[S] Change in coreboot[main]: cpu/x86/64bit/mode_switch: Add workaround for FSP bug

[Patrick Rudolph (Code Review)]

Patrick Rudolph has uploaded this change for review. ( https://review.coreboot.org/c/coreboot/+/85789?usp=email )

Change subject: cpu/x86/64bit/mode_switch: Add workaround for FSP bug
......................................................................

cpu/x86/64bit/mode_switch: Add workaround for FSP bug

FSP is able to backup and restore the bootloader IDT on entry/exit.
Even though it sets up it's 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
FSP might die on entry. To workaround this issue do:

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

Change-Id: I56367d8153aa10a9b1bcaa5ffde8ebe202e8c00c
Signed-off-by: Patrick Rudolph 
---
M src/cpu/x86/64bit/mode_switch.S
1 file changed, 19 insertions(+), 2 deletions(-)

git pull ssh://review.coreboot.org:29418/coreboot refs/changes/89/85789/1

diff --git a/src/cpu/x86/64bit/mode_switch.S b/src/cpu/x86/64bit/mode_switch.S
index 9555cef..71d47ef 100644
--- a/src/cpu/x86/64bit/mode_switch.S
+++ b/src/cpu/x86/64bit/mode_switch.S
@@ -19,15 +19,25 @@
    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 */
+	/* Align stack */
    movabs	$0xfffffffffffffff0, %rax
    andq	%rax, %rsp
+
+	/* New IDT to stack */
+	push	$0
+	push	$0
+
+	/* Make room for arguments on stack */
    sub	$16, %rsp

-	/* Arguments to stack */
+	/* Protected mode arguments to stack */
    movl	%edi, 12(%rsp)
    movl	%esi, 0(%rsp)
    movl	%edx, 4(%rsp)
@@ -36,6 +46,9 @@
    /* 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(%esp)
+
    /* Fetch function to call */
    movl	12(%esp), %ebx

@@ -52,6 +65,10 @@
    /* Restore stack pointer */
    mov	%rbp, %rsp

+	/* Restore IDT */
+	lidt	(%rsp)
+	add	$16, %rsp
+
    /* Restore registers */
    pop	%rbx
    movl	%ebx, %gs