cpu/intel/car: Make stack guards more useful on C_ENV_BOOTBLOCK

With C_ENVIRONMENT_BOOTBLOCK, CONFIG_DCACHE_BSP_STACK_SIZE needs to be
set to define a stack region that can be shared over all stages using
CAR. It makes sense to use that Kconfig option's value instead of a
hardcoded value. This will result in less false positives when the
stack size is big, for instance with FSP using the coreboot stack.

In many configurations with C_ENVIRONMENT_BOOTBLOCK the stack_base is
at the base of CAR. If the stack grows too large it operates out of
CAR, typically resulting in a hang. Therefore the stack guards are
extended to cover 256 bytes at the base to at least provide a warning
when the romstage is dangerously close of running out of stack.

Change-Id: I2ce1dda4d1f254e6c36de4d3fea26e12c34195ff
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/34976
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Aaron Durbin <adurbin@chromium.org>

src/cpu/intel/car/romstage.c

@@ -21,6 +21,8 @@
 #include <program_loading.h>
 #include <timestamp.h>
 
+/* If we do not have a constrained _car_stack region size, use the
+   following as a guideline for acceptable stack usage. */
 #define DCACHE_RAM_ROMSTAGE_STACK_SIZE 0x2000
 
 static struct postcar_frame early_mtrrs;
@@ -43,16 +45,18 @@ static void prepare_and_run_postcar(struct postcar_frame *pcf)
 static void romstage_main(unsigned long bist)
 {
 	int i;
-	const int num_guards = 4;
+	const int num_guards = 64;
 	const u32 stack_guard = 0xdeadbeef;
 	u32 *stack_base;
 	u32 size;
+	const size_t stack_size = MAX(CONFIG_DCACHE_BSP_STACK_SIZE,
+				      DCACHE_RAM_ROMSTAGE_STACK_SIZE);
 
 	/* Size of unallocated CAR. */
 	size = ALIGN_DOWN(_car_stack_size, 16);
 
-	size = MIN(size, DCACHE_RAM_ROMSTAGE_STACK_SIZE);
+	size = MIN(size, stack_size);
-	if (size < DCACHE_RAM_ROMSTAGE_STACK_SIZE)
+	if (size < stack_size)
 		printk(BIOS_DEBUG, "Romstage stack size limited to 0x%x!\n",
 			size);