Re: [libvirt-users] How does libvirt interaction with KVM to create a VM?

On Thu, Jun 28, 2012 at 05:21:57PM +0800, Dennis Chen wrote: > All, > > These days I am trying to understand the interaction relationship > between the libvirt and KVM kernel module, eg. kvm_intel.ko. > > We know that KVM kernel module expose an entry in form of device > file "/dev/kvm" which can be accessed by user space application to > control, for example, create a VM using KVM_CREATE_VM with help of > ioctl. > > Now let's say the tool virsh based upon libvirt, we can create a > guest domain with the command looks like: > #virsh create guest.xml > Obviously, the above command will create a VM. But when I try to > investigate the libvirt code, I can't find any code play with the > "/dev/kvm" to send KVM_CREATE_VM ioctl code to KVM kernel module. > But I do found that the reference count of the kvm_intel.ko changed > before the virsh create command launched and after. > > So my question is: how does the libvirt interaction with KVM to > create a VM? Anybody can give me some tips about that, eg, the > corresponding codes in libvirt? The '/dev/kvm' device is the low level kernel interface for creating virtual domains. This is not actually used by libvirt at all. The QEMU binary has code that talks to /dev/kvm, so all libvirt does is to spawn a QEMU process which in turns creates the virtual machine All the libvirt code for this part is under $GIT/src/qemu/ in particular the qemu_command.c and qemu_process.c files Daniel