TLB/PermissionValidator.py

   1 from nmigen import Signal, Module, If, Else
   2 from nmigen.cli import main
   3
   4 # The expected form of the data is
   5 # Item (Bits)
   6 # Tag (N - 79) / ASID (78 - 64) / PTE (63 - 0)
   7
   8 # The purpose of this Module is to check the Permissions of a given PTE
   9 # against the requested access permissions.
  10 # This module will either validate (by setting the valid bit HIGH) the request
  11 # or find a permission fault and invalidate (by setting the valid bit LOW)
  12 # the request
  13 class PermissionValidator():
  14     def __init__(self, data_size):
  15         # Input
  16         self.data = Signal(data_size);
  17         self.xwr = Signal(3) # Execute, Write, Read
  18         self.super = Signal(1) # Supervisor Mode
  19         self.super_access = Signal(1) # Supervisor Access
  20         self.asid = Signal(15) # Address Space IDentifier (ASID)
  21
  22         # Output
  23         self.valid = Signal(1) # Denotes if the permissions are correct
  24
  25     def elaborate(self, platform):
  26         m = Module()
  27         m.d.comb += [
  28             # Check if ASID matches OR entry is global
  29             If(data[64:78] == self.asid or data[5] == 1,
  30                # Check Execute, Write, Read (XWR) Permissions
  31                If(data[3] == self.xwr[2] and data[2] == self.xwr[1] \
  32                   and data[1] == self.xwr[0],
  33                   # Check if supervisor
  34                   If(self.super == 1,
  35                      # Check if entry is in user mode
  36                      # Check if supervisor has access
  37                      If(data[4] == 0,
  38                         self.valid.eq(1)
  39                      ).Elif(self.super_access == 1,
  40                         self.valid.eq(1)
  41                      ).Else(
  42                         self.valid.eq(0)
  43                      )
  44                   ).Else(
  45                       # Check if entry is in user mode
  46                       If(data[4] == 1,
  47                          self.valid.eq(1)
  48                       ).Else(
  49                          self.valid.eq(0)
  50                       )
  51                   )
  52                ).Else(
  53                    self.valid.eq(0)
  54                )
  55             ).Else(
  56                 self.valid.eq(0)
  57             )
  58         ]