arch/x86: Restrict use of _car_global[start|end]

Restrict the use of symbol names _car_global_[start|end]
to be used exclusively with CAR_GLOBAL_MIGRATION=y.
They just alias the start and end of .bss section in CAR.

Change-Id: I36c858a4f181516d4c61f9fd1d5005c7d2c06057
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/35034
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-by: Marshall Dawson <marshalldawson3rd@gmail.com>

src/arch/x86/assembly_entry.S

@@ -32,11 +32,11 @@ _start:
 	/* reset stack pointer to CAR stack */
 	mov	$_car_stack_end, %esp
 
-	/* clear CAR_GLOBAL area as it is not shared */
+	/* clear .bss section as it is not shared */
 	cld
 	xor	%eax, %eax
-	movl	$(_car_global_end), %ecx
+	movl	$(_ebss), %ecx
-	movl	$(_car_global_start), %edi
+	movl	$(_bss), %edi
 	sub	%edi, %ecx
 	shrl	$2, %ecx
 	rep	stosl

src/arch/x86/car.ld

@@ -64,11 +64,13 @@
         . += 80;
         _car_ehci_dbg_info_end = .;
 
-	/* _car_global_start and _car_global_end provide symbols to per-stage
+	/* _bss and _ebss provide symbols to per-stage
 	 * variables that are not shared like the timestamp and the pre-ram
 	 * cbmem console. This is useful for clearing this area on a per-stage
 	 * basis when more than one stage uses cache-as-ram for CAR_GLOBALs. */
-	_car_global_start = .;
+
+	. = ALIGN(ARCH_POINTER_ALIGN_SIZE);
+	_bss = .;
 #if ENV_STAGE_HAS_BSS_SECTION
 	/* Allow global uninitialized variables for stages without CAR teardown. */
 	*(.bss)
@@ -76,10 +78,12 @@
 	*(.sbss)
 	*(.sbss.*)
 #else
+	_car_global_start = .;
 	*(.car.global_data);
+	_car_global_end = .;
 #endif
 	. = ALIGN(ARCH_POINTER_ALIGN_SIZE);
-	_car_global_end = .;
+	_ebss = .;
 	_car_unallocated_start = .;
 
 #if !CONFIG(C_ENVIRONMENT_BOOTBLOCK)

src/arch/x86/include/arch/early_variables.h

@@ -20,6 +20,15 @@
 
 #if ENV_ROMSTAGE && CONFIG(CAR_GLOBAL_MIGRATION)
 
+/*
+ * The _car_global_[start|end]symbols cover CAR data which is relocatable
+ * once memory comes online. Variables with CAR_GLOBAL decoration
+ * reside within this region.
+ */
+extern char _car_global_start[];
+extern char _car_global_end[];
+#define _car_global_size (_car_global_end - _car_global_start)
+
 asm(".section .car.global_data,\"w\",@nobits");
 asm(".previous");
 #ifdef __clang__

src/arch/x86/include/arch/symbols.h

@@ -24,8 +24,8 @@ extern char _car_region_end[];
 #define _car_region_size (_car_region_end - _car_region_start)
 
 /*
- * This is the stack used under CONFIG_C_ENVIRONMENT_BOOTBLOCK for
+ * This is the stack area used for all stages that execute when cache-as-ram
- * all stages that execute when cache-as-ram is up.
+ * is up. Area is not cleared in between stages.
  */
 extern char _car_stack_start[];
 extern char _car_stack_end[];
@@ -38,13 +38,4 @@ extern char _car_ehci_dbg_info_end[];
 #define _car_ehci_dbg_info_size \
 	(_car_ehci_dbg_info_end - _car_ehci_dbg_info_start)
 
-/*
- * The _car_global_[start|end]symbols cover CAR data which is relocatable
- * once memory comes online. Variables with CAR_GLOBAL decoration
- * reside within this region.
- */
-extern char _car_global_start[];
-extern char _car_global_end[];
-#define _car_global_size (_car_global_end - _car_global_start)
-
 #endif

src/soc/intel/common/block/cpu/car/cache_as_ram_fsp.S

@@ -89,11 +89,11 @@ CAR_init_done:
 	/* Setup bootblock stack */
 	mov	%edx, %esp
 
-	/* clear CAR_GLOBAL area as it is not shared */
+	/* clear .bss section as it is not shared */
 	cld
 	xor	%eax, %eax
-	movl	$(_car_global_end), %ecx
+	movl	$(_ebss), %ecx
-	movl	$(_car_global_start), %edi
+	movl	$(_bss), %edi
 	sub	%edi, %ecx
 	shrl	$2, %ecx
 	rep	stosl

src/soc/intel/quark/romstage/fsp2_0.c

@@ -115,7 +115,7 @@ void platform_fsp_memory_init_params_cb(FSPM_UPD *fspm_upd, uint32_t version)
 			aupd->StackBase);
 		printk(BIOS_SPEW, "|                   |\n");
 		printk(BIOS_SPEW, "+-------------------+ 0x%p\n",
-			_car_global_end);
+			_car_unallocated_start);
 		printk(BIOS_SPEW, "| coreboot data     |\n");
 		printk(BIOS_SPEW, "+-------------------+ 0x%p\n",
 			_car_stack_end);