2. As Jonathan Behrens has already noted, some systems depend on being able to set mstatus.MXR = 1 temporarily to read S/U-executable instructions, for emulation purposes. The proposal should be modified to say that any S/U-mode-only PMP region that grants execute permission to S/U modes (bit X is set), implicitly grants read permission to M mode when MXR = 1.
Correction: I believe that should say "... implicitly grants read permission to S/U modes when in M mode and MXR = 1". This is relevant only when MPRV = 1 and MPP = 0 or 1, so it's a rather narrow case. Hopefully I've got it right this time.
