Rearrange endpoint number layout

The constants ANY, NONE, and SELF are now a function of the way the
endpoint number is split between a generation number and a process
slot number, rather than the other way around.  This allows for the
use of bit masking and shifting instead of the previous (and more
expensive) multiplication and division.

Change-Id: Id890eea74435444128c75eb0c89816b948f43c0b

include/lib.h

@@ -18,7 +18,7 @@
 #include <minix/com.h>
 #include <minix/type.h>
 #include <minix/callnr.h>
-
+#include <minix/endpoint.h>
 #include <minix/ipc.h>
 
 struct ps_strings; /* forward declaration for minix_stack_fill. */

include/minix/com.h

@@ -35,28 +35,10 @@
 #ifndef _MINIX_COM_H
 #define _MINIX_COM_H 
 
-/*===========================================================================*
- *          	    		Magic process numbers			     *
- *===========================================================================*/
-
-/* These may not be any valid endpoint (see <minix/endpoint.h>). */
-#define ANY	((endpoint_t) 0x7ace)	/* used to indicate 'any process' */
-#define NONE 	((endpoint_t) 0x6ace)   /* used to indicate 'no process at all' */
-#define SELF	((endpoint_t) 0x8ace) 	/* used to indicate 'own process' */
-#define _MAX_MAGIC_PROC (SELF)	/* used by <minix/endpoint.h> 
-				   to determine generation size */
-
 /*===========================================================================*
  *            	Process numbers of processes in the system image	     *
  *===========================================================================*/
 
-/* The values of several task numbers depend on whether they or other tasks
- * are enabled. They are defined as (PREVIOUS_TASK - ENABLE_TASK) in general.
- * ENABLE_TASK is either 0 or 1, so a task either gets a new number, or gets
- * the same number as the previous task and is further unused. Note that the
- * order should correspond to the order in the task table defined in table.c. 
- */
-
 /* Kernel tasks. These all run in the same address space. */
 #define ASYNCM	((endpoint_t) -5) /* notifies about finished async sends */
 #define IDLE    ((endpoint_t) -4) /* runs when no one else can run */

include/minix/endpoint.h

@@ -7,22 +7,65 @@
 #include <limits.h>
 #include <minix/type.h>
 
-/* The point of the padding in 'generation size' is to 
- * allow for certain bogus endpoint numbers such as NONE, ANY, etc.
+/*
+ * Endpoints are split into two parts: a process slot and a generation number.
  *
- * The _MAX_MAGIC_PROC is defined by <minix/com.h>. That include
- * file defines some magic process numbers such as ANY and NONE,
- * and must never be a valid endpoint number. Therefore we make sure
- * the generation size is big enough to start the next generation
- * above the highest magic number.
+ * The process slot number identifies the slot in various process tables.
+ * It is positive or zero for user processes, and negative for kernel tasks.
+ * Constants dictate that with the current endpoint layout, the maximum range
+ * of process slot numbers is [-MAX_NR_TASKS,MAX_NR_PROCS>.  The used part of
+ * the range is currently [-NR_TASKS,NR_PROCS> -- these two constants may be
+ * changed within the maximum range without changing the endpoint layout.
+ *
+ * The generation number is a per-slot number that gets increased by one every
+ * time a slot is reused for a new process.  The generation number minimizes
+ * the chance that the endpoint of a dead process can (accidentially) be used
+ * to communicate with a different, live process.  Preventing such accidents
+ * is essential for proper system service restartability support.
+ *
+ * The split between the two parts of the endpoint is such that when the
+ * generation number is zero, the endpoint number equals the process slot
+ * number, even for negative task numbers.  This is required for the endpoint
+ * numbers hardcoded in <minix/com.h>, and it makes endpoint numbers easy to
+ * read in general.
+ *
+ * There are three special endpoint numbers: ANY, NONE, and SELF.  These
+ * numbers are used to identify "any process", "no process at all", and
+ * "own process", respectively.  They fall outside the normal range of
+ * process slot numbers, and are always of generation zero.
  */
-#define _ENDPOINT_GENERATION_SIZE (MAX_NR_TASKS+_MAX_MAGIC_PROC+1)
-#define _ENDPOINT_MAX_GENERATION  (INT_MAX/_ENDPOINT_GENERATION_SIZE-1)
+
+/*
+ * The following constant defines the split between the two parts of the
+ * endpoint numbers.  It can be adjusted to allow for either more processes
+ * or more per-process generation numbers.  Changing it will change the
+ * endpoint number layout, and thus break binary compatibility with existing
+ * processes.
+ */
+#define _ENDPOINT_GENERATION_SHIFT	15
+
+/* Derived constants. */
+#define _ENDPOINT_GENERATION_SIZE	(1 << _ENDPOINT_GENERATION_SHIFT)
+/* INT_MAX is used here to prevent signedness issues with the macros below. */
+#define _ENDPOINT_MAX_GENERATION	(INT_MAX/_ENDPOINT_GENERATION_SIZE-1)
+#define _ENDPOINT_SLOT_TOP	(_ENDPOINT_GENERATION_SIZE-MAX_NR_TASKS)
+
+/* The special endpoint numbers, and the resulting maximum slot number. */
+#define ANY		((endpoint_t) (_ENDPOINT_SLOT_TOP - 1))
+#define NONE		((endpoint_t) (_ENDPOINT_SLOT_TOP - 2))
+#define SELF		((endpoint_t) (_ENDPOINT_SLOT_TOP - 3))
+#define MAX_NR_PROCS		      (_ENDPOINT_SLOT_TOP - 3)	/* (int)SELF */
+
+/* Sanity check. */
+#if NR_PROCS > MAX_NR_PROCS
+#error "NR_PROCS exceeds MAX_NR_PROCS, increase _ENDPOINT_GENERATION_SHIFT"
+#endif
 
 /* Generation + Process slot number <-> endpoint. */
-#define _ENDPOINT(g, p) ((endpoint_t)((g) * _ENDPOINT_GENERATION_SIZE + (p)))
-#define _ENDPOINT_G(e) (((e)+MAX_NR_TASKS) / _ENDPOINT_GENERATION_SIZE)
+#define _ENDPOINT(g, p) \
+	((endpoint_t)(((g) << _ENDPOINT_GENERATION_SHIFT) + (p)))
+#define _ENDPOINT_G(e) (((e)+MAX_NR_TASKS) >> _ENDPOINT_GENERATION_SHIFT)
 #define _ENDPOINT_P(e) \
-	((((e)+MAX_NR_TASKS) % _ENDPOINT_GENERATION_SIZE) - MAX_NR_TASKS)
+	((((e)+MAX_NR_TASKS) & (_ENDPOINT_GENERATION_SIZE - 1)) - MAX_NR_TASKS)
 
 #endif

test/test39.c

@@ -23,7 +23,7 @@ void test39a()
   subtest = 1;
 
   for (g = 0; g <= _ENDPOINT_MAX_GENERATION; g++) {
-	for (p = -MAX_NR_TASKS; p < _NR_PROCS; p++) {
+	for (p = -MAX_NR_TASKS; p < MAX_NR_PROCS; p++) {
 		endpoint_t ept;
 		int mg, mp;
 		ept = _ENDPOINT(g, p);