libvirt users - Jun 2012 - How does libvirt interaction with KVM to create a VM?

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?

BRs.
Dennis

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
-- 
|: http://berrange.com      -o-    http://www.flickr.com/photos/dberrange/ :|
|: http://libvirt.org              -o-             http://virt-manager.org :|
|: http://autobuild.org       -o-         http://search.cpan.org/~danberr/ :|
|: http://entangle-photo.org       -o-       http://live.gnome.org/gtk-vnc :|

On 06/28/2012 05:21 PM, 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?
http://libvirt.org/git/?p=libvirt.git;a=blob;f=src/qemu/qemu_driver.c;h=2f934040ea7d996574ac447498a195d440ec0783;hb=HEAD

114 /* device for kvm ioctls */
115 #define KVM_DEVICE "/dev/kvm"

1022 static int kvmGetMaxVCPUs(void) {
1023     int maxvcpus = 1;
1024
1025     int r, fd;
1026
1027     fd = open(KVM_DEVICE, O_RDONLY);
1028     if (fd < 0) {
1029         virReportSystemError(errno, _("Unable to open %s"), 
KVM_DEVICE);
1030         return -1;
1031     }
1032
1033     r = ioctl(fd, KVM_CHECK_EXTENSION, KVM_CAP_NR_VCPUS);
1034     if (r > 0)
1035         maxvcpus = r;
1036
1037     VIR_FORCE_CLOSE(fd);
1038     return maxvcpus;
1039 }


http://libvirt.org/git/?p=libvirt.git;a=blob;f=src/qemu/qemu_process.c;h=c5140c3c92a9ceec8cb7778c79c8542f60eca486;hb=HEAD

Please see qemuProcessStart() implemetation:

3288 int qemuProcessStart(virConnectPtr conn,
3289                      struct qemud_driver *driver,
3290                      virDomainObjPtr vm,
3291                      const char *migrateFrom,
3292                      int stdin_fd,
3293                      const char *stdin_path,
3294                      virDomainSnapshotObjPtr snapshot,
3295                      enum virNetDevVPortProfileOp vmop,
3296                      unsigned int flags)
......
>
> BRs.
> Dennis
>
> _______________________________________________
> libvirt-users mailing list
> libvirt-users at redhat.com
> https://www.redhat.com/mailman/listinfo/libvirt-users