PERFORCE change 31101 for review

[Marcel Moolenaar]

http://perforce.freebsd.org/chv.cgi?CH=31101

Change 31101 by marcel at marcel_nfs on 2003/05/13 16:17:14

	Don't create a RID map that covers the full range imposed
	by the number of bits implemented by the processor for the
	region ID. We only need 19 bits to cover 5 regions per
	process (5*PID_MAX=0x7a11b). Since the minimum number of
	bits guaranteed to be implemented is 18, we either need a
	32KB map (16-bit RID) or otherwise a 64KB map (all other
	cases). Don't create a 2MB map simply because the processor
	has 24-bits in the region ID...

Affected files ...

.. //depot/projects/ia64_epc/sys/ia64/ia64/pmap.c#14 edit

Differences ...

==== //depot/projects/ia64_epc/sys/ia64/ia64/pmap.c#14 (text+ko) ====

@@ -329,6 +329,24 @@
 
 	/*
 	 * Setup RIDs. RIDs 0..7 are reserved for the kernel.
+	 *
+	 * We currently need at least 19 bits in the RID because PID_MAX
+	 * can only be encoded in 17 bits and we need RIDs for 5 regions
+	 * per process. With PID_MAX equalling 99999 this means that we
+	 * need to be able to encode 499995 (=5*PID_MAX).
+	 * The Itanium processor only has 18 bits and the architected
+	 * minimum is exactly that. So, we cannot use a PID based scheme
+	 * in those cases. Enter pmap_ridmap...
+	 * We should avoid the map when running on a processor that has
+	 * implemented enough bits. This means that we should pass the
+	 * process/thread ID to pmap. This we currently don't do, so we
+	 * use the map anyway. However, we don't want to allocate a map
+	 * that is large enough to cover the range dictated by the number
+	 * of bits in the RID, because that may result in a RID map of
+	 * 2MB in size for a 24-bit RID. A 64KB map is enough.
+	 * The bottomline: we create a 32KB map when the processor only
+	 * implements 18 bits (or when we can't figure it out). Otherwise
+	 * we create a 64KB map.
 	 */
 	res = ia64_call_pal_static(PAL_VM_SUMMARY, 0, 0, 0);
 	if (res.pal_status != 0) {
@@ -339,14 +357,11 @@
 		ridbits = (res.pal_result[1] >> 8) & 0xff;
 		if (bootverbose)
 			printf("Processor supports %d Region ID bits\n",
-			       ridbits);
+			    ridbits);
 	}
+	if (ridbits > 19)
+		ridbits = 19;
 
-	/*
-	 * XXX: If we have enough bits in the RID to use the thread ID then
-	 * we should avoid the overhead of the map and create unique RIDs
-	 * based on the thread ID.
-	 */
  	pmap_ridmax = (1 << ridbits);
 	pmap_ridmapsz = pmap_ridmax / 64;
 	pmap_ridmap = (u_int64_t *)pmap_steal_memory(pmap_ridmax / 8);