From 18eb4a59d6d41af599f0d2fba2d6a81ad5cccbe4 Mon Sep 17 00:00:00 2001
From: Marcin Undak <mcinek@gmail.com>
Date: Thu, 7 Oct 2021 15:54:48 -0400
Subject: [PATCH] Kernel: Print current CPU exception level after booting on
 Aarch64

This is a first step to switch to exception level 1.

This is also my first patch for SerenityOS :)
---
 Kernel/Prekernel/Arch/aarch64/Aarch64_asm_utils.S | 12 ++++++++++++
 Kernel/Prekernel/Arch/aarch64/Aarch64_asm_utils.h |  9 +++++++++
 Kernel/Prekernel/Arch/aarch64/init.cpp            |  7 +++++++
 Kernel/Prekernel/CMakeLists.txt                   |  1 +
 4 files changed, 29 insertions(+)
 create mode 100644 Kernel/Prekernel/Arch/aarch64/Aarch64_asm_utils.S
 create mode 100644 Kernel/Prekernel/Arch/aarch64/Aarch64_asm_utils.h

diff --git a/Kernel/Prekernel/Arch/aarch64/Aarch64_asm_utils.S b/Kernel/Prekernel/Arch/aarch64/Aarch64_asm_utils.S
new file mode 100644
index 0000000000..334848f9e7
--- /dev/null
+++ b/Kernel/Prekernel/Arch/aarch64/Aarch64_asm_utils.S
@@ -0,0 +1,12 @@
+/*
+ * Copyright (c) 2021, Marcin Undak <mcinek@gmail.com>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+.global get_current_exception_level
+get_current_exception_level:
+  mrs x0, CurrentEL
+  lsr x0, x0, #2
+  and x0, x0, #0x3
+  ret
diff --git a/Kernel/Prekernel/Arch/aarch64/Aarch64_asm_utils.h b/Kernel/Prekernel/Arch/aarch64/Aarch64_asm_utils.h
new file mode 100644
index 0000000000..e3d4b54ce6
--- /dev/null
+++ b/Kernel/Prekernel/Arch/aarch64/Aarch64_asm_utils.h
@@ -0,0 +1,9 @@
+/*
+ * Copyright (c) 2021, Marcin Undak <mcinek@gmail.com>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+extern "C" uint8_t get_current_exception_level();
diff --git a/Kernel/Prekernel/Arch/aarch64/init.cpp b/Kernel/Prekernel/Arch/aarch64/init.cpp
index 85662770c5..a478bd98c4 100644
--- a/Kernel/Prekernel/Arch/aarch64/init.cpp
+++ b/Kernel/Prekernel/Arch/aarch64/init.cpp
@@ -1,10 +1,12 @@
 /*
  * Copyright (c) 2021, Nico Weber <thakis@chromium.org>
+ * Copyright (c) 2021, Marcin Undak <mcinek@gmail.com>
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
 
 #include <AK/Types.h>
+#include <Kernel/Prekernel/Arch/aarch64/Aarch64_asm_utils.h>
 #include <Kernel/Prekernel/Arch/aarch64/Mailbox.h>
 #include <Kernel/Prekernel/Arch/aarch64/Timer.h>
 #include <Kernel/Prekernel/Arch/aarch64/UART.h>
@@ -25,6 +27,11 @@ extern "C" [[noreturn]] void init()
     uart.print_num(firmware_version);
     uart.print_str("\r\n");
 
+    auto exception_level = get_current_exception_level();
+    uart.print_str("Current CPU exception level: EL");
+    uart.print_num(exception_level);
+    uart.print_str("\r\n");
+
     auto& timer = Prekernel::Timer::the();
     u64 start_musec = 0;
     for (;;) {
diff --git a/Kernel/Prekernel/CMakeLists.txt b/Kernel/Prekernel/CMakeLists.txt
index e370e2ad59..c156ae4447 100644
--- a/Kernel/Prekernel/CMakeLists.txt
+++ b/Kernel/Prekernel/CMakeLists.txt
@@ -11,6 +11,7 @@ if ("${SERENITY_ARCH}" STREQUAL "aarch64")
         Arch/aarch64/MMIO.cpp
         Arch/aarch64/Timer.cpp
         Arch/aarch64/UART.cpp
+        Arch/aarch64/Aarch64_asm_utils.S
         Arch/aarch64/boot.S
         Arch/aarch64/init.cpp
     )