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# Tutorial 04 - Zero Overhead Abstraction
## tl;dr
- All hand-written assembly is replaced by Rust code from the [cortex-a] crate, which provides
zero-overhead abstractions and wraps the `unsafe` parts.
[cortex-a]: https://github.com/rust-embedded/cortex-a
## Diff to previous
4 years ago
```diff
diff -uNr 03_hacky_hello_world/Cargo.toml 04_zero_overhead_abstraction/Cargo.toml
--- 03_hacky_hello_world/Cargo.toml
+++ 04_zero_overhead_abstraction/Cargo.toml
4 years ago
@@ -18,3 +18,8 @@
##--------------------------------------------------------------------------------------------------
4 years ago
[dependencies]
+
+# Platform specific dependencies
+[target.'cfg(target_arch = "aarch64")'.dependencies]
+cortex-a = { version = "5.x.x" }
+
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diff -uNr 03_hacky_hello_world/src/_arch/aarch64/cpu/smp.rs 04_zero_overhead_abstraction/src/_arch/aarch64/cpu/smp.rs
--- 03_hacky_hello_world/src/_arch/aarch64/cpu/smp.rs
+++ 04_zero_overhead_abstraction/src/_arch/aarch64/cpu/smp.rs
@@ -0,0 +1,22 @@
+// SPDX-License-Identifier: MIT OR Apache-2.0
+//
+// Copyright (c) 2018-2020 Andre Richter <andre.o.richter@gmail.com>
+
+//! Architectural symmetric multiprocessing.
+
+use cortex_a::regs::*;
+
+//--------------------------------------------------------------------------------------------------
+// Public Code
+//--------------------------------------------------------------------------------------------------
+
+/// Return the executing core's id.
+#[inline(always)]
+pub fn core_id<T>() -> T
+where
+ T: From<u8>,
+{
+ const CORE_MASK: u64 = 0b11;
+
+ T::from((MPIDR_EL1.get() & CORE_MASK) as u8)
+}
diff -uNr 03_hacky_hello_world/src/_arch/aarch64/cpu.rs 04_zero_overhead_abstraction/src/_arch/aarch64/cpu.rs
--- 03_hacky_hello_world/src/_arch/aarch64/cpu.rs
+++ 04_zero_overhead_abstraction/src/_arch/aarch64/cpu.rs
@@ -4,8 +4,35 @@
4 years ago
//! Architectural processor code.
-// Assembly counterpart to this file.
-global_asm!(include_str!("cpu.S"));
+use crate::{bsp, cpu};
+use cortex_a::{asm, regs::*};
+
+//--------------------------------------------------------------------------------------------------
+// Boot Code
+//--------------------------------------------------------------------------------------------------
+
+/// The entry of the `kernel` binary.
+///
+/// The function must be named `_start`, because the linker is looking for this exact name.
+///
+/// # Safety
+///
+/// - Linker script must ensure to place this function where it is expected by the target machine.
+/// - We have to hope that the compiler omits any stack pointer usage before the stack pointer is
+/// actually set (`SP.set()`).
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+#[no_mangle]
+pub unsafe fn _start() -> ! {
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+ use crate::runtime_init;
+
+ // Expect the boot core to start in EL2.
+ if bsp::cpu::BOOT_CORE_ID == cpu::smp::core_id() {
+ SP.set(bsp::memory::boot_core_stack_end() as u64);
4 years ago
+ runtime_init::runtime_init()
+ } else {
+ // If not core0, infinitely wait for events.
+ wait_forever()
+ }
+}
//--------------------------------------------------------------------------------------------------
// Public Code
@@ -14,13 +41,7 @@
4 years ago
/// Pause execution on the core.
#[inline(always)]
pub fn wait_forever() -> ! {
- unsafe {
- loop {
- #[rustfmt::skip]
- asm!(
- "wfe",
- options(nomem, nostack, preserves_flags)
- );
4 years ago
- }
+ loop {
+ asm::wfe()
}
}
diff -uNr 03_hacky_hello_world/src/_arch/aarch64/cpu.S 04_zero_overhead_abstraction/src/_arch/aarch64/cpu.S
--- 03_hacky_hello_world/src/_arch/aarch64/cpu.S
+++ 04_zero_overhead_abstraction/src/_arch/aarch64/cpu.S
@@ -1,21 +0,0 @@
-// SPDX-License-Identifier: MIT OR Apache-2.0
-//
-// Copyright (c) 2018-2020 Andre Richter <andre.o.richter@gmail.com>
-
-.section ".text._start"
-
-.global _start
-
-_start:
- mrs x1, mpidr_el1 // Read Multiprocessor Affinity Register
- and x1, x1, #3 // Clear all bits except [1:0], which hold core id
- cbz x1, 2f // Jump to label 2 if we are core 0
-1: wfe // Wait for event
- b 1b // In case an event happened, jump back to 1
-2: // If we are here, we are core0
- ldr x1, =_start // Load address of function "_start()"
- mov sp, x1 // Set start of stack to before our code, aka first
- // address before "_start()"
- bl runtime_init // Jump to the "runtime_init()" kernel function
- b 1b // We should never reach here. But just in case,
- // park this core aswell
diff -uNr 03_hacky_hello_world/src/bsp/raspberrypi/cpu.rs 04_zero_overhead_abstraction/src/bsp/raspberrypi/cpu.rs
--- 03_hacky_hello_world/src/bsp/raspberrypi/cpu.rs
+++ 04_zero_overhead_abstraction/src/bsp/raspberrypi/cpu.rs
@@ -0,0 +1,12 @@
4 years ago
+// SPDX-License-Identifier: MIT OR Apache-2.0
+//
+// Copyright (c) 2018-2020 Andre Richter <andre.o.richter@gmail.com>
+
+//! BSP Processor code.
+
+//--------------------------------------------------------------------------------------------------
+// Public Definitions
+//--------------------------------------------------------------------------------------------------
+
+/// Used by `arch` code to find the early boot core.
+pub const BOOT_CORE_ID: usize = 0;
diff -uNr 03_hacky_hello_world/src/bsp/raspberrypi/memory.rs 04_zero_overhead_abstraction/src/bsp/raspberrypi/memory.rs
--- 03_hacky_hello_world/src/bsp/raspberrypi/memory.rs
+++ 04_zero_overhead_abstraction/src/bsp/raspberrypi/memory.rs
@@ -17,9 +17,25 @@
}
//--------------------------------------------------------------------------------------------------
+// Public Definitions
+//--------------------------------------------------------------------------------------------------
4 years ago
+
+/// The board's memory map.
+#[rustfmt::skip]
+pub(super) mod map {
+ pub const BOOT_CORE_STACK_END: usize = 0x8_0000;
+}
+
+//--------------------------------------------------------------------------------------------------
// Public Code
//--------------------------------------------------------------------------------------------------
+/// Exclusive end address of the boot core's stack.
+#[inline(always)]
+pub fn boot_core_stack_end() -> usize {
+ map::BOOT_CORE_STACK_END
+}
+
/// Return the inclusive range spanning the .bss section.
///
/// # Safety
4 years ago
diff -uNr 03_hacky_hello_world/src/bsp/raspberrypi.rs 04_zero_overhead_abstraction/src/bsp/raspberrypi.rs
--- 03_hacky_hello_world/src/bsp/raspberrypi.rs
+++ 04_zero_overhead_abstraction/src/bsp/raspberrypi.rs
@@ -5,4 +5,5 @@
4 years ago
//! Top-level BSP file for the Raspberry Pi 3 and 4.
pub mod console;
+pub mod cpu;
pub mod memory;
4 years ago
diff -uNr 03_hacky_hello_world/src/cpu/smp.rs 04_zero_overhead_abstraction/src/cpu/smp.rs
--- 03_hacky_hello_world/src/cpu/smp.rs
+++ 04_zero_overhead_abstraction/src/cpu/smp.rs
@@ -0,0 +1,10 @@
+// SPDX-License-Identifier: MIT OR Apache-2.0
+//
+// Copyright (c) 2018-2020 Andre Richter <andre.o.richter@gmail.com>
+
+//! Symmetric multiprocessing.
+
+#[cfg(target_arch = "aarch64")]
+#[path = "../_arch/aarch64/cpu/smp.rs"]
+mod arch_cpu_smp;
+pub use arch_cpu_smp::*;
diff -uNr 03_hacky_hello_world/src/cpu.rs 04_zero_overhead_abstraction/src/cpu.rs
--- 03_hacky_hello_world/src/cpu.rs
+++ 04_zero_overhead_abstraction/src/cpu.rs
@@ -8,3 +8,5 @@
#[path = "_arch/aarch64/cpu.rs"]
mod arch_cpu;
pub use arch_cpu::*;
+
+pub mod smp;
diff -uNr 03_hacky_hello_world/src/main.rs 04_zero_overhead_abstraction/src/main.rs
--- 03_hacky_hello_world/src/main.rs
+++ 04_zero_overhead_abstraction/src/main.rs
@@ -92,9 +92,8 @@
//! - `crate::memory::*`
4 years ago
//! - `crate::bsp::memory::*`
-#![feature(asm)]
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#![feature(format_args_nl)]
-#![feature(global_asm)]
+#![feature(naked_functions)]
#![feature(panic_info_message)]
#![no_main]
#![no_std]
@@ -116,7 +115,8 @@
///
/// - Only a single core must be active and running this function.
unsafe fn kernel_init() -> ! {
- println!("[0] Hello from Rust!");
+ println!("[0] Hello from pure Rust!");
- panic!("Stopping here.")
+ println!("[1] Stopping here.");
+ cpu::wait_forever()
}
diff -uNr 03_hacky_hello_world/src/runtime_init.rs 04_zero_overhead_abstraction/src/runtime_init.rs
--- 03_hacky_hello_world/src/runtime_init.rs
+++ 04_zero_overhead_abstraction/src/runtime_init.rs
@@ -30,7 +30,6 @@
4 years ago
/// # Safety
///
/// - Only a single core must be active and running this function.
-#[no_mangle]
pub unsafe fn runtime_init() -> ! {
4 years ago
zero_bss();
```