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README.md

Tutorial 02 - Runtime Init

tl;dr

  • We extend boot.s to call into Rust code for the first time. Before the jump to Rust happens, a bit of runtime init work is done.
  • The Rust code being called just halts execution with a call to panic!().
  • Check out make qemu again to see the additional code run.

Notable additions

  • More additions to the linker script:
    • New sections: .rodata, .got, .data, .bss.
    • A dedicated place for linking boot-time arguments that need to be read by _start().
  • _start() in _arch/__arch_name__/cpu/boot.s:
    1. Halts core if core != core0.
    2. Initializes the DRAM by zeroing the bss section.
    3. Sets up the stack pointer.
    4. Jumps to the _start_rust() function, defined in arch/__arch_name__/cpu/boot.rs.
  • _start_rust():
    • Calls kernel_init(), which calls panic!(), which eventually halts core0 as well.
  • The library now uses the aarch64-cpu crate, which provides zero-overhead abstractions and wraps unsafe parts when dealing with the CPU's resources.
    • See it in action in _arch/__arch_name__/cpu.rs.

Diff to previous

diff -uNr 01_wait_forever/Cargo.toml 02_runtime_init/Cargo.toml--- 01_wait_forever/Cargo.toml+++ 02_runtime_init/Cargo.toml@@ -1,6 +1,6 @@ [package] name = "mingo" -version = "0.1.0"+version = "0.2.0" authors = ["Andre Richter <andre.o.richter@gmail.com>"] edition = "2021" @@ -21,3 +21,7 @@ ##-------------------------------------------------------------------------------------------------- [dependencies] ++# Platform specific dependencies+[target.'cfg(target_arch = "aarch64")'.dependencies]+aarch64-cpu = { version = "9.x.x" }diff -uNr 01_wait_forever/Makefile 02_runtime_init/Makefile--- 01_wait_forever/Makefile+++ 02_runtime_init/Makefile@@ -181,6 +181,7 @@ $(call color_header, "Launching objdump") @$(DOCKER_TOOLS) $(OBJDUMP_BINARY) --disassemble --demangle \ --section .text \ + --section .rodata \ $(KERNEL_ELF) | rustfilt ##------------------------------------------------------------------------------ diff -uNr 01_wait_forever/src/_arch/aarch64/cpu/boot.rs 02_runtime_init/src/_arch/aarch64/cpu/boot.rs--- 01_wait_forever/src/_arch/aarch64/cpu/boot.rs+++ 02_runtime_init/src/_arch/aarch64/cpu/boot.rs@@ -14,4 +14,19 @@ use core::arch::global_asm; // Assembly counterpart to this file. -global_asm!(include_str!("boot.s"));+global_asm!(+ include_str!("boot.s"),+ CONST_CORE_ID_MASK = const 0b11+);++//--------------------------------------------------------------------------------------------------+// Public Code+//--------------------------------------------------------------------------------------------------++/// The Rust entry of the `kernel` binary.+///+/// The function is called from the assembly `_start` function.+#[no_mangle]+pub unsafe fn _start_rust() -> ! {+ crate::kernel_init()+}diff -uNr 01_wait_forever/src/_arch/aarch64/cpu/boot.s 02_runtime_init/src/_arch/aarch64/cpu/boot.s--- 01_wait_forever/src/_arch/aarch64/cpu/boot.s+++ 02_runtime_init/src/_arch/aarch64/cpu/boot.s@@ -3,6 +3,22 @@ // Copyright (c) 2021-2023 Andre Richter <andre.o.richter@gmail.com> //-------------------------------------------------------------------------------------------------- +// Definitions+//--------------------------------------------------------------------------------------------------++// Load the address of a symbol into a register, PC-relative.+//+// The symbol must lie within +/- 4 GiB of the Program Counter.+//+// # Resources+//+// - https://sourceware.org/binutils/docs-2.36/as/AArch64_002dRelocations.html+.macro ADR_REL register, symbol+ adrp \register, \symbol+ add \register, \register, #:lo12:\symbol+.endm++//-------------------------------------------------------------------------------------------------- // Public Code //-------------------------------------------------------------------------------------------------- .section .text._start @@ -11,6 +27,34 @@ // fn _start() //------------------------------------------------------------------------------ _start: + // Only proceed on the boot core. Park it otherwise.+ mrs x0, MPIDR_EL1+ and x0, x0, {CONST_CORE_ID_MASK}+ ldr x1, BOOT_CORE_ID // provided by bsp/__board_name__/cpu.rs+ cmp x0, x1+ b.ne .L_parking_loop++ // If execution reaches here, it is the boot core.++ // Initialize DRAM.+ ADR_REL x0, __bss_start+ ADR_REL x1, __bss_end_exclusive++.L_bss_init_loop:+ cmp x0, x1+ b.eq .L_prepare_rust+ stp xzr, xzr, [x0], #16+ b .L_bss_init_loop++ // Prepare the jump to Rust code.+.L_prepare_rust:+ // Set the stack pointer.+ ADR_REL x0, __boot_core_stack_end_exclusive+ mov sp, x0++ // Jump to Rust code.+ b _start_rust+ // Infinitely wait for events (aka "park the core"). .L_parking_loop: wfe diff -uNr 01_wait_forever/src/_arch/aarch64/cpu.rs 02_runtime_init/src/_arch/aarch64/cpu.rs--- 01_wait_forever/src/_arch/aarch64/cpu.rs+++ 02_runtime_init/src/_arch/aarch64/cpu.rs@@ -0,0 +1,26 @@+// SPDX-License-Identifier: MIT OR Apache-2.0+//+// Copyright (c) 2018-2023 Andre Richter <andre.o.richter@gmail.com>++//! Architectural processor code.+//!+//! # Orientation+//!+//! Since arch modules are imported into generic modules using the path attribute, the path of this+//! file is:+//!+//! crate::cpu::arch_cpu++use aarch64_cpu::asm;++//--------------------------------------------------------------------------------------------------+// Public Code+//--------------------------------------------------------------------------------------------------++/// Pause execution on the core.+#[inline(always)]+pub fn wait_forever() -> ! {+ loop {+ asm::wfe()+ }+}diff -uNr 01_wait_forever/src/bsp/raspberrypi/cpu.rs 02_runtime_init/src/bsp/raspberrypi/cpu.rs--- 01_wait_forever/src/bsp/raspberrypi/cpu.rs+++ 02_runtime_init/src/bsp/raspberrypi/cpu.rs@@ -0,0 +1,14 @@+// SPDX-License-Identifier: MIT OR Apache-2.0+//+// Copyright (c) 2018-2023 Andre Richter <andre.o.richter@gmail.com>++//! BSP Processor code.++//--------------------------------------------------------------------------------------------------+// Public Definitions+//--------------------------------------------------------------------------------------------------++/// Used by `arch` code to find the early boot core.+#[no_mangle]+#[link_section = ".text._start_arguments"]+pub static BOOT_CORE_ID: u64 = 0;diff -uNr 01_wait_forever/src/bsp/raspberrypi/kernel.ld 02_runtime_init/src/bsp/raspberrypi/kernel.ld--- 01_wait_forever/src/bsp/raspberrypi/kernel.ld+++ 02_runtime_init/src/bsp/raspberrypi/kernel.ld@@ -3,6 +3,8 @@ * Copyright (c) 2018-2023 Andre Richter <andre.o.richter@gmail.com> */ +__rpi_phys_dram_start_addr = 0;+ /* The physical address at which the the kernel binary will be loaded by the Raspberry's firmware */ __rpi_phys_binary_load_addr = 0x80000; @@ -13,21 +15,65 @@ * 4 == R * 5 == RX * 6 == RW + *+ * Segments are marked PT_LOAD below so that the ELF file provides virtual and physical addresses.+ * It doesn't mean all of them need actually be loaded. */ PHDRS { - segment_code PT_LOAD FLAGS(5);+ segment_boot_core_stack PT_LOAD FLAGS(6);+ segment_code PT_LOAD FLAGS(5);+ segment_data PT_LOAD FLAGS(6); } SECTIONS { - . = __rpi_phys_binary_load_addr;+ . = __rpi_phys_dram_start_addr;++ /***********************************************************************************************+ * Boot Core Stack+ ***********************************************************************************************/+ .boot_core_stack (NOLOAD) :+ {+ /* ^ */+ /* | stack */+ . += __rpi_phys_binary_load_addr; /* | growth */+ /* | direction */+ __boot_core_stack_end_exclusive = .; /* | */+ } :segment_boot_core_stack /*********************************************************************************************** - * Code+ * Code + RO Data + Global Offset Table ***********************************************************************************************/ .text : { KEEP(*(.text._start)) + *(.text._start_arguments) /* Constants (or statics in Rust speak) read by _start(). */+ *(.text._start_rust) /* The Rust entry point */+ *(.text*) /* Everything else */ } :segment_code ++ .rodata : ALIGN(8) { *(.rodata*) } :segment_code++ /***********************************************************************************************+ * Data + BSS+ ***********************************************************************************************/+ .data : { *(.data*) } :segment_data++ /* Section is zeroed in pairs of u64. Align start and end to 16 bytes */+ .bss (NOLOAD) : ALIGN(16)+ {+ __bss_start = .;+ *(.bss*);+ . = ALIGN(16);+ __bss_end_exclusive = .;+ } :segment_data++ /***********************************************************************************************+ * Misc+ ***********************************************************************************************/+ .got : { *(.got*) }+ ASSERT(SIZEOF(.got) == 0, "Relocation support not expected")++ /DISCARD/ : { *(.comment*) } } diff -uNr 01_wait_forever/src/bsp/raspberrypi.rs 02_runtime_init/src/bsp/raspberrypi.rs--- 01_wait_forever/src/bsp/raspberrypi.rs+++ 02_runtime_init/src/bsp/raspberrypi.rs@@ -4,4 +4,4 @@ //! Top-level BSP file for the Raspberry Pi 3 and 4. -// Coming soon.+pub mod cpu;diff -uNr 01_wait_forever/src/cpu.rs 02_runtime_init/src/cpu.rs--- 01_wait_forever/src/cpu.rs+++ 02_runtime_init/src/cpu.rs@@ -4,4 +4,13 @@ //! Processor code. +#[cfg(target_arch = "aarch64")]+#[path = "_arch/aarch64/cpu.rs"]+mod arch_cpu;+ mod boot; ++//--------------------------------------------------------------------------------------------------+// Architectural Public Reexports+//--------------------------------------------------------------------------------------------------+pub use arch_cpu::wait_forever;diff -uNr 01_wait_forever/src/main.rs 02_runtime_init/src/main.rs--- 01_wait_forever/src/main.rs+++ 02_runtime_init/src/main.rs@@ -104,7 +104,9 @@ //! //! 1. The kernel's entry point is the function `cpu::boot::arch_boot::_start()`. //! - It is implemented in `src/_arch/__arch_name__/cpu/boot.s`. +//! 2. Once finished with architectural setup, the arch code calls `kernel_init()`.+#![feature(asm_const)] #![no_main] #![no_std] @@ -112,4 +114,11 @@ mod cpu; mod panic_wait; -// Kernel code coming next tutorial.+/// Early init code.+///+/// # Safety+///+/// - Only a single core must be active and running this function.+unsafe fn kernel_init() -> ! {+ panic!()+}diff -uNr 01_wait_forever/src/panic_wait.rs 02_runtime_init/src/panic_wait.rs--- 01_wait_forever/src/panic_wait.rs+++ 02_runtime_init/src/panic_wait.rs@@ -4,6 +4,7 @@ //! A panic handler that infinitely waits. +use crate::cpu; use core::panic::PanicInfo; //-------------------------------------------------------------------------------------------------- @@ -12,5 +13,5 @@ #[panic_handler] fn panic(_info: &PanicInfo) -> ! { - unimplemented!()+ cpu::wait_forever() }
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