How to Use ddb(4)?

[Intron]

I have add relevant options into my kernel (7.0-CURRENT) configuration
file:

makeoptions	DEBUG=-g
options 	KDB
options 	DDB
options 	GDB
options 	INVARIANTS
options 	INVARIANT_SUPPORT
options 	WITNESS
options 	WITNESS_SKIPSPIN

When kernel panics, the prompt "db>" will appear. But at this time
I cannot control my computer with keyboard any longer. What's wrong
with me?

In my mind, both 7.0-CURRENT and 6.x-STABLE halt a year ago are
all right in ddb(4).

By the way, the loader(8) option "-a" (to let user decide which file
system is taken as root file system) is also unusable.

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                                                 From Beijing, China