Commit 9751d762 authored Jun 21, 2012 by H. Peter Anvin

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

parent 2b1b712f

arch/x86/realmode/rm/reboot.S

+6 −2

Original line number	Diff line number	Diff line
		@@ -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