Dmitry Torokhov writes:

Not every keyboard, but all keycodes above KEY_MIN_INTERESTING which is
KEY_MUTE, so that interested handlers could match on devices they are
interested in without first opening them or poking through sysfs.

/Shouldn't/ they be reading sysfs attributes to find that information
out though?  Isn't modalias there to help modprobe find the right module
that wants to bind to this device, which doesn't happen for input
devices?  If user space is looking at this information then isn't it
getting it by reading from sysfs anyway?

What in user space looks at input devices other than X and Wayland?  And
those aren't looking for particular "interesting" keys are they?

I don't know why Xen keyboard exports that many keycodes ;) In general,
my recommendation is to mirror the physical device when possible, and
instantiate several devices so there is 1:1 relationship between virtual
and physical devices.

Xen guys: any input as to why it supports so many "interesting" keys?