Finally, we are into the vCPU execution flow which helps us to put everything together and understand what happens under the hood.
I hope you didn't forget that the QEMU creates a posix thread for a vCPU of the guest and ioctl(), which is responsible for running a CPU and has the KVM_RUN arg (#define KVM_RUN_IO(KVMIO, 0x80)). vCPU thread executes ioctl(.., KVM_RUN, ...) to run the guest code. As these are posix threads, the Linux kernel can schedule these threads as with any other process/thread in the system.
Let us see how it all works:
Qemu-kvm User Space:
kvm_init_vcpu ()
kvm_arch_init_vcpu()
qemu_init_vcpu()
qemu_kvm_start_vcpu()
qemu_kvm_cpu_thread_fn()
while (1) {
if (cpu_can_run(cpu)) {
r = kvm_cpu_exec(cpu);
}
}
kvm_cpu_exec (CPUState *cpu)
-> run_ret = kvm_vcpu_ioctl(cpu, KVM_RUN, 0);According to the underlying architecture and hardware, different structures...