TLB/src/PermissionValidator.py

   1 from nmigen import Signal
   2 from nmigen.cli import main
   3
   4 class PermissionValidator():
   5     """ The purpose of this Module is to check the Permissions of a given PTE
   6         against the requested access permissions.
   7
   8         This module will either validate (by setting the valid bit HIGH)
   9         the request or find a permission fault and invalidate (by setting
  10         the valid bit LOW) the request
  11     """
  12
  13     def __init__(self, data_size):
  14         """ Arguments:
  15             * data_size: (bit count) The size of the data words being processed
  16
  17             Return:
  18             1. Data is valid ->  valid is HIGH
  19             2. Data is not valid -> valid is LOW
  20         """
  21         # Input
  22         self.data = Signal(data_size);
  23         self.xwr = Signal(3) # Execute, Write, Read
  24         self.super = Signal(1) # Supervisor Mode
  25         self.super_access = Signal(1) # Supervisor Access
  26         self.asid = Signal(15) # Address Space IDentifier (ASID)
  27
  28         # Output
  29         self.valid = Signal(1) # Denotes if the permissions are correct
  30
  31     def elaborate(self, platform):
  32         m = Module()
  33         m.d.comb += [
  34             # Check if ASID matches OR entry is global
  35             If(data[64:78] == self.asid or data[5] == 1,
  36                # Check Execute, Write, Read (XWR) Permissions
  37                If(data[3] == self.xwr[2] and data[2] == self.xwr[1] \
  38                   and data[1] == self.xwr[0],
  39                   # Check if supervisor
  40                   If(self.super == 1,
  41                      # Check if entry is in user mode
  42                      # Check if supervisor has access
  43                      If(data[4] == 0,
  44                         self.valid.eq(1)
  45                      ).Elif(self.super_access == 1,
  46                         self.valid.eq(1)
  47                      ).Else(
  48                         self.valid.eq(0)
  49                      )
  50                   ).Else(
  51                       # Check if entry is in user mode
  52                       If(data[4] == 1,
  53                          self.valid.eq(1)
  54                       ).Else(
  55                          self.valid.eq(0)
  56                       )
  57                   )
  58                ).Else(
  59                    self.valid.eq(0)
  60                )
  61             ).Else(
  62                 self.valid.eq(0)
  63             )
  64         ]