Stateen bits designated for custom use?

[henry-hsieh]

Current spec has following description:

Initially, the following bits are defined in mstateen0, hstateen0, and sstateen0:
bit 0 - Custom state
bit 1 - fcsr for Zfinx and related extensions (Zdinx, etc.)
Bit 0 controls access to any and all custom state.

Does it means stateen won't reserve some bits for custom extensions, and the custom extensions should have their own enablement mechanisms? If the spaces in stateen are not critical (have room for custom extension), is it possible for the standard open some bits for custom use alike upper 16-bit of mip?

It's a little painful that some custom extensions can be granted to user mode, and others can not be granted. If they all share the same switch, the hardware couldn't distinguish whether the extension can be used by user mode.

Thanks!