                 Information about /proc/ppc_htab
=====================================================================

This document and the related code was written by me (Cort Dougan), please
email me (cort@cs.nmt.edu) if you have questions, comments or corrections.

This entry in the proc directory is readable by all users but only
writable by root.

The ppc_htab interface is a user level way of accessing the
performance monitoring registers as well as providing information
about the PTE hash table.

1. Reading

  Reading this file will give you information about the memory management
  hash table that serves as an extended tlb for page translation on the
  powerpc.  It will also give you information about performance measurement
  specific to the cpu that you are using.

  Explanation of the 604 Performance Monitoring Fields:
    MMCR0 - the current value of the MMCR0 register
    PMC1
    PMC2 - the value of the performance counters and a
           description of what events they are counting
           which are based on MMCR0 bit settings.
  Explanation of the PTE Hash Table fields:

    Size - hash table size in Kb.
    Buckets -  number of buckets in the table.
    Address - the virtual kernel address of the hash table base.
    Entries - the number of ptes that can be stored in the hash table.
    User/Kernel - how many pte's are in use by the kernel or user at that time.
    Overflows - How many of the entries are in their secondary hash location.
    Percent full - ratio of free pte entries to in use entries.
    Reloads - Count of how many hash table misses have occurred
              that were fixed with a reload from the linux tables.
              Should always be 0 on 603 based machines.
    Non-error Misses - Count of how many hash table misses have occurred
              that were completed with the creation of a pte in the linux
              tables with a call to do_page_fault().
    Error Misses - Number of misses due to errors such as bad address
              and permission violations.  This includes kernel access of
              bad user addresses that are fixed up by the trap handler.

  Note that calculation of the data displayed from /proc/ppc_htab takes
  a long time and spends a great deal of time in the kernel.  It would
  be quite hard on performance to read this file constantly.  In time
  there may be a counter in the kernel that allows successive reads from
  this file only after a given amount of time has passed to reduce the
  possibility of a user slowing the system by reading this file.

2. Writing

  Writing to the ppc_htab allows you to change the characteristics of
  the powerpc PTE hash table and setup performance monitoring.

  Resizing the PTE hash table is not enabled right now due to many
  complications with moving the hash table, rehashing the entries
  and many many SMP issues that would have to be dealt with.

  Write options to ppc_htab:
  
   - To set the size of the hash table to 64Kb:

      echo 'size 64' > /proc/ppc_htab

     The size must be a multiple of 64 and must be greater than or equal to
     64.

   - To turn off performance monitoring:

      echo 'off' > /proc/ppc_htab

   - To reset the counters without changing what they're counting:

      echo 'reset' > /proc/ppc_htab

     Note that counting will continue after the reset if it is enabled.

   - To count only events in user mode or only in kernel mode:

      echo 'user' > /proc/ppc_htab
       ...or...
      echo 'kernel' > /proc/ppc_htab

     Note that these two options are exclusive of one another and the
     lack of either of these options counts user and kernel.
     Using 'reset' and 'off' reset these flags.

   - The 604 has 2 performance counters which can each count events from
     a specific set of events.  These sets are disjoint so it is not
     possible to count _any_ combination of 2 events.  One event can
     be counted by PMC1 and one by PMC2.

     To start counting a particular event use:

      echo 'event' > /proc/ppc_htab

     and choose from these events:

     PMC1
     ----
      'ic miss' - instruction cache misses
      'dtlb' - data tlb misses (not hash table misses)

     PMC2
     ----
      'dc miss' - data cache misses
      'itlb' - instruction tlb misses (not hash table misses)
      'load miss time' - cycles to complete a load miss

3. Bugs

  Doing a 'less' or 'more' on ppc_htab results in a segmentation violation.
  I'm not sure of the cause but in the mean time 'cat' works adequately for
  reading the file.

  The PMC1 and PMC2 counters can overflow and give no indication of that
  in /proc/ppc_htab.

