[libvirt] ask for help to explain the network about tap

[Yong Sheng Gong]

Hi,
I don't know who helps qemu-kvm to create and open the tap file and when if I define an interface of a kvm vm like:
    <interface type='bridge'>
      <mac address='52:54:00:e4:2e:c1'/>
      <source bridge='br200'/>
      <model type='virtio'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x03' function='0x0'/>
    </interface>

The qemu-kvm process's command line will be:
/usr/libexec/qemu-kvm -S -M pc-0.14 -enable-kvm -m 1024 -smp 1,sockets=1,cores=1,threads=1 -name test -uuid 4b39bb5e-3994-9529-b554-2610f905945d -nodefconfig -nodefaults -chardev socket,id=charmonitor,path=/var/lib/libvirt/qemu/test.monitor,server,nowait -mon chardev=charmonitor,id=monitor,mode=control -rtc base=utc -drive file=/var/lib/libvirt/images/test.img,if=none,id=drive-virtio-disk0,format=raw,cache=none,aio=threads -device virtio-blk-pci,bus=pci.0,addr=0x5,drive=drive-virtio-disk0,id=virtio-disk0,bootindex=1 -drive if=none,media=cdrom,id=drive-ide0-1-0,readonly=on,format=raw -device ide-drive,bus=ide.1,unit=0,drive=drive-ide0-1-0,id=ide0-1-0 -netdev tap,fd=24,id=hostnet0 -device virtio-net-pci,netdev=hostnet0,id=net0,mac=52:54:00:e4:2e:c1,bus=pci.0,addr=0x3 -chardev pty,id=charserial0 -device isa-serial,chardev=charserial0,id=serial0 -usb -device usb-tablet,id=input0 -vnc 127.0.0.1:0 -vga cirrus -device intel-hda,id=sound0,bus=pci.0,addr=0x4 -device hda-duplex,id=sound0-codec0,bus=sound0.0,cad=0 -device virtio-balloon-pci,id=balloon0,bus=pci.0,addr=0x6

Note the -netdev tap,fd=24

If I use predefined tap dev name to define:
    <interface type='ethernet'>
      <mac address='f6:19:e0:6c:84:c3'/>
      <script path=''/>
      <target dev='tap10'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x03' function='0x0'/>
    </interface>
The process will be run as:
/usr/libexec/qemu-kvm -S -M pc-0.14 -enable-kvm -m 1024 -smp 1,sockets=1,cores=1,threads=1 -name test-clone -uuid c188ef83-82a4-23bb-dd81-b364e0b4d055 -nodefconfig -nodefaults -chardev socket,id=charmonitor,path=/var/lib/libvirt/qemu/test-clone.monitor,server,nowait -mon chardev=charmonitor,id=monitor,mode=control -rtc base=utc -drive file=/var/lib/libvirt/images/test-clone.img,if=none,id=drive-virtio-disk0,format=raw,cache=none,aio=threads -device virtio-blk-pci,bus=pci.0,addr=0x5,drive=drive-virtio-disk0,id=virtio-disk0,bootindex=1 -drive if=none,media=cdrom,id=drive-ide0-1-0,readonly=on,format=raw -device ide-drive,bus=ide.1,unit=0,drive=drive-ide0-1-0,id=ide0-1-0 -netdev tap,ifname=tap10,script=,id=hostnet0 -device rtl8139,netdev=hostnet0,id=net0,mac=f6:19:e0:6c:84:c3,bus=pci.0,addr=0x3 -chardev pty,id=charserial0 -device isa-serial,chardev=charserial0,id=serial0 -usb -device usb-tablet,id=input0 -vnc 127.0.0.1:1 -vga cirrus -device intel-hda,id=sound0,bus=pci.0,addr=0x4 -device hda-duplex,id=sound0-codec0,bus=sound0.0,cad=0 -device virtio-balloon-pci,id=balloon0,bus=pci.0,addr=0x6
please note the -netdev tap,ifname=tap10

Thanks
Yong Sheng Gong