aarch64: Add WIP qemu raspi3 machine support

- Only initialize CPU 0 and spin on other CPUs (based on https://github.com/rust-embedded/rust-raspberrypi-OS-tutorials/)
- Adjust physical start address of RAM to correct value for raspi3 (again based on the raspberry tutorial)
- Add feature to map write_char() to use a special address for usage with qemu. The aarch64 solution doesn't seem to work on qemu. Didn't look to closely though.
 The feature is enabled by default for make arch=aarch64 for now. A toggle is future work

Cargo.toml

@@ -6,6 +6,9 @@ license = "MIT/Apache-2.0"
 readme = "README.md"
 edition = "2018"
 
+[features]
+aarch64-qemu-stdout = []    # Output to special qemu address 0x3F20_1000 instead of trying to use uart
+
 [dependencies]
 bitflags = "1.2.*"
 

Makefile

@@ -10,6 +10,11 @@ opt := --release
 rdir := release
 endif
 
+# Todo - make this feature toggleable
+ifeq ($(arch), aarch64)
+opt += --features "aarch64-qemu-stdout"
+endif
+
 CONVERT :=
 RN :=
 ifdef COMSPEC

src/arch/aarch64/entry.rs

@@ -12,6 +12,7 @@ extern "C" {
 }
 
 const BOOT_STACK_SIZE: usize = 4096;
+const BOOT_CORE_ID: u64 = 0;	// ID of CPU for booting on SMP systems - this might be board specific in the future
 
 #[link_section = ".data"]
 static STACK: [u8; BOOT_STACK_SIZE] = [0; BOOT_STACK_SIZE];
@@ -51,16 +52,16 @@ fn tcr_size(x: u64) -> u64 {
 	(((64) - (x)) << 16) | (((64) - (x)) << 0)
 }
 
+global_asm!(include_str!("entry.s"));
+
 #[inline(never)]
 #[no_mangle]
-#[naked]
-pub unsafe extern "C" fn _start() -> ! {
+pub unsafe fn _start_rust() -> ! {
 	// Pointer to stack base
 	llvm_asm!("mov sp, $0"
 		:: "r"(&STACK[BOOT_STACK_SIZE - 0x10] as *const u8 as usize)
         :: "volatile");
-
-	pre_init();
+	pre_init()
 }
 
 unsafe fn pre_init() -> ! {
@@ -172,3 +173,9 @@ unsafe fn pre_init() -> ! {
 	// we should never reach this  point
 	loop {}
 }
+
+pub unsafe fn wait_forever() -> ! {
+	loop {
+		asm!("wfe")
+	}
+}
\ No newline at end of file

src/arch/aarch64/entry.s

+// Adapted from https://github.com/rust-embedded/rust-raspberrypi-OS-tutorials/blob/master/02_runtime_init/src/_arch/aarch64/cpu/boot.s
+
+.equ _core_id_mask, 0b11    //Assume 4 core raspi3
+
+.section .text._start
+
+_start:
+	// Only proceed on the boot core. Park it otherwise.
+	mrs	x1, MPIDR_EL1
+	and	x1, x1, _core_id_mask
+	mov	x2, 0      // Assume CPU 0 is responsible for booting
+	cmp	x1, x2
+	b.ne	1f
+
+	// If execution reaches here, it is the boot core. Now, prepare the jump to Rust code.
+
+	// Jump to Rust code.
+	b	_start_rust
+
+	// Infinitely wait for events (aka "park the core").
+1:	wfe
+	b	1b
+
+.size	_start, . - _start
+.type	_start, function
+.global	_start
\ No newline at end of file

src/arch/aarch64/link.ld

@@ -3,7 +3,7 @@ OUTPUT_ARCH("aarch64")
 ENTRY(_start)
 
 /* start address of the RAM, below belongs to the flash */
-phys = 0x40080000;
+phys = 0x00080000;
 
 SECTIONS
 {

src/console.rs

@@ -14,6 +14,14 @@ pub struct Console;
 /// a message to HermitCore's console.
 impl fmt::Write for Console {
 	/// Print a single character.
+	#[cfg(feature = "aarch64-qemu-stdout")]
+	fn write_char(&mut self, c: char) -> fmt::Result {
+		unsafe {
+			core::ptr::write_volatile(0x3F20_1000 as *mut u8, c as u8); //qemu raspi3
+		}
+		Ok(())
+	}
+	#[cfg(not(feature = "aarch64-qemu-stdout"))]
 	fn write_char(&mut self, c: char) -> fmt::Result {
 		arch::output_message_byte(c as u8);
 		Ok(())

src/lib.rs

@@ -10,6 +10,8 @@
 #![feature(const_raw_ptr_to_usize_cast)]
 #![feature(lang_items)]
 #![feature(llvm_asm)]
+#![feature(global_asm)]
+#![feature(asm)]
 #![feature(panic_info_message)]
 #![feature(specialization)]
 #![feature(naked_functions)]