This is the right thing (also done by head.s).

diff --exclude-from=/home/ldb/src/linux-exclude -urNd a/arch/i386/kernel/cpu/common.c b/arch/i386/kernel/cpu/common.c
--- a/arch/i386/kernel/cpu/common.c	2002-08-06 10:26:11.000000000 +0200
+++ b/arch/i386/kernel/cpu/common.c	2002-08-07 17:47:05.000000000 +0200
@@ -451,7 +451,7 @@
 	 */
 	if (cpu) {
 		memcpy(cpu_gdt_table[cpu], cpu_gdt_table[0], GDT_SIZE);
-		cpu_gdt_descr[cpu].size = GDT_SIZE;
+		cpu_gdt_descr[cpu].size = GDT_SIZE - 1;
 		cpu_gdt_descr[cpu].address = (unsigned long)cpu_gdt_table[cpu];
 	}