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USB 2.0 Quick Start
The QEMU EHCI Adapter can be used with and without companion
controllers. See below for the companion controller mode.
When not running in companion controller mode there are two completely
separate USB busses: 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 '-usb' switch will make qemu create the UHCI controller as part 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 a individual name, for example
'-device usb-ehci,id=ehci". This will give you a USB 2.0 bus named
I strongly recomment to also use -device to attach usb devices because
you can specify the bus they should be attached to this way. Here is
a complete example:
qemu -M pc ${otheroptions} \
-drive if=none,id=usbstick,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
Companion controller support has been added recently. The operational
model described above with two completely separate busses still works
fine. Additionally the UHCI and OHCI controllers got the ability to
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, just
try ...
qemu -readconfig docs/ich9-ehci-uhci.cfg
... then use "bus=ehci.0" to assign your usb devices to that bus.
xhci controller support
There is also xhci host controller support available. It got a lot
less testing than ehci and there are a bunch of known limitations, so
ehci may work better for you. On the other hand the xhci hardware
design is much more virtualization-friendly, thus xhci emulation uses
less resources (especially cpu). If you want to give xhci a try
use this to add the host controller ...
qemu -device nec-usb-xhci,id=xhci
... then use "bus=xhci.0" when assigning usb devices.
More USB tips & tricks
Recently the usb pass through driver (also known as usb-host) and the
qemu usb subsystem gained a few capabilities which are available only
via qdev properties, i,e. when using '-device'.
physical port addressing
First you can (for all usb devices) 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 four root
ports (1-4), 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=...
You can do basically the same in the monitor using the device_add
command. If you want to unplug devices too you should specify some
unique id which you can use to refer to the device ...
(qemu) device_add usb-tablet,bus=usb-bus.0,port=1,id=my-tablet
(qemu) device_del my-tablet
... when unplugging it with device_del.
USB pass through hints
The usb-host driver has a bunch of properties to specify the device
which should be passed to the guest:
hostbus=<nr> -- Specifies the bus number the device must be attached
hostaddr=<nr> -- Specifies the device address the device got
assigned by the guest os.
hostport=<str> -- Specifies the physical port the device is attached
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:
(1) vendorid+productid -- match for a specific device, pass it to
the guest when it shows up somewhere in the host.
(2) hostbus+hostport -- match for a specific physical port in the
host, any device which is plugged in there gets passed to the
(3) hostbus+hostaddr -- most useful for ad-hoc pass through as the
hostaddr isn't stable, the next time you plug in the device it
gets a new one ...
Note that USB 1.1 devices are handled by UHCI/OHCI and USB 2.0 by
EHCI. That means a device plugged into the very same physical port
may show up on different busses depending on the speed. The port I'm
using for testing is bus 1 + port 1 for 2.0 devices and bus 3 + port 1
for 1.1 devices. Passing through any device plugged into that port
and also assign them to the correct bus can be done this way:
qemu -M pc ${otheroptions} \
-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
Gerd Hoffmann <>