libpayload: x86: Move Multiboot header to include file

This moves the multiboot header into its own include file, simplifying
head.S and making it easier to include/exclude the multiboot header
based on config options.

BUG=b:242829490
TEST=Able to build and boot google/rex.

Change-Id: I59a22dfe36044b4dd64a5b028a134be7a7d02a48
Signed-off-by: Subrata Banik <subratabanik@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/82533
Reviewed-by: Julius Werner <jwerner@chromium.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

2 files changed, 58 insertions, 27 deletions

diff --git a/payloads/libpayload/arch/x86/head.S b/payloads/libpayload/arch/x86/head.S
index 2bac700bda..99842cd54b 100644
--- a/payloads/libpayload/arch/x86/head.S
+++ b/payloads/libpayload/arch/x86/head.S
@@ -38,36 +38,13 @@
  * change anything.
  */
 _entry:
-	jmp _init
-
-	.align 4
-
-#define MB_MAGIC 0x1BADB002
-#define MB_FLAGS 0x00010003
-
-mb_header:
-	.long MB_MAGIC
-	.long MB_FLAGS
-	.long -(MB_MAGIC + MB_FLAGS)
-	.long mb_header
-	.long _start
-	.long _edata
-	.long _end
-	.long _init
-
-/*
- * This function saves off the previous stack and switches us to our
- * own execution environment.
- */
-_init:
-	/* No interrupts, please. */
-	cli
 
+	/* Add multiboot header and jump around it when building with multiboot support. */
 #if CONFIG(LP_MULTIBOOT)
-	/* Store EAX and EBX */
-	movl %eax, loader_eax
-	movl %ebx, loader_ebx
+	#include "multiboot_header.inc"
 #endif
+	/* No interrupts, please. */
+	cli
 
 	/* save pointer to coreboot tables */
 	movl 4(%esp), %eax
diff --git a/payloads/libpayload/arch/x86/multiboot_header.inc b/payloads/libpayload/arch/x86/multiboot_header.inc
new file mode 100644
index 0000000000..e321dc42b2
--- /dev/null
+++ b/payloads/libpayload/arch/x86/multiboot_header.inc
@@ -0,0 +1,54 @@
+/*
+ *
+ * Copyright (C) 2008 Advanced Micro Devices, Inc.
+ * Copyright (C) 2017 Patrick Rudolph <siro@das-labor.org>
+ * Copyright (C) 2024 Google Inc.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ * 3. The name of the author may not be used to endorse or promote products
+ *    derived from this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
+ * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
+ * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+ * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+ * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+ * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+ * SUCH DAMAGE.
+ */
+
+#define MB_MAGIC 0x1BADB002
+#define MB_FLAGS 0x00010003
+
+	jmp _init
+
+/*
+ * Note: The Multiboot standard requires Multiboot header to be placed
+ * below 0x2000 in the resulting image.  See:
+ * http://www.gnu.org/software/grub/manual/multiboot/html_node/OS-image-format.html
+ */
+mb_header:
+	.long MB_MAGIC
+	.long MB_FLAGS
+	.long -(MB_MAGIC + MB_FLAGS)
+	.long mb_header
+	.long _start
+	.long _edata
+	.long _end
+	.long _init
+
+_init:
+	/* Store EAX and EBX */
+	movl %eax, loader_eax
+	movl %ebx, loader_ebx