.
    >
    > [[sourced from a IBM developer blog post](https://www.ibm.com/developerworks/library/l-qemu-development/index.html)]
    **If the above explaination wasn't enough**:

    The `qemu-system-*` binaries e.g. qemu-system-i386 or qemu-system-x86_64 are used depending on guest OS's architecture. [[sourced from the ArchLinux Wiki KVM page](https://wiki.archlinux.org/index.php/QEMU#Enabling_KVM)]

    Whereas `qemu-arch` is for running the specified arch under Linux without starting an actual virtual machine with that architecture. [[Sourced from a GNU email thread](https://lists.gnu.org/archive/html/qemu-discuss/2016-04/msg00013.html) and [a ServerFault forum discussion](https://serverfault.com/questions/767212/difference-between-qemu-kvm-qemu-system-x86-64-qemu-x86-64)]

    Note: `qemu-kvm` is the equivalent to `qemu-system-x86_64 --enable kvm`. [[sourced from a GNU email thread](https://lists.gnu.org/archive/html/qemu-discuss/2012-02/msg00018.html)]


    ## [Logging into a QEMU VM](#login-to-qemu-kvm)
    ..
    ..
    ..
    ..
    ..
    ..


    ## [Managing a QEMU VM](#managing-a-qemu-vm)

    ### [The QEMU Console](#the-qemu-console)

    When a VM instance is booted without the `-ncurses` option, [enter the QEMU console by pressing](https://stackoverflow.com/questions/14165158/how-to-switch-to-qemu-monitor-console-when-running-with-curses) <kbd>esc</kdd> + 2 and <kbd>esc</kbd> + 1 to switch back.

    With the `-nographic` option, press <kbd>ctrl</kbd> + a then c to switch back and forth between the QEMU console and the VM.

    If none of the above options were used, press <kbd>ctrl</kbd> + <kbd>alt</kbd> then 2 to enter the console and with <kbd>ctrl</kbd> + <kbd>alt</kbd> then 1 to go back.

    ### [Shutdown and Starting up the Same Instance](#shutdown-Startup)

    **Shutting down** is simply clicking [x] on the tool-bar of the Qemu monitor console or doing <kbd>CTRL</kbd>+<kbd>c</kbd> on the terminal where the `qemu-kvm` command was initiated.
    It is also possible to shutdown the VM by pressing <kbd>CTRL</kbd>+<kbd>a</kbd> and then <kbd>c</kbd> when focus is on the QEMU monitor console (this is if the `-nographic` option was used).
    Cached memory on the VM will not be saved but whatever that was saved to disk is persistent since all data is saved with the `rhcos.qcow2.qemu` file; a raw disk image.

    **Starting up** is by running same command again, as long as the `-drive file=` option was provided with the same disk image as before. If a new clean VM of the same OS is needed, download or use `qemu-img create` a new raw disk image of the same OS.
    Then provide that file within the `-drive file=` option. Essentially, the raw disk image is the identity of a particular VM.

    ### [Saving the State of a VM](#save-state)

    It is possible to make **snapshots** of a VM at a given state, see page 101 of the [OpenSUSE Virtualization with KVM book](https://www.suse.com/documentation/opensuse121/pdfdoc/book_kvm/book_kvm.pdf).
    Note that the snapshots will be saved within a given raw disk image. It is not available globally.

    ---

    - change image file example
    - add vnc logging
    - add logging details