lib: ramdetect: Register exception handlers for ARMv8

Register exception handlers to avoid a Synchronous External Abort
that is raised when you try to access a non-memory address on ARMv8.
An exception handler can jump over the faulting instruction.
This is the feature only for QEMU/AArch64.

Signed-off-by: Asami Doi <d0iasm.pub@gmail.com>
Change-Id: I09a306ca307ba4027d9758c3debc2e7c844c66b8
Reviewed-on: https://review.coreboot.org/c/coreboot/+/34774
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Raul Rangel <rrangel@chromium.org>

src/arch/arm64/Makefile.inc

@@ -107,6 +107,7 @@ romstage-$(CONFIG_ARM64_USE_ARCH_TIMER) += arch_timer.c
 romstage-y += memset.S
 romstage-y += memcpy.S
 romstage-y += memmove.S
+romstage-y += ramdetect.c
 romstage-y += romstage.c
 romstage-y += transition.c transition_asm.S
 
@@ -131,6 +132,7 @@ ramstage-y += div0.c
 ramstage-y += eabi_compat.c
 ramstage-y += boot.c
 ramstage-y += tables.c
+ramstage-y += ramdetect.c
 ramstage-$(CONFIG_ARM64_USE_ARCH_TIMER) += arch_timer.c
 ramstage-y += memset.S
 ramstage-y += memcpy.S

src/arch/arm64/ramdetect.c

@@ -0,0 +1,41 @@
+/*
+ * This file is part of the coreboot project.
+ *
+ * SPDX-License-Identifier: GPL-2.0-or-later
+ */
+
+#include <types.h>
+#include <device/mmio.h>
+#include <ramdetect.h>
+#include <arch/exception.h>
+#include <arch/transition.h>
+
+static enum {
+	ABORT_CHECKER_NOT_TRIGGERED,
+	ABORT_CHECKER_TRIGGERED,
+} abort_state = ABORT_CHECKER_NOT_TRIGGERED;
+
+static int abort_checker(struct exc_state *state, uint64_t vector_id)
+{
+	if (raw_read_esr_el3() >> 26 != 0x25)
+		return EXC_RET_IGNORED; /* Not a data abort. */
+
+	abort_state = ABORT_CHECKER_TRIGGERED;
+	state->elx.elr += sizeof(uint32_t); /* Jump over faulting instruction. */
+	raw_write_elr_el3(state->elx.elr);
+	return EXC_RET_HANDLED;
+}
+
+static struct exception_handler sync_el0 = {.handler = &abort_checker};
+
+int probe_mb(const uintptr_t dram_start, const uintptr_t size)
+{
+	uintptr_t addr = dram_start + (size * MiB) - sizeof(uint32_t);
+	void *ptr = (void *)addr;
+
+	abort_state = ABORT_CHECKER_NOT_TRIGGERED;
+	exception_handler_register(EXC_VID_CUR_SP_EL0_SYNC, &sync_el0);
+	read32(ptr);
+	exception_handler_unregister(EXC_VID_CUR_SP_EL0_SYNC, &sync_el0);
+	return abort_state == ABORT_CHECKER_NOT_TRIGGERED;
+}

src/include/ramdetect.h

@@ -11,6 +11,12 @@
  * GNU General Public License for more details.
  */
 
+
+/*
+ * Used in probe_ramsize(). This is a weak function and it's overridden for
+ * ARMv8. Return 1 when DRAM exists at the address, otherwise return 0.
+ */
+int probe_mb(const uintptr_t dram_start, const uintptr_t size);
 /*
  * Probe an area if it's read/writable.
  * Primary use case is the detection of DRAM amount on emulators.

src/lib/ramdetect.c

@@ -19,7 +19,7 @@
 
 #define OVERLAP(a, b, s, e) ((b) > (s) && (a) < (e))
 
-static int probe_mb(const uintptr_t dram_start, const uintptr_t size)
+int __weak probe_mb(const uintptr_t dram_start, const uintptr_t size)
 {
 	uintptr_t addr = dram_start + (size * MiB) - sizeof(uint32_t);
 	static const uint32_t patterns[] = {