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

[Daniel P. Berrange]

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
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