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

[Eric Blake]

On 06/29/2012 12:25 AM, Dennis Chen wrote:

>>>
>>> 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
>>

> Yes, I noticed the code above in libvirt, but it's not related with my
> question in the first message...

Actually, yes it is.  That's the code that libvirtd runs in reaction to
your client code making an RPC code via libvirt.so.

> 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", ...);

Not StartProcess, so much as execve(), as wrapped in our virCommand API
(src/util/command.c).  If you want to see the exact command line that we
constructed, look at /var/log/libvirt/qemu/$dom.log for the domain name
that you are starting.  It is a rather hairy command line; this is an
example from one of my VMs:

LC_ALL=C PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin
QEMU_AUDIO_DRV=none /usr/bin/qemu-kvm -S -M pc-0.14 -cpu qemu32
-enable-kvm -m 768 -smp 1,sockets=1,cores=1,threads=1 -name fedora-local
-uuid 24ab99db-bf50-085d-d3e5-7528243ae9b1 -nodefconfig -nodefaults
-chardev
socket,id=charmonitor,path=/var/lib/libvirt/qemu/fedora-local.monitor,server,nowait
-mon chardev=charmonitor,id=monitor,mode=control -rtc base=utc
-no-shutdown -device piix3-usb-uhci,id=usb,bus=pci.0,addr=0x1.0x2 -drive
file=/var/lib/libvirt/images/fedora-local.img,if=none,id=drive-virtio-disk0,format=raw
-device
virtio-blk-pci,scsi=off,bus=pci.0,addr=0x4,drive=drive-virtio-disk0,id=virtio-disk0,bootindex=1
-drive
file=/var/lib/libvirt/images/top.img,if=none,id=drive-virtio-disk1,format=qcow2
-device
virtio-blk-pci,scsi=off,bus=pci.0,addr=0x3,drive=drive-virtio-disk1,id=virtio-disk1
-drive if=none,id=drive-ide0-1-0,readonly=on,format=raw -device
ide-cd,bus=ide.1,unit=0,drive=drive-ide0-1-0,id=ide0-1-0,bootindex=2
-netdev tap,fd=22,id=hostnet0,vhost=on,vhostfd=23 -device
virtio-net-pci,netdev=hostnet0,id=net0,mac=52:54:00:65:82:9e,bus=pci.0,addr=0x5
-chardev pty,id=charserial0 -device
isa-serial,chardev=charserial0,id=serial0 -device usb-tablet,id=input0
-vnc 127.0.0.1:1 -k en-us -vga cirrus -device
ES1370,id=sound0,bus=pci.0,addr=0x6 -device
virtio-balloon-pci,id=balloon0,bus=pci.0,addr=0x7

In turn, it is that hairy command line that starts the qemu process, and
qemu then takes over the actual communication with the kernel via
/dev/kvm to use KVM virtualization.

-- 
Eric Blake   eblake at redhat.com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org



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