I want to have a linux VM guest that can present itself to the host as a USB device. Specifically, I want to be able to use the USB gadget api for linux on the guest, so that, loading g_mass_storage (kernel module for linux as usb mass storage device), will talk to a usb device controller on the guest and present a usb mass storage device on the bus.

Virtualbox probably won't help much, because it connects to the host via a virtual usb hub. Here, its required to have a usb device controller on the guest.

Hello and welcome to LQ.


If I get the question correctly, you are asking about a usb passthrough in a virtual machine. Some of them can do that and your host may need to have support for it too.

I could be not understanding the question however too.

In a physical system, attaching a USB keyboard would mean that, a chip with USB device controller functionality is attached to the USB hub of the host. My objective, here, is to have this USB keyboard as a VM guest. The gadget api has linux modules that can emulate several USB classes if the hardware has a usb controller. I need a guest which, with this gadget api, can emulate an usb device and present itself to the host.

Here, the guest needs a virtual usb controller. Also, I think a virtual hub on the host is required that is virtually connected to the guest's usb controller.

A usb passthrough would mean that a device connected to the host's hub is passed on to the guest's virtual hub, which I do not want.

Can this be done?

No, you can't. "is to have this USB keyboard as a VM guest."


In your example a usb keyboard reports to the host hardware, OS and to the VM as such. It is coded to work as a keyboard and work transparently to the vm client. Unless you code qemu to act like you want, I rather doubt you can get a usb keyboard to connect to the client as a usb device (without passthrough.) If I recall the vm presents a ps/2 keyboard to the client. You'd have to check on that.