---
title: KVM Architecture
url: https://doc.liz6.com/en/linux-kernel/13-kvm-and-virtualization/01-kvm-architecture
locale: en
area: linux-kernel
tags:
- linux-kernel
- kvm-and-virtualization
date: 2026-06-30
modified: 2026-07-16
description: 'Coverage: KVM as Linux hypervisor → VMX/SVM hardware virtualization → vCPU lifecycle → VM exit handling → KVM + QEMU collaboration model → /dev/kvm ioctl API Kernel version: 2.6.20 ~ 6.x'
---
# KVM Architecture
> Coverage: KVM as Linux hypervisor → VMX/SVM hardware virtualization → vCPU lifecycle → VM exit handling → KVM + QEMU collaboration model → /dev/kvm ioctl API
> Kernel version: 2.6.20 ~ 6.x
## Overview
KVM (Kernel-based Virtual Machine) transforms the Linux kernel into a type-1 hypervisor—leveraging Intel VMX or AMD SVM hardware virtualization extensions to allow guests to run directly on physical CPUs in VMX non-root mode. Each VM is a QEMU process in user space, and each vCPU is a thread within that process.
## Architecture Layers
## VM Entry / VM Exit
## vCPU Thread Model
```c
// Each vCPU = one QEMU thread, executing a loop:
while (running) {
ioctl(vcpu_fd, KVM_RUN, 0);
// Returns: VM exit
switch (vcpu->kvm_run->exit_reason) {
case KVM_EXIT_IO: handle_io(vcpu); break;
case KVM_EXIT_MMIO: handle_mmio(vcpu); break;
case KVM_EXIT_IRQ_WINDOW_OPEN: inject_interrupt(vcpu); break;
// ...
}
}
```
### Scheduler Interaction
```c
// KVM knows when a vCPU is preempted via preempt notifier:
// → kvm_sched_out(): vCPU is scheduled out
// → kvm_sched_in(): vCPU is scheduled in
// Used for: steal time accounting, TSC offset calculation
```
## /dev/kvm Interface
```c
// Create VM:
int kvm_fd = open("/dev/kvm", O_RDWR);
int vm_fd = ioctl(kvm_fd, KVM_CREATE_VM, 0);
// Register memory (guest physical address space):
struct kvm_userspace_memory_region region = {
.slot = 0, .guest_phys_addr = 0, .memory_size = 1ULL << 30,
.userspace_addr = (__u64)guest_ram_ptr, .flags = 0 };
ioctl(vm_fd, KVM_SET_USER_MEMORY_REGION, ®ion);
// Create vCPU:
int vcpu_fd = ioctl(vm_fd, KVM_CREATE_VCPU, cpu_id);
// Get KVM version/capabilities:
int version = ioctl(kvm_fd, KVM_GET_API_VERSION, 0);
struct kvm_cpuid2 cpuid = { .nent = N };
ioctl(kvm_fd, KVM_GET_SUPPORTED_CPUID, &cpuid);
```
## Interrupt Injection
```c
// KVM injects interrupts to the guest:
// 1. Set interrupt info fields in VMCS (VMX) or VMCB (SVM)
// 2. Set interrupt window → guest responds immediately after VM entry
// 3. If vCPU is not running (scheduled out) → set kick flags
// Interrupt sources:
// - virtio devices (msix)
// - Emulated devices (8259A PIC, IOAPIC)
// - IPI (multiple vCPUs within guest)
// - Timer (hrtimer → KVM timer)
```
## References
- **Source Code**: `virt/kvm/kvm_main.c` (core), `arch/x86/kvm/vmx/vmx.c` (Intel VMX), `arch/x86/kvm/svm/svm.c` (AMD SVM)
- **Kernel Documentation**: `Documentation/virt/kvm/`
- **LWN**: "KVM: the Linux virtual machine monitor"
*Keywords: KVM, VMX, SVM, vCPU, VM exit, VMLAUNCH, VMRESUME, /dev/kvm, KVM_RUN, interrupt injection*