x86-64, reboot: Be more paranoid in 64-bit reboot=bios

Be a bit more paranoid in the transition back to 16-bit mode.  In
particular, in case the kernel is residing above the 4 GiB mark,
switch to the trampoline GDT, and make the jump after turning off
paging a far jump.  In theory, none of this should matter, but it is
exactly the kind of things that broken SMM or virtualization software
could trip up on.
Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Link: http://lkml.kernel.org/r/tip-jopx7y6g6dbcx4tpal8q0jlr@git.kernel.org

arch/x86/realmode/rm/reboot.S

@@ -22,14 +22,18 @@
 ENTRY(machine_real_restart_asm)
 
 #ifdef CONFIG_X86_64
+	/* Switch to trampoline GDT as it is guaranteed < 4 GiB */
+	movl	$__KERNEL_DS, %eax
+	movl	%eax, %ds
+	lgdtl	pa_tr_gdt
 
 	/* Disable paging to drop us out of long mode */
 	movl	%cr0, %eax
 	andl	$~X86_CR0_PG, %eax
 	movl	%eax, %cr0
-	jmp	1f	/* "A branch" may be needed here, assume near is OK */
+	ljmpl	$__KERNEL32_CS, $pa_machine_real_restart_paging_off
 
-1:
+GLOBAL(machine_real_restart_paging_off)
 	xorl	%eax, %eax
 	xorl	%edx, %edx
 	movl	$MSR_EFER, %ecx