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

[Dennis Chen]

On 06/28/2012 07:03 PM, Alex Jia wrote:
> 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 }
Yes, I noticed the code above in libvirt, but it's not related with my 
question in the first message...
> 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
>>
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I guess the qemuProcessStart() is used to spawn a QEMU process, but 
seems that lot's of tricks in its implementation, essentially, does this 
function spawn the qemu process looks like, eg:

StartProcess("/usr/bin/qemu-system-x86_64", ...);

BRs,
Dennis