When kernel remaps to high memory in amd64 locore, the GDT used before

becomes invalid. As such, split it in two parts, one for use when system
boots in low memory, and one for use when it jumps to high memory.

sys/arch/amd64/amd64/locore.S

@@ -1,4 +1,4 @@
-/*	$NetBSD: locore.S,v 1.55 2009/11/27 03:23:04 rmind Exp $	*/
+/*	$NetBSD: locore.S,v 1.56 2010/04/18 15:24:54 jym Exp $	*/
 
 /*
  * Copyright-o-rama!
@@ -269,13 +269,25 @@ _C_LABEL(biosextmem):	.long	REALEXTMEM
 #define	RELOC(x)	_RELOC(_C_LABEL(x))
 
 #ifndef XEN
-	.globl	gdt64
+	.globl	gdt64_lo
+	.globl	gdt64_hi
 
-gdt64:
-	.word	gdt64_end-gdt64_start
+#define GDT64_LIMIT gdt64_end-gdt64_start-1
+
+/* Temporary gdt64, with base address in low memory */
+gdt64_lo:
+	.word	GDT64_LIMIT
 	.quad	_RELOC(gdt64_start)
 .align 64
 
+/* Temporary gdt64, with base address in high memory */
+gdt64_hi:
+	.word	GDT64_LIMIT
+	.quad	gdt64_start
+.align 64
+
+#undef GDT64_LIMIT
+
 gdt64_start:
 	.quad 0x0000000000000000	/* always empty */
 	.quad 0x00af9a000000ffff	/* kernel CS */
@@ -638,7 +650,7 @@ compat:
 	 * in it to do that.
 	 */
 
-	movl	$RELOC(gdt64),%eax
+	movl	$RELOC(gdt64_lo),%eax
 	lgdt	(%eax)
 	movl	$RELOC(farjmp64),%eax
 	ljmp	*(%eax)
@@ -654,9 +666,17 @@ longmode:
 	 */
 	movabsq	$longmode_hi,%rax
 	jmp	*%rax
+
 longmode_hi:
+
+	/*
+	 * We left the identity mapped area. Base address of
+	 * the temporary gdt64 should now be in high memory.
+	 */
+	movq	$RELOC(gdt64_hi),%rax
+	lgdt	(%rax)
+
 	/*
-	 * We have arrived.
 	 * There's no need anymore for the identity mapping in low
 	 * memory, remove it.
 	 */