A RV64 with usingVM=false config will throw firrtl exception

[DecodeTheEncoded]

It's true that this is an unsual config, but maybe it's worth initiating an issue, I setup a RV64 with usingVM being false, and the firrtl lowering will complain about the following:
PTW.scala:298: High index 26 is out of range [0, 20] in class freechips.rocketchip.rocket.PT
The loc in line 298 of PTW.scala is :
val (r_tag, r_idx) = Split(Cat(r_req.vstage1, r_req.addr(maxSVAddrBits-pgIdxBits-1, 0)), idxBits)
the r_req.addr is vpnBits width, and it's (paddrBits + 1) min xLen when usingVM is false, but the up index maxSVAddrBits-pgIdxBits-1 seems always expect the usingVM is true. Therefore there is an out-of-range exception.
I don't know if it's worth fixing, but just post here so that others can be aware of this.
Thanks for your work.