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+                 Information about /proc/ppc_htab
+=====================================================================
+
+This document and the related code was written by me (Cort Dougan), please
+email me (cort@fsmlabs.com) if you have questions, comments or corrections.
+
+Last Change: 2.16.98
+
+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
+
+  The PMC1 and PMC2 counters can overflow and give no indication of that
+  in /proc/ppc_htab.