TLB/src/PermissionValidator.py

   1 from nmigen import Signal
   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 #
  10 # Arguments:
  11 #  data_size: (bit count) The size of the data words being processed
  12 #
  13 # Return:
  14 #  1. Data is valid ->  valid is HIGH
  15 #  2. Data is not valid -> valid is LOW
  16 class PermissionValidator():
  17     def __init__(self, data_size):
  18         # Input
  19         self.data = Signal(data_size);
  20         self.xwr = Signal(3) # Execute, Write, Read
  21         self.super = Signal(1) # Supervisor Mode
  22         self.super_access = Signal(1) # Supervisor Access
  23         self.asid = Signal(15) # Address Space IDentifier (ASID)
  24
  25         # Output
  26         self.valid = Signal(1) # Denotes if the permissions are correct
  27
  28     def elaborate(self, platform):
  29         m = Module()
  30         m.d.comb += [
  31             # Check if ASID matches OR entry is global
  32             If(data[64:78] == self.asid or data[5] == 1,
  33                # Check Execute, Write, Read (XWR) Permissions
  34                If(data[3] == self.xwr[2] and data[2] == self.xwr[1] \
  35                   and data[1] == self.xwr[0],
  36                   # Check if supervisor
  37                   If(self.super == 1,
  38                      # Check if entry is in user mode
  39                      # Check if supervisor has access
  40                      If(data[4] == 0,
  41                         self.valid.eq(1)
  42                      ).Elif(self.super_access == 1,
  43                         self.valid.eq(1)
  44                      ).Else(
  45                         self.valid.eq(0)
  46                      )
  47                   ).Else(
  48                       # Check if entry is in user mode
  49                       If(data[4] == 1,
  50                          self.valid.eq(1)
  51                       ).Else(
  52                          self.valid.eq(0)
  53                       )
  54                   )
  55                ).Else(
  56                    self.valid.eq(0)
  57                )
  58             ).Else(
  59                 self.valid.eq(0)
  60             )
  61         ]