arm64: Seed the stack at stage_entry

Seed the stack in order to avoid boot process from complaining false stack
overflow.

BUG=chrome-os-partner:30824
BRANCH=None
TEST=Compiles successfully for rush and stack overflow error fixed in boot flow

Change-Id: I5d29d24eb5270d38a35a32171881b1aab8bf32e5
Signed-off-by: Patrick Georgi <pgeorgi@chromium.org>
Original-Commit-Id: 26e53568e82ad8418c20c2410f0cbc5c444c9917
Original-Change-Id: Ie51e1bcd263e3b886feb2e0e9c7d544f23c3444e
Original-Signed-off-by: Furquan Shaikh <furquan@google.com>
Original-Reviewed-on: https://chromium-review.googlesource.com/210594
Original-Tested-by: Furquan Shaikh <furquan@chromium.org>
Original-Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Original-Commit-Queue: Furquan Shaikh <furquan@chromium.org>
Reviewed-on: http://review.coreboot.org/8942
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>

src/arch/arm64/stage_entry.S

@@ -20,6 +20,29 @@
 
 #include <arch/asm.h>
 
+ENTRY(seed_stack)
+	/*
+	 * Initialize the stack to a known value. This is used to check for
+	 * stack overflow later in the boot process.
+	 */
+	ldr	x0, .stack_bottom
+	ldr	x1, .stack_top
+	ldr	x2, =0xdeadbeefdeadbeef
+	ldr	x3, =0x8
+
+init_stack_loop:
+	str	x2, [x0]
+	add	x0, x0, x3
+	cmp	x0, x1
+	bne	init_stack_loop
+
+load_stack:
+	b	arm64_init
+	.align	4
+	.stack_bottom:
+		.quad _stack
+ENDPROC(seed_stack)
+
 ENTRY(arm64_el3_startup)
 	/* Set all bits in SCTLR_EL3 to 0 except RES1 and RES0 */
 	mrs     x0, SCTLR_EL3
@@ -37,7 +60,7 @@ ENTRY(arm64_el3_startup)
 	isb
 
 	/* Load up the stack if non-zero. */
-	ldr	x0, .stack
+	ldr	x0, .stack_top
 	cmp	x0, #0
 	b.eq	1f
 	mov	sp, x0
@@ -58,10 +81,10 @@ ENTRY(arm64_el3_startup)
 	 * to the Kconfig option for cpu0. However, this code can be relocated
 	 * and reused to start up secondary cpus.
 	 */
-	.stack:
+	.stack_top:
 		.quad _estack
 	.entry:
-		.quad arm64_init
+		.quad seed_stack
 ENDPROC(arm64_el3_startup)
 .global arm64_el3_startup_end
 arm64_el3_startup_end: