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pc64: disable interrupts in mmuwalk() for checkmmu()

we have to disable interrupts during mmuwalk() of user pages
as we can get preempted during mmu walk and the original
m->pml4 might become one of a different process.

--- a/sys/src/9/pc64/mmu.c

+++ b/sys/src/9/pc64/mmu.c

@@ -517,12 +517,17 @@

 void

 checkmmu(uintptr va, uintptr pa)

 {

-	uintptr *pte;

+	uintptr *pte, old;

+	int x;

+	x = splhi();

 	pte = mmuwalk(m->pml4, va, 0, 0);

-	if(pte != 0 && (*pte & PTEVALID) != 0 && PPN(*pte) != pa)

-		print("%ld %s: va=%#p pa=%#p pte=%#p\n",

-			up->pid, up->text, va, pa, *pte);

+	if(pte == 0 || ((old = *pte) & PTEVALID) == 0 || PPN(old) == pa){

+		splx(x);

+		return;

+	}

+	splx(x);

+	print("%ld %s: va=%#p pa=%#p pte=%#p\n", up->pid, up->text, va, pa, old);

 }

 uintptr