The satp register has reserved values.  Some implementers will, no doubt, want to define non-standard behavior based on satp.  I would like to propose that we define some of the reserved values in satp as Custom now so that those who do so won't head in diverging directions.

For RV64, there are 11 reserved values of the Mode field.  I propose that the encodings 14 and 15 be defined as Custom.

For RV32, values with satp[31] clear and satp[30:0] non-zero are reserved.  I propose that values with satp[31] clear and satp[30:29]=0x3 be defined as Custom.

The RV32 encoding is not perfect.  To have the largest contiguous space free, we need to pick bits at the top or bottom.  Top bits encroach on what's now ASID space while bottom bits encroach on what's now PPN space.  The former seemed a bit better.

       Bill