Another nice goodie coming in the qemu 2.6 release is support for linux evdev devices. qemu can pick up input events directly from the devices now, which is quite useful in case you don't use gtk/sdl/vnc/spice for display output. Most common use case for this is vga pass-through. It's used this way:
qemu -object input-linux,id=$name,evdev=/dev/input/event$nr
Note that udev creates symlinks with static names in
/dev/input/by-path which you might find more convinient to use. It is possible to pass through multiple devices (i.e. mouse and keyboard).
Hitting both control keys at the same time will acquire and release the keyboard grab, i.e. it'll switch the keyboard between guest and host. With the grab_all option enabled the keyboard hotkeys will toggle the grab for all other input devices too, so you can switch keyboard and mouse together. For the ps/2 devices on my thinkpad the command line for that looks like this:
qemu -object input-linux,id=kbd,evdev=/dev/input/by-path/platform-i8042-serio-0-event-kbd,grab_all=true
Note: On my laptop
platform-i8042-serio-1-event-mouse is the touchpad which can't be used.
When using this with usb devices take care that they often have multiple functions, and each of them gets its own input device. The usb nano receiver for my wireless microsoft mouse has three event devices (probably because they use the same thing for all devices not only the mouse):
nilsson root ~# ls /dev/input/by-id/