clean up handling of vmap 0

Reference: /n/atom/patch/applied/vmapzero
Date: Tue Jan 21 17:24:43 CET 2014
Signed-off-by: quanstro@quanstro.net

--- /sys/src/nix/k10/mmu.c	Tue Jan 21 17:24:39 2014
+++ /sys/src/nix/k10/mmu.c	Tue Jan 21 17:24:40 2014
@@ -606,6 +606,9 @@
 	pa -= o;
 	sz = ROUNDUP(size+o, PGSZ);
 
+	if(pa+sz == 0 || pa >= -KSEG2)
+		return nil;
+
 	/*
 	 * This is incomplete; the checks are not comprehensive
 	 * enough.  Assume requests for low memory are already mapped.
@@ -614,7 +617,6 @@
 		return KADDR(pa+o);
 	if(pa < 1ull*MiB)
 		return nil;
-
 	/*
 	 * only adralloc the actual request.  pci bars can be less than 1 page.
 	 * take it on faith that they don't overlap.
@@ -662,26 +664,29 @@
  */
 typedef struct Remap Remap;
 struct Remap {
-	uintmem	map[150];
+	uintmem	map[300];
 	usize	n;
 };
 static	Remap	smap;
 
 void*
-vmapoverlap(uintmem pa0, usize size)
+vmapoverlap(uintmem pa0, usize sz)
 {
 	int i, o;
 	uintmem pa, p;
 
 	o = pa0 - (pa0 & ~(PGSZ-1));
 	pa = pa0 - o;
-	size = ROUNDUP(size+o, PGSZ);
+	sz = ROUNDUP(sz+o, PGSZ);
+
+	if(pa+sz == 0 || pa >= -KSEG2)
+		return nil;
 
-	if(pa+size < 1ull*MiB)
+	if(pa+sz < 1ull*MiB)
 		return KADDR(pa);
 	if(pa < 1ull*MiB)
 		return nil;
-	for(p = pa; p < pa+size; p += PGSZ){
+	for(p = pa; p < pa+sz; p += PGSZ){
 		for(i = 0; i < smap.n; i++)
 			if(p == smap.map[i])
 				break;