QEMU 10.1.0

KVM is basically three components.

* An abstraction over second level page tables to map some of a host user process as what the guest thinks of as physical memory.

* An abstraction to jump into the context that uses those page tables, and traps back out in the case of anything that the hardware would normally handle, but the hypervisor wants to handle manually instead.

* A collection of mechanisms to handle some of those traps in kernel space to avoid having to context switch back out to the host user process if the kind of trap is common enough, both in the sense of the trap itself happens often enough to show up on perf graphs, as well as the abstraction being exercised is relatively standard (think interrupt controllers and timers).

Let me know if you have any other questions.

If you use it rarely, I can high recommend the excellent QuickEMU [0]

Any VM is just a `quickget ubuntu 24.04` and `quickemu --vm ubuntu-24.04.conf` away. The conf file is just a yaml that is very readable and can give you more cores/ram/disk easily. Just run `quickget` to get a list of OS's to download.

[0] https://github.com/quickemu-project/quickemu

Yeah I also found myself curious as to how KVM actually works, I found these helpful

https://www.kernel.org/doc/ols/2007/ols2007v1-pages-225-230.... http://www.haifux.org/lectures/312/High-Level%20Introduction... https://zserge.com/posts/kvm/

> I've always wondered though how it works with KVM

Other people have given some more comprehensive explanations, but I'll try to put it as simply as possible.

Plain QEMU has a CPU emulation layer called TCG. The machine basically consists of memory (RAM and MMIO devices) and CPUs (CPU registers and state). When QEMU has set up the machine and is ready to run, it calls TCG to say "given this memory and this initial CPU register state, start running instructions". When you use QEMU with KVM, the TCG emulation layer is swapped out with KVM and it asks KVM to start running instructions. That's it. KVM exposes APIs that caller can specify guest memory and initial CPU register state, and a call to run that CPU with that memory.

Going a bit further, the hardware virtualization functions that KVM uses have the ability to map that memory with a second level of translation which lets KVM present it to the guest at the locations it expects, and to prevent the guest from accessing any memory that it should not. The hardware also has the ability to run the CPU in a mode where it has the normal set of registers (which is what QEMU wants), but it maintains some additional hypervisor control registers not available to the guest, and those can ensure the guest can't take complete control of the CPU (for example, the guest OS can "disable interrupts" with the usual MSR or similar bit and that does prevent the guest from getting interrupts, but that it does not disable hypervisor directed interrupts, so the hypervisor can always take back control of the CPU with a hypervisor-IPI or hypervisor timer interrupt).

Further still: when running in plain QEMU mode, devices are emulated by registering MMIO ranges in the memory address space and emulated loads and stores have code to detect these regions and instead of performing a simple load or store, they call into device model code which handles it accordingly. When you plug KVM in, you can still use these emulated devices. These are modeled by using that second level page table to put "not-valid" mappings in those MMIO ranges. These cause the CPU to trigger a page fault when it tries to access them, and KVM sees this, looks up the table of memory registered by QEMU, and sees that it is an address which QEMU wants to handle, so it returns from the KVM_RUN system call with result code that indicates there was an MMIO read/write that needs to be handled. QEMU then directs this into its emulated device model. Then when QEMU has performed that device emulation, it calls back into KVM to continue running the CPU.

It's all pretty clever. The really astounding thing is that most of the basic concepts for all this stuff were developed/discovered/invented like 50+ years ago.

qemu/kvm in enabling the cloud is huge but that's not the only place it really makes a tremendous difference. One example where it's essential is new OS development. They all basically first target the qemu machine with its virtual hardware. It makes development much faster compared to running on real hardawre while easily enabling debug output without needing cables and the like.

Resources-wise there's not really any "stealing" going on. The people/companies who care about KVM and the virtualization use cases work on that, and the people/companies who care about emulation work on those parts. If QEMU didn't support virtualization then it's not like the people currently working on QEMU virtualization would shift over to emulation support: they'd be working on some other project instead to achieve their VM goals.

Xen is still used massively too.

Not everything uses qemu. Some do. More use KVM. Not everything does.

Example: https://firecracker-microvm.github.io/

Yes, it's possible and supported. QEMU can emulate an aarch64 system, and Google provides aarch64 Android builds for virtual machines specifically, called "Cuttlefish". Search for keywords "Android Cuttlefish QEMU" for instructions.

The official Android "emulator" supplied by Google is qemu. If you're not satisfied with it for some reason, IIRC I used these images some years ago on top of vanilla qemu:

https://www.fosshub.com/Android-x86.html

They don't seem to be well supported anymore, and there aren't many prebuilt alternatives. One can always compile AOSP from source, though Google does not make this easy.

WFM