minor fixes

Reference: /n/patches.lsub.org/patch/vboxmmu
Date: Sat May 26 12:47:37 CES 2012
Signed-off-by: paurea@lsub.org

--- /sys/src/nix/k10/mmu.c	Fri May 11 12:10:06 2012
+++ /sys/src/nix/k10/mmu.c	Sat May 26 12:44:47 2012
@@ -22,10 +22,15 @@
 void
 mmuflushtlb(u64int)
 {
+	PTE *pte;
+	int i;
 
 	m->tlbpurge++;
 	if(m->pml4->daddr){
-		memset(UINT2PTR(m->pml4->va), 0, m->pml4->daddr*sizeof(PTE));
+		pte = UINT2PTR(m->pml4->va);
+		for(i = 0; i < m->pml4->daddr; i++)
+			if(pte[i] & PteP)
+				pte[i] = 0;
 		m->pml4->daddr = 0;
 	}
 	cr3put(m->pml4->pa);