pc/l.s: load virtual gdt pointer after mmu switch

before removing the double map at 0, load our
initial gdt pointer with its new KZERO based
virtual address.

this is prerequisite for handling traps early during
bootup before mmuinit() loads the final gdt.

sys/src/9/pc/l.s

@@ -71,8 +71,7 @@ TEXT _multibootentry(SB), $0
 	STD
 	REP; MOVSB
 	MOVL	BX, multibootptr-KZERO(SB)
-	MOVL	$_startPADDR(SB), AX
-	ANDL	$~KZERO, AX
+	MOVL	$_startPADDR-KZERO(SB), AX
 	JMP*	AX
 
 /* multiboot structure pointer (physical address) */
@@ -98,8 +97,7 @@ TEXT _startPADDR(SB), $0
 	CLI					/* make sure interrupts are off */
 
 	/* set up the gdt so we have sane plan 9 style gdts. */
-	MOVL	$tgdtptr(SB), AX
-	ANDL	$~KZERO, AX
+	MOVL	$tgdtptr-KZERO(SB), AX
 	MOVL	(AX), GDTR
 	MOVW	$1, AX
 	MOVW	AX, MSW
@@ -158,9 +156,11 @@ TEXT m0idtptr(SB), $0
 	WORD $(256*8-1)
 	LONG $IDTADDR
 
-TEXT mode32bit(SB), $0
-	/* At this point, the GDT setup is done. */
+TEXT vtgdtptr(SB), $0
+	WORD	$(3*8)
+	LONG	$tgdt(SB)
 
+TEXT mode32bit(SB), $0
 	MOVL	$((CPU0END-CPU0PDB)>>2), CX
 	MOVL	$PADDR(CPU0PDB), DI
 	XORL	AX, AX
@@ -231,6 +231,9 @@ _setpte:
  * be initialised here.
  */
 TEXT _startpg(SB), $0
+	MOVL	$vtgdtptr(SB), AX
+	MOVL	(AX), GDTR
+
 	MOVL	$0, (PDO(0))(CX)		/* undo double-map of KZERO at 0 */
 	MOVL	CX, CR3				/* load and flush the mmu */