x86: prepare cache-as-ram to allow multiple stages

In order to do a verification of romstage on x86 one needs to
run verstage which verifies romstage (and the memory init code).
However, x86 doesn't have SRAM like every other modern SoC so
managing the cache-as-ram region is especially critical.

First move all of the "shared" objects to the beginning of
the .car.data section. This change then ensures that each stage
using car.ld to link has the same consistent view of the addresses
of these fixed-sized objects in cache-as-ram. The CAR_GLOBALs can
be unique per stage. However, these variables are expected to have
a value of zero at the start of each stage. In order to allow a
stage to provide those semantics outside of the initial cache-as-arm
setup routine add _car_global_start and _car_global_end symbols.
Those symbols can be used to clear the CAR_GLOBALs for that stage.

Note that the timestamp region can't be moved out similarly to the
pre-ram cbmem console because the object storage of the timestamp
cache is used *after* cache-as-ram is torn down to indicate if the
cache should be used or not. Therefore, that timestamp needs to
migrated to ram. A logic change in src/lib/timestamp.c could
alleviate this requirement, but that task wasn't tackled in this
patch.

BUG=chrome-os-partner:44827
BRANCH=None
TEST=Built and booted glados.

Change-Id: I15e9f6b0c632ee5a2369da0709535d6cb0d94f61
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: http://review.coreboot.org/11740
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Georgi <pgeorgi@google.com>
diff --git a/src/arch/x86/car.ld b/src/arch/x86/car.ld
index c30c802..5da9dcf 100644
--- a/src/arch/x86/car.ld
+++ b/src/arch/x86/car.ld
@@ -22,15 +22,29 @@
 /* This file is included inside a SECTIONS block */
 . = CONFIG_DCACHE_RAM_BASE;
 .car.data . (NOLOAD) : {
+	/* The pre-ram cbmem console as well as the timestamp region are fixed
+	 * in size. Therefore place them at the beginning .car.data section
+	 * so that multiple stages (romstage and verstage) have a consistent
+	 * link address of these shared objects. */
+	PRERAM_CBMEM_CONSOLE(., (CONFIG_LATE_CBMEM_INIT ? 0 : 0xc00))
 	_car_data_start = .;
+	/* The timestamp implementation relies on this storage to be around
+	 * after migration. One of the fields indicates not to use it as the
+	 * backing store once cbmem comes online. Therefore, this data needs
+	 * to reside in the migrated area (between _car_data_start and
+	 * _car_data_end). */
 #if IS_ENABLED(CONFIG_HAS_PRECBMEM_TIMESTAMP_REGION)
 	TIMESTAMP(., 0x100)
 #endif
+	/* _car_global_start and _car_global_end 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 = .;
 	*(.car.global_data);
 	. = ALIGN(ARCH_POINTER_ALIGN_SIZE);
+	_car_global_end = .;
 	_car_data_end = .;
-
-	PRERAM_CBMEM_CONSOLE(., (CONFIG_LATE_CBMEM_INIT ? 0 : 0xc00))
 }
 
 /* Global variables are not allowed in romstage
@@ -48,4 +62,4 @@
 	*(.sbss.*)
 }
 
-_bogus = ASSERT((CONFIG_DCACHE_RAM_SIZE == 0) || (SIZEOF(.car.data) + 0xc00 <= CONFIG_DCACHE_RAM_SIZE), "Cache as RAM area is too full");
+_bogus = ASSERT((CONFIG_DCACHE_RAM_SIZE == 0) || (SIZEOF(.car.data) <= CONFIG_DCACHE_RAM_SIZE), "Cache as RAM area is too full");