A collection of tips on using usb with qemu.

Picking a host adapter

The short answer for this one is: Unless you are running an operating system museum just use -device qemu-xhci.

Any recent operating system should support xhci out-of-the box. The only OS without xhci support which is still in widespread use is Windows 7.

In case your qemu version doesn't support qemu-xhci you can use nec-usb-xhci instead.

The -usb command line switch adds usb controllers matching the emulated hardware platform. So for the 'pc' machine type (emulating a 20+ year old i440FX chipset) this is a uhci host adapter (supporting usb1). For the 'q35' machine type (emulating a almost 10 year old Q35 chipset) it is ehci (for usb2 devices) with uhci companions (for usb1 devices). This is what you can use when running old guests which lack xhci support.

When using xhci you should better not use -usb, because you would get two usb busses then. Which is a valid configuration, but requires naming the usb host adapter and specifying the usb bus when adding usb devices if you want avoid qemu picking a random usb bus:

-device qemu-xhci,id=xhci -device usb-tablet,bus=xhci.0

With a single usb bus you can just say -device usb-tablet and be done with it.

Not enough usb ports?

Qemu can emulate an usb hub (-device usb-hub). But the hub supports usb1 only, so you should avoid using it. Better solution is to just increase the number of root ports. xhci has four root ports by default, but it supports up to 15 ports. And in case this still isn't enough a second xhci adapter can be added to the virtual machine.

To create a host adapter with 8 ports use -device qemu-xhci,p2=8,p3=8. The libvirt configuration is: 

<controller type='usb' model='qemu-xhci' ports='8'/>

In case you wonder why qemu-xhci needs both p2 and p3 parameters: p2 specifies the number of usb2 ports (which support usb1 too), and p3 specifies the number of usb3 ports. It is possible to assign different counts here. When using -device qemu-xhci,p2=8,p3=4 you'll get an xhci adapter where ports 1-4 support both usb2 and usb3 and ports 5-8 are usb2-only. Can be used to force a usb3-capable usb device into usb2 mode by plugging it into a usb2-only xhci port. There should rarely be a need to actually do that in practice though.