Raise guest page fault if GPA is out of range

Based on this statement from priv spec 5.5.1 (regarding Sv39x4):
"Address bits 63:41 must all be zeros, or else a guest-page-fault
exception occurs."

riscv/mmu.cc

@@ -267,9 +267,13 @@ reg_t mmu_t::s2xlate(reg_t gva, reg_t gpa, access_type type, access_type trap_ty
   if (vm.levels == 0)
     return gpa;
 
+  int maxgpabits = vm.levels * vm.idxbits + vm.widenbits + PGSHIFT;
+  reg_t maxgpa = (1ULL << maxgpabits) - 1;
+
   bool mxr = proc->state.sstatus->readvirt(false) & MSTATUS_MXR;
 
   reg_t base = vm.ptbase;
+  if ((gpa & ~maxgpa) == 0) {
   for (int i = vm.levels - 1; i >= 0; i--) {
     int ptshift = i * vm.idxbits;
     int idxbits = (i == (vm.levels - 1)) ? vm.idxbits + vm.widenbits : vm.idxbits;
@@ -328,6 +332,7 @@ reg_t mmu_t::s2xlate(reg_t gva, reg_t gpa, access_type type, access_type trap_ty
       return page_base | (gpa & page_mask);
     }
   }
+  }
 
   switch (trap_type) {
     case FETCH: throw trap_instruction_guest_page_fault(gva, gpa >> 2, 0);