riscv64-unknown-elf-gdb: E14, cannot fetch $stvec

[mapleFU]

I'm learning 6.S081 myself, and when I read the Isolation & system call entry/exit, using riscv64-unknown-elf-gdb will receive an error:

Could not fetch register "stvec"; remote failure reply 'E14'

My gdb version is:

riscv64-unknown-elf-gdb --version
Python Exception <type 'exceptions.ImportError'> No module named gdb: 
riscv64-unknown-elf-gdb: warning: 
Could not load the Python gdb module from `/usr/local/Cellar/riscv-gnu-toolchain/gnu/share/gdb/python'.
Limited Python support is available from the _gdb module.
Suggest passing --data-directory=/path/to/gdb/data-directory.

GNU gdb (GDB) 8.3.0.20190516-git
Copyright (C) 2019 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

[mapleFU]

Oh, I made a mistake, seems that if system is not in kernel, print/x stvec will failed.