aarch64: Add feature to use stdout via qemu special address

- Since the feature is only used in aarch64 subsystem no need to additionally check for aarch64 target
Signed-off-by: Jonathan Schwender <schwenderjonathan@gmail.com>

Cargo.toml

@@ -41,13 +41,14 @@ name = "measure_startup_time"
 harness = false
 
 [features]
-default = ["pci", "acpi", "fsgsbase", "smp"]
+default = ["pci", "acpi", "fsgsbase", "smp", "aarch64-qemu-stdout"]
 vga = []
 newlib = []
 pci = []
 acpi = []
 smp = []
 fsgsbase = []
+aarch64-qemu-stdout = []	# Doesn't do anything on x86 
 
 [dependencies]
 bitflags = "1.3"

src/arch/aarch64/kernel/mod.rs

@@ -149,6 +149,7 @@ pub fn get_cmdline() -> VirtAddr {
 pub fn message_output_init() {
 	percore::init();
 
+	#[cfg(not(feature = "aarch64-qemu-stdout"))]
 	if environment::is_single_kernel() {
 		// We can only initialize the serial port here, because VGA requires processor
 		// configuration first.
@@ -159,6 +160,11 @@ pub fn message_output_init() {
 }
 
 pub fn output_message_byte(byte: u8) {
+	#[cfg(feature = "aarch64-qemu-stdout")]
+	unsafe {
+		core::ptr::write_volatile(0x3F20_1000 as *mut u8, byte); 
+	}
+	#[cfg(not(feature = "aarch64-qemu-stdout"))]
 	if environment::is_single_kernel() {
 		// Output messages to the serial port and VGA screen in unikernel mode.
 		unsafe {