USB emulation — QEMU documentation

# USB emulation

QEMU can emulate a PCI UHCI, OHCI, EHCI or XHCI USB controller. You can plug virtual USB devices or real host USB devices (only works with certain host operating systems). QEMU will automatically create and connect virtual USB hubs as necessary to connect multiple USB devices.

## USB controllers

### XHCI controller support

QEMU has XHCI host adapter support. The XHCI hardware design is much more virtualization-friendly when compared to EHCI and UHCI, thus XHCI emulation uses less resources (especially CPU). So if your guest supports XHCI (which should be the case for any operating system released around 2010 or later) we recommend using it:

> qemu -device qemu-xhci

XHCI supports USB 1.1, USB 2.0 and USB 3.0 devices, so this is the only controller you need. With only a single USB controller (and therefore only a single USB bus) present in the system there is no need to use the bus= parameter when adding USB devices.

### EHCI controller support

The QEMU EHCI Adapter supports USB 2.0 devices. It can be used either standalone or with companion controllers (UHCI, OHCI) for USB 1.1 devices. The companion controller setup is more convenient to use because it provides a single USB bus supporting both USB 2.0 and USB 1.1 devices. See next section for details.

When running EHCI in standalone mode you can add UHCI or OHCI controllers for USB 1.1 devices too. Each controller creates its own bus though, so there are two completely separate USB buses: One USB 1.1 bus driven by the UHCI controller and one USB 2.0 bus driven by the EHCI controller. Devices must be attached to the correct controller manually.

The easiest way to add a UHCI controller to a `pc` machine is the `-usb` switch. QEMU will create the UHCI controller as function of the PIIX3 chipset. The USB 1.1 bus will carry the name `usb-bus.0`.

You can use the standard `-device` switch to add a EHCI controller to your virtual machine. It is strongly recommended to specify an ID for the controller so the USB 2.0 bus gets an individual name, for example `-device usb-ehci,id=ehci`. This will give you a USB 2.0 bus named `ehci.0`.

When adding USB devices using the `-device` switch you can specify the bus they should be attached to. Here is a complete example:

    qemu-system-x86_64 -M pc ${otheroptions}                        \
        -drive if=none,id=usbstick,format=raw,file=/path/to/image   \
        -usb                                                        \
        -device usb-ehci,id=ehci                                    \
        -device usb-tablet,bus=usb-bus.0                            \
        -device usb-storage,bus=ehci.0,drive=usbstick

This attaches a USB tablet to the UHCI adapter and a USB mass storage device to the EHCI adapter.

### Companion controller support

The UHCI and OHCI controllers can attach to a USB bus created by EHCI as companion controllers. This is done by specifying the `masterbus` and `firstport` properties. `masterbus` specifies the bus name the controller should attach to. `firstport` specifies the first port the controller should attach to, which is needed as usually one EHCI controller with six ports has three UHCI companion controllers with two ports each.

There is a config file in docs which will do all this for you, which you can use like this:

    qemu-system-x86_64 -readconfig docs/config/ich9-ehci-uhci.cfg

Then use `bus=ehci.0` to assign your USB devices to that bus.

Using the `-usb` switch for `q35` machines will create a similar USB controller configuration.

## Connecting USB devices

USB devices can be connected with the `-device usb-...` command line option or the `device_add` monitor command. Available devices are:

`usb-mouse`

  Virtual Mouse. This will override the PS/2 mouse emulation when activated.

`usb-tablet`

  Pointer device that uses absolute coordinates (like a touchscreen). This means QEMU is able to report the mouse position without having to grab the mouse. Also overrides the PS/2 mouse emulation when activated.

`usb-storage,drive=drive_id`

  Mass storage device backed by drive_id (see the Disk Images chapter in the System Emulation Users Guide). This is the classic bulk-only transport protocol used by 99% of USB sticks. This example shows it connected to an XHCI USB controller and with a drive backed by a raw format disk image:

      qemu-system-x86_64 [...]                                   \
       -drive if=none,id=stick,format=raw,file=/path/to/file.img \
       -device nec-usb-xhci,id=xhci                              \
       -device usb-storage,bus=xhci.0,drive=stick

`usb-uas`

  USB attached SCSI device. This does not create a SCSI disk, so you need to explicitly create a `scsi-hd` or `scsi-cd` device on the command line, as well as using the `-drive` option to specify what those disks are backed by. One `usb-uas` device can handle multiple logical units (disks). This example creates three logical units: two disks and one cdrom drive:

      qemu-system-x86_64 [...]                                         \
       -drive if=none,id=uas-disk1,format=raw,file=/path/to/file1.img  \
       -drive if=none,id=uas-disk2,format=raw,file=/path/to/file2.img  \
       -drive if=none,id=uas-cdrom,media=cdrom,format=raw,file=/path/to/image.iso \
       -device nec-usb-xhci,id=xhci                                    \
       -device usb-uas,id=uas,bus=xhci.0                               \
       -device scsi-hd,bus=uas.0,scsi-id=0,lun=0,drive=uas-disk1       \
       -device scsi-hd,bus=uas.0,scsi-id=0,lun=1,drive=uas-disk2       \
       -device scsi-cd,bus=uas.0,scsi-id=0,lun=5,drive=uas-cdrom

`usb-bot`

  Bulk-only transport storage device. This presents the guest with the same USB bulk-only transport protocol interface as `usb-storage`, but the QEMU command line option works like `usb-uas` and does not automatically create SCSI disks for you. `usb-bot` supports up to 16 LUNs. Unlike `usb-uas`, the LUN numbers must be continuous, i.e. for three devices you must use 0+1+2. The 0+1+5 numbering from the `usb-uas` example above won’t work with `usb-bot`.

`usb-mtp,rootdir=dir`

  Media transfer protocol device, using dir as root of the file tree that is presented to the guest.

`usb-host,hostbus=bus,hostaddr=addr`

  Pass through the host device identified by bus and addr

`usb-host,vendorid=vendor,productid=product`

  Pass through the host device identified by vendor and product ID

`usb-wacom-tablet`

  Virtual Wacom PenPartner tablet. This device is similar to the `tablet` above but it can be used with the tslib library because in addition to touch coordinates it reports touch pressure.

`usb-kbd`

  Standard USB keyboard. Will override the PS/2 keyboard (if present).

`usb-serial,chardev=id`

  Serial converter. This emulates an FTDI FT232BM chip connected to host character device id.

`usb-braille,chardev=id`

  Braille device. This emulates a Baum Braille device USB port. id has to specify a character device defined with `-chardev …,id=id`. One will normally use BrlAPI to display the braille output on a BRLTTY-supported device with

      qemu-system-x86_64 [...] -chardev braille,id=brl -device usb-braille,chardev=brl

  or alternatively, use the following equivalent shortcut:

      qemu-system-x86_64 [...] -usbdevice braille

`usb-net[,netdev=id]`

  Network adapter that supports CDC ethernet and RNDIS protocols. id specifies a netdev defined with `-netdev …,id=id`. For instance, user-mode networking can be used with

      qemu-system-x86_64 [...] -netdev user,id=net0 -device usb-net,netdev=net0

`usb-ccid`

  Smartcard reader device

`usb-audio`

  USB audio device

`u2f-{emulated,passthru}`

  Universal Second Factor (U2F) USB Key Device

`canokey`

  An Open-source Secure Key implementing FIDO2, OpenPGP, PIV and more. For more information, see CanoKey QEMU.

### Physical port addressing

For all the above USB devices, by default QEMU will plug the device into the next available port on the specified USB bus, or onto some available USB bus if you didn’t specify one explicitly. If you need to, you can also specify the physical port where the device will show up in the guest. This can be done using the `port` property. UHCI has two root ports (1,2). EHCI has six root ports (1-6), and the emulated (1.1) USB hub has eight ports.

Plugging a tablet into UHCI port 1 works like this:

    -device usb-tablet,bus=usb-bus.0,port=1

Plugging a hub into UHCI port 2 works like this:

    -device usb-hub,bus=usb-bus.0,port=2

Plugging a virtual USB stick into port 4 of the hub just plugged works this way:

    -device usb-storage,bus=usb-bus.0,port=2.4,drive=...

In the monitor, the `device_add` command also accepts a ``port` property specification. If you want to unplug devices too you should specify some unique id which you can use to refer to the device. You can then use `device_del` to unplug the device later. For example:

    (qemu) device_add usb-tablet,bus=usb-bus.0,port=1,id=my-tablet
    (qemu) device_del my-tablet

## Hotplugging USB storage

The `usb-bot` and `usb-uas` devices can be hotplugged. In the hotplug case they are added with `attached = false` so the guest will not see the device until the `attached` property is explicitly set to true. That allows you to attach one or more scsi devices before making the device visible to the guest. The workflow looks like this:

1. `device-add usb-bot,id=foo`

2. `device-add scsi-{hd,cd},bus=foo.0,lun=0`

3. optionally add more devices (luns 1 … 15)

4. `scripts/qmp/qom-set foo.attached = true`

## Using host USB devices on a Linux host

WARNING: this is an experimental feature. QEMU will slow down when using it. USB devices requiring real time streaming (i.e. USB Video Cameras) are not supported yet.

1. If you use an early Linux 2.4 kernel, verify that no Linux driver is actually using the USB device. A simple way to do that is simply to disable the corresponding kernel module by renaming it from `mydriver.o` to `mydriver.o.disabled`.

2. Verify that `/proc/bus/usb` is working (most Linux distributions should enable it by default). You should see something like that:

       ls /proc/bus/usb
       001  devices  drivers

3. Since only root can access to the USB devices directly, you can either launch QEMU as root or change the permissions of the USB devices you want to use. For testing, the following suffices:

       chown -R myuid /proc/bus/usb

4. Launch QEMU and do in the monitor:

       info usbhost
         Device 1.2, speed 480 Mb/s
           Class 00: USB device 1234:5678, USB DISK

   You should see the list of the devices you can use (Never try to use hubs, it won’t work).

5. Add the device in QEMU by using:

       device_add usb-host,vendorid=0x1234,productid=0x5678

   Normally the guest OS should report that a new USB device is plugged. You can use the option `-device usb-host,...` to do the same.

6. Now you can try to use the host USB device in QEMU.

When relaunching QEMU, you may have to unplug and plug again the USB device to make it work again (this is a bug).

### `usb-host` properties for specifying the host device

The example above uses the `vendorid` and `productid` to specify which host device to pass through, but this is not the only way to specify the host device. `usb-host` supports the following properties:

`hostbus=<nr>`

  Specifies the bus number the device must be attached to

`hostaddr=<nr>`

  Specifies the device address the device got assigned by the guest os

`hostport=<str>`

  Specifies the physical port the device is attached to

`vendorid=<hexnr>`

  Specifies the vendor ID of the device

`productid=<hexnr>`

  Specifies the product ID of the device.

In theory you can combine all these properties as you like. In practice only a few combinations are useful:

- `vendorid` and `productid` – match for a specific device, pass it to the guest when it shows up somewhere in the host.

- `hostbus` and `hostport` – match for a specific physical port in the host, any device which is plugged in there gets passed to the guest.

- `hostbus` and `hostaddr` – most useful for ad-hoc pass through as the hostaddr isn’t stable. The next time you plug the device into the host it will get a new hostaddr.

Note that on the host USB 1.1 devices are handled by UHCI/OHCI and USB 2.0 by EHCI. That means different USB devices plugged into the very same physical port on the host may show up on different host buses depending on the speed. Supposing that devices plugged into a given physical port appear as bus 1 + port 1 for 2.0 devices and bus 3 + port 1 for 1.1 devices, you can pass through any device plugged into that port and also assign it to the correct USB bus in QEMU like this:

    qemu-system-x86_64 -M pc [...]                            \
         -usb                                                 \
         -device usb-ehci,id=ehci                             \
         -device usb-host,bus=usb-bus.0,hostbus=3,hostport=1  \
         -device usb-host,bus=ehci.0,hostbus=1,hostport=1

### `usb-host` properties for reset behavior

The `guest-reset` and `guest-reset-all` properties control whenever the guest is allowed to reset the physical usb device on the host. There are three cases:

`guest-reset=false`

  The guest is not allowed to reset the (physical) usb device.

`guest-reset=true,guest-resets-all=false`

  The guest is allowed to reset the device when it is not yet initialized (aka no usb bus address assigned). Usually this results in one guest reset being allowed. This is the default behavior.

`guest-reset=true,guest-resets-all=true`

  The guest is allowed to reset the device as it pleases.

The reason for this existing are broken usb devices. In theory one should be able to reset (and re-initialize) usb devices at any time. In practice that may result in shitty usb device firmware crashing and the device not responding any more until you power-cycle (aka un-plug and re-plug) it.

What works best pretty much depends on the behavior of the specific usb device at hand, so it’s a trial-and-error game. If the default doesn’t work, try another option and see whenever the situation improves.

### record usb transfers

All usb devices have support for recording the usb traffic. This can be enabled using the `pcap=<file>` property, for example:

`-device usb-mouse,pcap=mouse.pcap`

The pcap files are compatible with the linux kernels usbmon. Many tools, including `wireshark`, can decode and inspect these trace files.