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This feature modifies the current single-port virtio-console device to guests running on top of qemu and kvm. It exposes multiple ports to the guest in the form of simple char devices for simple IO between the guest and host userspaces. It also allows for multiple such devices to be exposed, lifting the current single device restriction.


Current status

  • Targeted release: Fedora 13
  • Last updated: 2010-02-08
  • Percentage of completion: 100%

Detailed Description

Also known as 'vmchannel', a transport mechanism is needed for communication between the host userspace and guest userspace for achieving things like making clipboard copy/paste work seamlessly across the host and guest, locking the guest screen in case the vnc session to the guest is closed and so on. This can be used in offline cases as well, for example with libguestfs to probe which file systems the guest uses, the apps installed, etc.

Virtio-serial is just the transport protocol that will enable such applications to be written. It has two parts: (a) device emulation in qemu that presents a virtio-pci device to the guest and (b) a guest driver that presents a char device interface to userspace applications.

Benefit to Fedora

Some benefits include:

  • a communication channel between the guest and the host even when guest networking is disabled
  • applications can be written on top of the transport to achieve better guest-host cooperation, eg., clipboard copy/paste support between the host and the guest
  • libguestfs currently uses a low-performing vmchannel interface. Updating libguestfs to use this interface will achieve better speeds and flexibility.
  • SPICE will be able to use this infrastructure.
  • Anaconda logs could be sent to the host for messages that are emitted before networking is setup.


Changes are required in QEMU and Linux. A few udev rules will have to be added to dynamically create ports for the device and specify default permissions.

How To Test

There are quite a few ways to test the functionality:

  • Spawn a virtioconsole with new guest kernel and qemu images. Work on the console.
  • Spawn multiple virtioconsoles.
  • Transfer files from the host to the guest (and vice-versa) over virtio-serial ports and compare sha1sums of the files on the source and destination
  • Try all of the above at the same time
  • Try the above with different guest smp options
  • Try the testsuite at the link below for exercising various open/read/write/poll paths that test for various options

Git repository for testsuite:

When starting qemu or kvm guests from the command line, add something like this:

-device virtio-serial \
-chardev socket,path=/tmp/foo,server,nowait,id=foo \
-device virtserialport,chardev=foo,name=org.fedoraproject.port.0

This will create a device and expose one serial port to the guest. The guest kernel should load the virtio_console.ko module and expose a /dev/vport0p1 port to guest userspace app developers. A sysfs attribute is placed in /sys/class/virtio-ports/vport0p1/name that contains the text "org.fedoraproject.port.0". A udev rule will create a symlink in /dev/virtio-ports:

/dev/virtio-ports/org.fedoraproject.port.0 -> /dev/vport0p1

Data written into host's /tmp/foo (via a utility like socat) will then be relayed to the guest and a guest app should be able to read the data from /dev/vport0p1 or /dev/virtio-ports/org.fedoraproject.port.0.

/dev/vportNp0 is reserved for the first virtio console.

Testing console IO

Create virtio-console ports by:

-device virtio-serial
-chardev socket,path=/tmp/foo,server,nowait,id=foo \
-chardev socket,path=/tmp/bar,server,nowait,id=bar \
-device virtioconsole,chardev=foo,
-device virtioconsole,chardev=bar,

The console ports will be available in the guest as /dev/hvc0 and /dev/hvc1.

Spawn console ports via:

# agetty /dev/hvc0 9600 vt100
# agetty /dev/hvc1 9600 vt100

On the host:

$ socat /tmp/foo -
$ socat /tmp/bar -

Will get the two console terminals.

Invocation Options Available


The virtio-serial device itself has two options:

  • max_ports: This option can be used to set the maximum number of allowed ports for this device. The default is 31 (the current maximum). If more ports are needed, two such devices can be spawned. If it's expected that the number of ports ever needed across the lifetime of the guest will be lower than the default, specifying that value here will result in some resource savings (memory, MSI vectors). Example:
-device virtio-serial,max_ports=2
  • vectors: This option specifies the number of MSI vectors to be used for the device. It only makes sense for architectures that use the PCI bus. The default is to use as many vectors as there are ports (plus 1 for internal usage). If '0' is specified here, MSI will be disabled and a GSI interrupt will be used. Example:
-device virtio-serial,vectors=4
-device virtio-serial-pci,vectors=4

The default qdev options are available as well. For example, to create two virtio-serial devices and to differentiate between the two, something like the following will work:

-device virtio-serial,id=ser0,max_ports=2 -device virtio-serial,id=ser1,max_ports=3 \
-device virtserialport,bus=ser0.0,chardev=portchar0,id=port0,name=org.fedoraproject.port.0 \
-device virtioconsole,bus=ser1.0,chardev=conschar0,id=cons0,name=org.fedoraproject.console.0


Individual ports have port-specific options available:

  • nr: This option specifies the port number the port is to be given for enumeration within qemu. This option is useful when using hot-plug/unplug operations and migration. If this parameter is not specified, qemu will allocate a number to the port.
  • name: This option gives a 'name' to the port, which is exposed in the guest via sysfs and a symlink is created to the actual port, as described above.

The virtserialport and virtconsole ports have an extra parameter:

  • chardev: This parameter specifies the host-side qemu chardev the port is associated with for IO.

Hot-plug / Hot-unplug

Using the qemu monitor, ports can be hot-plugged as well as unplugged:

(qemu) device_add virtserialport,name=org.fedoraproject.port.0,id=port0
(qemu) device_del port0


A page describing the Linux and Windows Guest APIs and host qemu APIs along with some gotchas is put up at virtio-serial-API.

User Experience

Virtio-serial is just the transport and by itself won't be user-visible. Applications written on top of virtio-serial to communicate data between the host and guest OSes will bring user-visible changes.


The QEMU patches are accepted upstream and will be part of the next major release (post-0.12). The kernel patches are in Rusty's queue and will be part of the 2.6.35 release. A udev rule that creates the symlinks for port names is included in udev release 151 (with a bugfix, that should be part of 152).

Contingency Plan

QEMU currently contains an alternative implementation of vmchannel using userland networking which is slower but similar in feature set. Continue using that interface in case virtio-serial is not ready.


Release Notes

  • The virtio-console pci device is now equipped to handle multiple console ports as well as generic ports for guests running on top of qemu and kvm. This facilitates simple guest <-> host communication.

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