plat: Add _base_addr symbol to represent the reference base address

Export a linker script symbol called `_base_addr` to represent the exact
reference value used by the linker to resolve absolute references to
symbols.

Also, similarly to the other exported symbols, create a corresponding
macro, to avoid having to explicitly cast it for every reference.

Signed-off-by: Sergiu Moga <sergiu.moga@protonmail.com>

plat/common/include/uk/plat/common/sections.h

@@ -43,6 +43,11 @@
 /* _dtb: start of device tree */
 extern char _dtb[];
 
+/* _base_addr: this is the base address of the unikernel image. It is set at
+ * link time. It is used to compute other addresses, as reference to this.
+ */
+extern char _base_addr[];
+
 /* [_text, _etext]: contains .text.* sections */
 extern char _text[], _etext[];
 
@@ -75,6 +80,7 @@ extern char _end[];
 #define __uk_image_symbol(addr)    ((unsigned long)(addr))
 
 #define __DTB                   __uk_image_symbol(_dtb)
+#define __BASE_ADDR             __uk_image_symbol(_base_addr)
 #define __TEXT                  __uk_image_symbol(_text)
 #define __ETEXT                 __uk_image_symbol(_etext)
 #define __EH_FRAME_START        __uk_image_symbol(__eh_frame_start)

plat/kvm/arm/link64.lds.S

@@ -67,6 +67,8 @@ SECTIONS {
 	_start_ram_addr = .;
 	. = . + DTB_RESERVED_SIZE;
 
+	_base_addr = .;		/* Symbol to represent the load base address */
+
 	/* Code */
 	_text = .;
 	.text :

plat/kvm/x86/link64.lds.S

@@ -40,6 +40,8 @@ SECTIONS
 {
 	. = 0x100000;
 
+	_base_addr = .;		/* Symbol to represent the load base address */
+
 	/* Code */
 	_text = .;
 	.text :

plat/linuxu/arm64/entry64.S

@@ -35,6 +35,15 @@
 
 .section .text
 
+/* For now, although inaccurate, set the
+ * _base_addr symbol to _liblinuxuplat_start, since that is where most images
+ * happen to begin anyway. We do not use it in linuxu anyway, so we do not care.
+ * We only do this so that references to this symbol do not generate build
+ * errors, although it is not used.
+ */
+.globl	_base_addr
+.set	_base_addr, _liblinuxuplat_start
+
 .global _liblinuxuplat_start
 _liblinuxuplat_start:
 	ldr x0, [sp]		// argc to r0

plat/linuxu/x86/entry64.S

@@ -33,6 +33,15 @@
 
 .section .text
 
+/* For now, although inaccurate, set the
+ * _base_addr symbol to _liblinuxuplat_start, since that is where most images
+ * happen to begin anyway. We do not use it in linuxu anyway, so we do not care.
+ * We only do this so that references to this symbol do not generate build
+ * errors, although it is not used.
+ */
+.globl	_base_addr
+.set	_base_addr, _liblinuxuplat_start
+
 .global _liblinuxuplat_start
 _liblinuxuplat_start:
 	xorl %ebp, %ebp		# mark the outmost frame (clear the frame pointer)

plat/xen/arm/link32.lds.S

@@ -49,6 +49,9 @@ SECTIONS
 	_boot_stack_end	= 0x404000;
 	_page_dir	= 0x404000;	/* 16 KB translation table */
 	.		= 0x408000;
+
+	_base_addr = .;		/* Symbol to represent the load base address */
+
 	_text = .;			/* Text and read-only data */
 	.text : {
 		*(.text.zimageboot)	/* zImage boot header & code

plat/xen/x86/link64.lds.S

@@ -43,6 +43,9 @@ ENTRY(_libxenplat_start)
 SECTIONS
 {
 	. = 0x0;
+
+	_base_addr = .;		/* Symbol to represent the load base address */
+
 	_text = .;			/* Text and read-only data */
 	.text : {
 		*(.text.boot)