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             * valid HIGH when permissions are correct
  19         """
  20         # Input
  21         self.data = Signal(data_size);
  22         self.xwr = Signal(3) # Execute, Write, Read
  23         self.supermode = Signal(1) # Supervisor Mode
  24         self.super_access = Signal(1) # Supervisor Access
  25         self.asid = Signal(15) # Address Space IDentifier (ASID)
  26
  27         # Output
  28         self.valid = Signal(1) # Denotes if the permissions are correct
  29
  30     def elaborate(self, platform):
  31         m = Module()
  32         # ASID match or Global Permission
  33         with m.If(data[64:78] == self.asid | data[5]):
  34             # Check Execute, Write, Read (XWR) Permissions
  35             with m.If(data[3] == self.xwr[2] \
  36                       & data[2] == self.xwr[1] \
  37                       & data[1] == self.xwr[0]):
  38                 # Supervisor Logic
  39                 with m.If(self.supermode):
  40                     # Valid if entry is not in user mode or supervisor
  41                     # has Supervisor User Memory (SUM) access via the
  42                     # SUM bit in the sstatus register
  43                     m.comb += self.valid.eq(~data[4] | self.super_access)
  44                 # User logic
  45                 with m.Else():
  46                     # Valid if the entry is in user mode only
  47                     m.comb += self.valid.eq(data[4])
  48             with m.Else():
  49                 m.comb += self.valid.eq(0)
  50         with m.Else():
  51             m.comb += self.valid.eq(0)