x86: Add 64-bit start-up code

Add code to start up U-Boot in 64-bit mode. It is fairly simple since we are
running from RAM and SPL has done the low-level init.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>

arch/x86/Makefile

@@ -3,8 +3,13 @@
 #
 
 ifeq ($(CONFIG_EFI_APP),)
+ifdef CONFIG_$(SPL_)X86_64
+head-y := arch/x86/cpu/start64.o
+else
 head-y := arch/x86/cpu/start.o
 endif
+endif
+
 head-$(CONFIG_$(SPL_)X86_16BIT_INIT) += arch/x86/cpu/start16.o
 head-$(CONFIG_$(SPL_)X86_16BIT_INIT) += arch/x86/cpu/resetvec.o
 

arch/x86/cpu/Makefile

@@ -8,7 +8,11 @@
 # SPDX-License-Identifier:	GPL-2.0+
 #
 
+ifeq ($(CONFIG_$(SPL_)X86_64),y)
+extra-y	= start64.o
+else
 extra-y	= start.o
+endif
 extra-$(CONFIG_$(SPL_)X86_16BIT_INIT) += resetvec.o start16.o
 obj-y	+= interrupts.o cpu.o cpu_x86.o call64.o setjmp.o
 

arch/x86/cpu/start64.S

@@ -0,0 +1,28 @@
+/*
+ * 64-bit x86 Startup Code
+ *
+ * (C) Copyright 216 Google, Inc
+ * Written by Simon Glass <sjg@chromium.org>
+ *
+ * SPDX-License-Identifier:	GPL-2.0+
+ */
+
+#include <config.h>
+
+.section .text
+.code64
+.globl _start
+.type _start, @function
+_start:
+	/* Set up memory using the existing stack */
+	mov	%rsp, %rdi
+	call	board_init_f_alloc_reserve
+	mov	%rax, %rsp
+
+	call	board_init_f_init_reserve
+
+	call	board_init_f
+	call	board_init_f_r
+
+	/* Should not return here */
+	jmp	.