QEMU can also be used purely for CPU emulation for user-level processes, allowing applications compiled for one architecture to be run on another. However, the default is to use DirectSound. QEMU has multiple operating modes: [2]. QEMU can run without a host kernel driver and yet gives acceptable performance. It uses dynamic translation to native code for reasonable speed, with support for self-modifying code and precise exceptions.
QEMU can save and restore the state of the virtual machine with all programs running. Guest operating-systems do not need patching in order to run inside QEMU. The virtual machine can interface with many types of physical host hardware.
USB devices can be completely emulated mass storage from image files, input devices , or the host's USB devices can be used however, this requires administrator privileges and does not work with all devices. Virtual disk images can be stored in a special format qcow or qcow2 that only take up disk space that the guest OS actually uses.
This way, an emulated GB disk may occupy only a few hundred megabytes on the host. The QCOW2 format also allows the creation of overlay images that record the difference from another unmodified base image file. What is Idempotency? Here's a Small Trick! Master Network Automation. Close You've successfully subscribed to Packet Coders. Close Success! Your account is fully activated, you now have access to all content.
Close Welcome back! You've successfully signed in. Close Nearly there! The result is that guest memory is copied into host storage. On the host side, the easiest way to emulate persistent storage is via treating a file in the host filesystem as raw data a mapping of offsets in the host file to disk offsets being accessed by the guest driver , but QEMU actually has the ability to glue together a lot of different host formats raw, qcow2 , qed, vhdx , … and protocols file system, block device, NBD , Ceph , gluster , … where any combination of host format and protocol can serve as the backend that is then tied to the QEMU emulation providing the guest device.
Thus, when you tell QEMU to use a host qcow2 file, the guest does not have to know qcow2, but merely has its normal driver make the same register reads and writes as it would on bare metal, which cause vmexits into QEMU code, then QEMU maps those accesses into reads and writes in the appropriate offsets of the qcow2 file.
The next thing to realize is that emulating IDE is not always the most efficient. Every time the guest writes to the control registers, it has to go through special handling, and vmexits slow down emulation.
Of course, different hardware models have different performance characteristics when virtualized. In general, however, what works best for real hardware does not necessarily work best for virtualization, and until recently, hardware was not designed to operate fast when emulated by software such as QEMU.
Therefore, QEMU includes paravirtualized devices that are designed specifically for this purpose. The QEMU developers have produced a specification for a set of hardware registers and the behavior for those registers which are designed to result in the minimum number of vmexits possible while still accomplishing what a hard disk must do, namely, transferring data between normal guest memory and persistent storage.
This specification is called virtio; using it requires installation of a virtio driver in the guest.
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