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	== QEMU Basics ==		== QEMU Basics ==

Admin: Created page with "Category:Wiki == QEMU Basics == === Architecture Emulation === QEMU can emulate different hardware architectures. For example, if you want to run a PowerPC-based system on an x86 host, you can use QEMU to emulate PowerPC hardware. qemu-system-ppc -cdrom install.iso -m 1024 This command starts a virtual machine emulating a PowerPC architecture with 1024 MB of RAM and an installation ISO image mounted as a CD-ROM. === Virtualization vs. Emulatio..."

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Created page with "Category:Wiki == QEMU Basics == === Architecture Emulation === QEMU can emulate different hardware architectures. For example, if you want to run a PowerPC-based system on an x86 host, you can use QEMU to emulate PowerPC hardware. <nowiki> qemu-system-ppc -cdrom install.iso -m 1024</nowiki> This command starts a virtual machine emulating a PowerPC architecture with 1024 MB of RAM and an installation ISO image mounted as a CD-ROM. === Virtualization vs. Emulatio..."

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[[Category:Wiki]]

== QEMU Basics ==

=== Architecture Emulation ===
QEMU can emulate different hardware architectures. For example, if you want to run a PowerPC-based system on an x86 host, you can use QEMU to emulate PowerPC hardware.

<nowiki>
qemu-system-ppc -cdrom install.iso -m 1024</nowiki>

This command starts a virtual machine emulating a PowerPC architecture with 1024 MB of RAM and an installation ISO image mounted as a CD-ROM.

=== Virtualization vs. Emulation ===
There are two main modes of operation in QEMU:

1. **Emulation**: The full hardware emulation where QEMU emulates the CPU and other hardware devices. This is slower but more flexible.
2. **Virtualization**: When QEMU is combined with KVM (Kernel-based Virtual Machine), it can run virtual machines with near-native performance by taking advantage of hardware virtualization features.

To enable KVM-based virtualization, you can use the following command:

<nowiki>
qemu-system-x86_64 -enable-kvm -m 2048 -cpu host -hda /path/to/disk_image</nowiki>

This enables KVM support, specifies 2 GB of memory, and runs a VM with a virtual disk image.

== QEMU Command-Line Interface ==

=== Common Command-Line Options ===
QEMU uses a command-line interface with various options for controlling the behavior of virtual machines. Below are some of the commonly used command-line options:

==== System Emulator ====
<nowiki>
qemu-system-<arch> [options]</nowiki>

Where `<arch>` can be replaced with the desired architecture (e.g., `x86_64`, `arm`, `mips`, etc.).

Some useful options include:
* `-m` – Set memory size (e.g., `-m 2048` sets 2 GB of RAM).
* `-hda` – Specify the hard disk image (e.g., `-hda /path/to/image`).
* `-cdrom` – Mount a CD-ROM image (e.g., `-cdrom /path/to/iso`).
* `-net` – Set network configuration (e.g., `-net nic -net user` for user-mode networking).
* `-enable-kvm` – Enable hardware virtualization support (for KVM users).

Example: Running a QEMU system emulation with a 64-bit x86 system and 2 GB of RAM:

<nowiki>
qemu-system-x86_64 -m 2048 -hda /path/to/disk.img -enable-kvm</nowiki>

==== User-mode Networking ====
QEMU provides user-mode networking for virtual machines. This type of networking is simple and does not require administrative privileges.

To enable user-mode networking, use the following command:

<nowiki>
qemu-system-x86_64 -m 1024 -hda /path/to/disk.img -net nic -net user</nowiki>

This creates a virtual network interface inside the virtual machine, which can access the host's network.

==== Bridge Networking ====
Bridge networking allows virtual machines to connect directly to the host's network, making them behave like regular machines on the network.

<nowiki>
qemu-system-x86_64 -m 1024 -hda /path/to/disk.img -net nic -net bridge,br=br0</nowiki>

This command creates a bridged network connection for the virtual machine, where `br0` is the name of the bridge on the host system.

== Virtual Machine Disk Images ==

=== Creating Virtual Disks ===
QEMU supports various disk image formats, such as QCOW2, RAW, VMDK, and VDI. The most common format is QCOW2 because it supports features like snapshots and compression.

To create a new disk image, use the `qemu-img` tool:

<nowiki>
qemu-img create -f qcow2 /path/to/disk.img 10G</nowiki>

This creates a 10 GB QCOW2 disk image. The `-f` option specifies the format (QCOW2 in this case).

=== Converting Disk Images ===
QEMU also allows converting between different disk image formats. To convert a QCOW2 disk to a raw format, use the following command:

<nowiki>
qemu-img convert -f qcow2 -O raw /path/to/disk.qcow2 /path/to/disk.raw</nowiki>

The `-O` option specifies the output format (raw, in this case).

=== Using Disk Images with QEMU ===
When starting a virtual machine, specify the disk image with the `-hda` option:

<nowiki>
qemu-system-x86_64 -m 2048 -hda /path/to/disk.img</nowiki>

QEMU will then boot the guest operating system from the specified disk image.

== Snapshots in QEMU ==

Snapshots allow you to save the state of a virtual machine at a specific point in time and revert to that state later.

=== Creating a Snapshot ===
To create a snapshot of a running virtual machine, use the following QEMU command:

<nowiki>
qemu-img snapshot -c snapshot_name /path/to/disk.img</nowiki>

This creates a snapshot of the virtual machine's current state, which can be useful for testing or recovery.

=== Reverting to a Snapshot ===
To revert a virtual machine to a specific snapshot, use:

<nowiki>
qemu-img snapshot -a snapshot_name /path/to/disk.img</nowiki>

This restores the disk image to the state it was in when the snapshot was created.

== Debugging and Tracing with QEMU ==

QEMU offers powerful debugging and tracing capabilities to help you troubleshoot and optimize your virtual machines.

=== Enabling QEMU Debugging ===
You can enable QEMU's built-in debugging facilities by passing the `-d` flag followed by the desired debug options.

Example for enabling CPU execution tracing:

<nowiki>
qemu-system-x86_64 -m 2048 -hda /path/to/disk.img -d cpu</nowiki>

This command enables detailed logging of the CPU execution, which can be helpful for performance tuning or debugging.

=== QEMU Tracing with `-trace` ===
For more advanced tracing, QEMU provides the `-trace` option, which can capture trace events during virtual machine execution.

Example of enabling tracing for memory accesses:

<nowiki>
qemu-system-x86_64 -m 2048 -hda /path/to/disk.img -trace memory</nowiki>

This records all memory access events, which can be analyzed for performance profiling or debugging.

== QEMU Monitor Interface ==

The QEMU monitor provides an interactive interface for controlling virtual machines while they are running. It allows you to issue commands like pausing the VM, changing device configurations, and more.

=== Accessing the Monitor ===
To access the monitor, use the `-monitor` option when starting QEMU:

<nowiki>
qemu-system-x86_64 -m 2048 -hda /path/to/disk.img -monitor telnet::4444,server,nowait</nowiki>

This starts a QEMU monitor server that listens on port 4444, which can be accessed via a Telnet client.

=== Common Monitor Commands ===
Some common commands available in the QEMU monitor include:

* `info` – Show information about the current state of the VM, such as devices, CPUs, and memory.
* `quit` – Exit the virtual machine.
* `stop` – Pause the virtual machine.
* `cont` – Resume a paused virtual machine.

== Useful Links ==
For further reading and resources, consult the following links:

* [Official QEMU Documentation](https://www.qemu.org/documentation/)
* [QEMU Git Repository](https://gitlab.com/qemu-project/qemu)
* [QEMU Wiki on GitHub](https://github.com/qemu/qemu/wiki)
* [QEMU User Manual](https://qemu-project.gitlab.io/qemu/docs/master/)
* [QEMU Forums](https://lists.nongnu.org/mailman/listinfo/qemu-devel)
* [Linux Kernel Virtual Machine (KVM)](https://www.linux-kvm.org/)

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