cpu/x86/64bit/mode_switch: Work around FSP bug

FSP, that is build against EDK2 2018 or newer, is able to back up and
restore the bootloader IDT on entry/exit. Even though it sets up its
own IDT, FSP checks the bootloader IDT size and deadloops without
warning if it's too big.

On x86_64 coreboot the IDT is naturally bigger than on x86_32 and thus
x86_32 FSP might die on entry. Work around this issue by:

* Back up and restore the IDT in protected_mode_call_wrapper
* Load zero IDT in protected mode before jumping to function

TEST: Can boot on SPR FSP (x86_32) using x86_64 coreboot with
      exceptions in romstage enabled.

Change-Id: I56367d8153aa10a9b1bcaa5ffde8ebe202e8c00c
Signed-off-by: Patrick Rudolph <patrick.rudolph@9elements.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/85789
Reviewed-by: Shuo Liu <shuo.liu@intel.com>
Reviewed-by: Jérémy Compostella <jeremy.compostella@intel.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Maximilian Brune <maximilian.brune@9elements.com>

src/cpu/x86/64bit/mode_switch.S

@@ -19,12 +19,22 @@ protected_mode_call_wrapper:
 	movl	%gs, %eax
 	push	%rax
 
+	/* Backup IDT to stack */
+	sub	$16, %rsp
+	sidt	(%rsp)
+
 	/* Store stack pointer */
 	mov	%rsp, %rbp
 
-	/* Align stack and make space for arguments */
+	/* New IDT to stack */
+	pushq	$0
+	pushq	$0
+
+	/* Align stack */
 	movabs	$0xfffffffffffffff0, %rax
 	andq	%rax, %rsp
+
+	/* Make room for arguments on stack */
 	sub	$16, %rsp
 
 	/* Arguments to stack */
@@ -36,6 +46,9 @@ protected_mode_call_wrapper:
 	/* Drop to protected mode */
 	#include <cpu/x86/64bit/exit32.inc>
 
+	/* Load zero IDT. x86_32 FSP doesn't like to find a x86_64 IDT */
+	lidt	-16(%ebp)
+
 	/* Fetch function to call */
 	movl	12(%esp), %ebx
 
@@ -52,6 +65,10 @@ protected_mode_call_wrapper:
 	/* Restore stack pointer */
 	mov	%rbp, %rsp
 
+	/* Restore IDT */
+	lidt	(%rsp)
+	add	$16, %rsp
+
 	/* Restore registers */
 	pop	%rbx
 	movl	%ebx, %gs