x86, oprom: ensure DF is always cleared

The Option ROM might mess with the EFLAGS register and break assumptions
the C part of coreboot implicitly has, e.g. the state of the direction
flag.

Prevent Option ROMs from confusing coreboot by restoring the old EFLAGS
value after the Option ROMs has finished and always clear the direction
flag before calling the C part of the interrupt handler.

Change-Id: I84663be6681b17f95f48d93f0b730e443336b4a8
Signed-off-by: Mathias Krause <minipli@googlemail.com>
Reviewed-on: http://review.coreboot.org/837
Tested-by: build bot (Jenkins)
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>

1 files changed, 37 insertions, 28 deletions

diff --git a/src/devices/oprom/x86_asm.S b/src/devices/oprom/x86_asm.S
index 488bfa6b91..56ebb3a885 100644
--- a/src/devices/oprom/x86_asm.S
+++ b/src/devices/oprom/x86_asm.S
@@ -75,35 +75,36 @@ __buffer = RELOCATED(.)
 	.globl __realmode_call
 __realmode_call = RELOCATED(.)
 	/* save all registers to the stack */
-	pushal
+	pusha
+	pushf
 
-	/* Move the protected mode stack to a safe place */
+	/* Move the protected mode stack pointer to a safe place */
 	movl	%esp, __stack
-	movl    %esp, %ebp
+	movl	%esp, %ebp
 
-	/* This function is called with regparm=0 and we have
-	 * to skip the 32 byte from pushal. Hence start at 36.
+	/* This function is called with regparm=0 and we have to
+	 * skip the 36 byte from pushf+pusha. Hence start at 40.
 	 */
 
 	/* entry point */
-	movl    36(%ebp), %eax
+	movl	40(%ebp), %eax
 	mov	%ax, __lcall_instr + 1
 	andl	$0xffff0000, %eax
 	shrl	$4, %eax
 	mov	%ax, __lcall_instr + 3
 
 	/* initial register values */
-	movl	40(%ebp), %eax
-	movl	%eax, __registers +  0 /* eax */
 	movl	44(%ebp), %eax
-	movl	%eax, __registers +  4 /* ebx */
+	movl	%eax, __registers +  0 /* eax */
 	movl	48(%ebp), %eax
-	movl	%eax, __registers +  8 /* ecx */
+	movl	%eax, __registers +  4 /* ebx */
 	movl	52(%ebp), %eax
-	movl	%eax, __registers + 12 /* edx */
+	movl	%eax, __registers +  8 /* ecx */
 	movl	56(%ebp), %eax
-	movl	%eax, __registers + 16 /* esi */
+	movl	%eax, __registers + 12 /* edx */
 	movl	60(%ebp), %eax
+	movl	%eax, __registers + 16 /* esi */
+	movl	64(%ebp), %eax
 	movl	%eax, __registers + 20 /* edi */
 
 	/* Activate the right segment descriptor real mode. */
@@ -193,41 +194,45 @@ __lcall_instr = RELOCATED(.)
 	/* restore proper idt */
 	lidt	idtarg
 
-	/* and exit */
+	/* restore stack pointer, eflags and register values */
 	movl	__stack, %esp
-	popal
+	popf
+	popa
 
+	/* and exit */
 	// TODO return AX from OPROM call
 	ret
 
 	.globl __realmode_interrupt
 __realmode_interrupt = RELOCATED(.)
 	/* save all registers to the stack */
-	pushal
-	/* save the stack */
+	pusha
+	pushf
+
+	/* save the stack pointer */
 	movl	%esp, __stack
-	movl    %esp, %ebp
+	movl	%esp, %ebp
 
-	/* This function is called with regparm=0 and we have
-	 * to skip the 32 byte from pushal. Hence start at 36.
+	/* This function is called with regparm=0 and we have to
+	 * skip the 36 byte from pushf+pusha. Hence start at 40.
 	 */
 
 	/* prepare interrupt calling code */
-	movl    36(%ebp), %eax
+	movl	40(%ebp), %eax
 	movb	%al, __intXX_instr + 1 /* intno */
 
 	/* initial register values */
-	movl	40(%ebp), %eax
-	movl	%eax, __registers +  0 /* eax */
 	movl	44(%ebp), %eax
-	movl	%eax, __registers +  4 /* ebx */
+	movl	%eax, __registers +  0 /* eax */
 	movl	48(%ebp), %eax
-	movl	%eax, __registers +  8 /* ecx */
+	movl	%eax, __registers +  4 /* ebx */
 	movl	52(%ebp), %eax
-	movl	%eax, __registers + 12 /* edx */
+	movl	%eax, __registers +  8 /* ecx */
 	movl	56(%ebp), %eax
-	movl	%eax, __registers + 16 /* esi */
+	movl	%eax, __registers + 12 /* edx */
 	movl	60(%ebp), %eax
+	movl	%eax, __registers + 16 /* esi */
+	movl	64(%ebp), %eax
 	movl	%eax, __registers + 20 /* edi */
 
 	/*  This configures CS properly for real mode. */
@@ -309,9 +314,10 @@ __intXX_instr = RELOCATED(.)
 	/* restore coreboot's 32-bit IDT */
 	lidt	idtarg
 
-	/* Exit */
+	/* restore stack pointer, eflags and register values and exit */
 	movl	__stack, %esp
-	popal
+	popf
+	popa
 	ret
 
 /* This is the 16-bit interrupt entry point called by the IDT stub code.
@@ -327,6 +333,9 @@ __interrupt_handler_16bit = RELOCATED(.)
 	push	%fs
 	push	%gs
 
+	/* Clear DF to not break ABI assumptions */
+	cld
+
 	/* Clean up the interrupt number. We could have done this in the stub,
 	 * but it would have cost 2 more bytes per stub entry.
 	 */