Change in coreboot[master]: x86/lapic/secondary.S: Align stack for _secondary_start

[Marshall Dawson (Code Review)]

Marshall Dawson has uploaded this change for review. ( https://review.coreboot.org/20537

Change subject: x86/lapic/secondary.S: Align stack for _secondary_start
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x86/lapic/secondary.S: Align stack for _secondary_start

At a process _start, the stack is expected to be aligned to a
16-byte boundary.  Upon entry to any function the stack frame
must have the end of any arguments also aligned.  In other words
the value of %esp+4 or %rsp+8 is always a multiple of 16 (1).

Align the stack down inside _secondary_start and preserve proper
alignment for the call to secondary_cpu_init.

Although 4-byte alignment is the minimum requirement for i386,
some AMD platforms use SSE instructions which expect 16-byte.

1) http://wiki.osdev.org/System_V_ABI
   See "Initial Stack and Register State" and "The Stack Frame"
   in the supplements.

BUG=chrome-os-partner:62841664

Change-Id: I72b7a474013e5caf67aedfabeb8d8d2553499b73
Signed-off-by: Marshall Dawson 
---
M src/cpu/x86/lapic/secondary.S
1 file changed, 4 insertions(+), 1 deletion(-)

git pull ssh://review.coreboot.org:29418/coreboot refs/changes/37/20537/1

diff --git a/src/cpu/x86/lapic/secondary.S b/src/cpu/x86/lapic/secondary.S
index 0c4c0d0..1bb957d 100644
--- a/src/cpu/x86/lapic/secondary.S
+++ b/src/cpu/x86/lapic/secondary.S
@@ -55,7 +55,6 @@
    ljmpl	$0x10, $__ap_protected_start

__ap_protected_start:
-
    movw	$0x18, %ax
    movw	%ax, %ds
    movw	%ax, %es
@@ -69,6 +68,10 @@
    /* Set the stack pointer, and flag that we are done */
    xorl	%eax, %eax
    movl	secondary_stack, %esp
+	andl	$0xfffffff0, %esp
+	pushl	$0x0 /* maintain 16-byte alignment for the call below */
+	pushl	$0x0
+	pushl	$0x0
    movl	secondary_cpu_index, %edi
    pushl	%edi
    movl	%eax, secondary_stack