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

On Fri, Jun 29, 2012 at 02:18:17PM +0800, Dennis Chen wrote: > On 06/28/2012 06:26 PM, Daniel P. Berrange wrote: >> 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 > Thanks Daniel, now I understand that the libvirt code will play with > QEMU binary, for example, qemu-system-x86_64, the latter will talk > with KVM module . But now the question is, I guess > qemuProcessStart() function was used to spawn a QEMU process, so I > built a virsh from the source package with "-g -O0" flag, I want to > gdb the virsh, but when I want to set a break point on > qemuProcessStart(), I encounter issue: > > #gdb virsh > (gdb) b main > Breakpoint 1 at 0x807d480: file virsh.c, line 20270. > (gdb) r > Starting program: /usr/bin/virsh > [Thread debugging using libthread_db enabled] > Using host libthread_db library "/lib/i386-linux-gnu/libthread_db.so.1". > > Breakpoint 1, main (argc=1, argv=0xbffff294) at virsh.c:20270 > 20270 { > (gdb) b qemuProcessStart > Function "qemuProcessStart" not defined. > Make breakpoint pending on future shared library load? (y or [n]) y > Breakpoint 2 (qemuProcessStart) pending. > > (gdb)c > > virsh # create guest.xml > Domain vdo created from vdo.xml > > virsh # > > continue instruction above doesn't hit the pending Breakpoint 2... > > So my question is, which kind of object file will be created from > $GIT/src/qemu folder during the build process, is it a .so file and > virsh will load it? virsh talks to the libvirtd daemon, the libvirtd daemon run those calls from the qemu driver. The documentation is relatively sparse, but reading what is available will help: http://libvirt.org/api.html http://libvirt.org/internals.html so if you want to step though that function you must gdb the libvirt daemon, not the client application. Daniel