x86_64 assembly: Don't touch %gs

With CPU_INFO_V2 enabled %gs holds the pointer to the cpu_info struct,
so don't clobber it. Backup and restore %gs where possible.
Fixes a crash in MPinit seen after calling FSP-S.

Change-Id: If9fc999b34530de5d8b6ad27b9af25fc552e9420
Signed-off-by: Patrick Rudolph <patrick.rudolph@9elements.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/59764
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>

src/cpu/x86/64bit/exit32.inc

@@ -68,7 +68,6 @@ __longmode_compatibility:
 	movl	%eax, %es
 	movl	%eax, %ss
 	movl	%eax, %fs
-	movl	%eax, %gs
 
 	/* Disable paging. */
 	movl	%cr0, %eax

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

@@ -15,6 +15,10 @@ protected_mode_call_narg:
 	push	%r14
 	push	%r15
 
+	/* Backup gs to stack */
+	movl	%gs, %eax
+	push	%rax
+
 	/* Arguments to stack */
 	push	%rdi
 	push	%rsi
@@ -23,9 +27,9 @@ protected_mode_call_narg:
 
 	#include <cpu/x86/64bit/exit32.inc>
 
-	movl	-48(%ebp), %eax	/* Argument count */
-	movl	-64(%ebp), %edx	/* Argument 0 */
-	movl	-72(%ebp), %ecx	/* Argument 1 */
+	movl	-56(%ebp), %eax	/* Argument count */
+	movl	-72(%ebp), %edx	/* Argument 0 */
+	movl	-80(%ebp), %ecx	/* Argument 1 */
 
 	/* Align the stack */
 	andl	$0xFFFFFFF0, %esp
@@ -46,7 +50,7 @@ protected_mode_call_narg:
 	pushl	%edx	/* Argument 0 */
 
 1:
-	movl	-56(%ebp), %ebx	/* Function to call */
+	movl	-64(%ebp), %ebx	/* Function to call */
 	call	*%ebx
 	movl	%eax, %ebx
 
@@ -57,6 +61,8 @@ protected_mode_call_narg:
 	movl	%ebx, %eax
 
 	/* Restore registers */
+	mov	-48(%rbp), %rbx
+	movl	%ebx, %gs
 	mov	-40(%rbp), %r15
 	mov	-32(%rbp), %r14
 	mov	-24(%rbp), %r13

src/security/intel/txt/getsec_enteraccs.S

@@ -227,6 +227,10 @@ cond_clear_var_mtrrs:
 	movd	%esp, %xmm0
 	movd	%ebp, %xmm1
 
+	/* Backup %gs used by CPU_INFO_V2 */
+	movl	%gs, %eax
+	movd	%eax, %xmm2
+
 	/*
 	 * Get function arguments.
 	 * It's important to pass the exact ACM size as it's used by getsec to verify
@@ -261,6 +265,8 @@ cond_clear_var_mtrrs:
 	movl	%eax, %es
 	movl	%eax, %ss
 	movl	%eax, %fs
+	/* Restore %gs used by CPU_INFO_V2 */
+	movd	%xmm2, %eax
 	movl	%eax, %gs
 
 	/* Disable cache */