arch/arm64: Add an alternative entry point for ramstage code

This change prepares an alternative entry point for the ARM64
ramstage. It is written in assembly language, avoids the usage of the
stack, and overrides the program stack pointer (SP register) if the
`preram_stack` and `postram_stack` point to different addresses.

Previous Boot Flow:
 - header.ld -> jump into `stage_entry` C code for ROMSTAGE onwards ->
     `stage_entry` being called and followed by `main` function

Updated Boot Flow:
 - header.ld -> jump into `_start` (assembly entry point) for
      ramstage specifically -> Update the existing SP (stack pointer)
      register if the `preram_` or `postram_` stack address is not
      same -> call into `stage_entry` and follow the `main` function.

BUG=b:456953373
BRANCH=None
TEST=Able to build google/bluey.

Change-Id: I4eec24aff1c9d01180c3452a3631dd344656c771
Signed-off-by: Subrata Banik <subratabanik@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/90403
Reviewed-by: Julius Werner <jwerner@chromium.org>
Reviewed-by: Jayvik Desai <jayvik@google.com>
Reviewed-by: Kapil Porwal <kapilporwal@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/arch/arm64/armv8/Makefile.mk

@@ -73,6 +73,7 @@ endif
 ################################################################################
 ifeq ($(CONFIG_ARCH_RAMSTAGE_ARMV8_64),y)
 
+ramstage-y += ramstage_entry.S
 ramstage-y += cache.c
 ramstage-y += cpu.S
 ramstage-y += exception.c

src/arch/arm64/armv8/ramstage_entry.S

@@ -0,0 +1,34 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+/*
+ * Ramstage entrypoint code for aarch64 (a.k.a. armv8)
+ */
+
+#include <arch/asm.h>
+
+ENTRY(_start)
+	/*
+	 * Note: x0 must be preserved as it contains the CBMEM top pointer
+	 * for the next stage entry.
+	 */
+	ldr	x10, =_preram_stack
+	ldr	x11, =_postram_stack
+
+	cmp	x10, x11
+	beq	stack_init_done
+
+	/* Initialize stack with sentinel value to later check overflow. */
+	ldr	x9, =0xdeadbeefdeadbeef
+	ldr	x10, =_epostram_stack
+1:
+	stp	x9, x9, [x11], #16
+	cmp	x11, x10
+	bne	1b
+
+	/* Update stack pointer with DRAM mapped stack base */
+	/* Leave a line of beef dead for easier visibility in stack dumps. */
+	sub	sp, x11, #16
+
+stack_init_done:
+	/* Call the C function. */
+	bl	stage_entry
+ENDPROC(_start)

src/arch/arm64/include/arch/header.ld

@@ -9,7 +9,7 @@ PHDRS
 	to_load PT_LOAD;
 }
 
-#if ENV_DECOMPRESSOR || ENV_BOOTBLOCK || ENV_RMODULE
+#if ENV_DECOMPRESSOR || ENV_BOOTBLOCK || ENV_RMODULE || ENV_RAMSTAGE
 ENTRY(_start)
 #else
 ENTRY(stage_entry)