TLB/PermissionValidator.py

   1 from nmigen import *
   2 from nmigen.cli import main
   3
   4 # The purpose of this Module is to check the Permissions of a given PTE
   5 # against the requested access permissions.
   6 # This module will either validate (by setting the valid bit HIGH) the request
   7 # or find a permission fault and invalidate (by setting the valid bit LOW)
   8 # the request
   9 class PermissionValidator():
  10     def __init__(self, data_size):
  11         # Input
  12         self.data = Signal(data_size);
  13         self.xwr = Signal(3) # Execute, Write, Read
  14         self.super = Signal(1) # Supervisor Mode
  15         self.super_access = Signal(1) # Supervisor Access
  16         self.asid = Signal(15) # Address Space IDentifier (ASID)
  17
  18         # Output
  19         self.valid = Signal(1) # Denotes if the permissions are correct
  20
  21     def elaborate(self, platform):
  22         m = Module()
  23         m.d.comb += [
  24             # Check if ASID matches OR entry is global
  25             If(data[98:113] == self.asid or data[5] == 1,
  26                # Check Execute, Write, Read (XWR) Permissions
  27                If(data[3] == self.xwr[2] and data[2] == self.xwr[1] and data[1] == self.xwr[0],
  28                   # Check if supervisor
  29                   If(self.super == 1,
  30                      # Check if entry is in user mode
  31                      # Check if supervisor has access
  32                      If(data[4] == 0,
  33                         self.valid.eq(1)
  34                      ).Elif(self.super_access == 1,
  35                         self.valid.eq(1)
  36                      ).Else(
  37                         self.valid.eq(0)
  38                      )
  39                   ).Else(
  40                       # Check if entry is in user mode
  41                       If(data[4] == 1,
  42                          self.valid.eq(1)
  43                       ).Else(
  44                          self.valid.eq(0)
  45                       )
  46                   )
  47                ).Else(
  48                    self.valid.eq(0)
  49                )
  50             ).Else(
  51                 self.valid.eq(0)
  52             )
  53         ]