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321 lines
10 KiB
Markdown
321 lines
10 KiB
Markdown
# Tutorial 02 - Runtime Init
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## tl;dr
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- We extend `boot.s` to call into Rust code for the first time. Before the jump
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to Rust happens, a bit of runtime init work is done.
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- The Rust code being called just halts execution with a call to `panic!()`.
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- Check out `make qemu` again to see the additional code run.
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## Notable additions
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- More additions to the linker script:
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- New sections: `.rodata`, `.got`, `.data`, `.bss`.
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- A dedicated place for linking boot-time arguments that need to be read by `_start()`.
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- `_start()` in `_arch/__arch_name__/cpu/boot.s`:
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1. Halts core if core != core0.
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1. Initializes the `DRAM` by zeroing the [bss] section.
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1. Sets up the `stack pointer`.
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1. Jumps to the `_start_rust()` function, defined in `arch/__arch_name__/cpu/boot.rs`.
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- `_start_rust()`:
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- Calls `kernel_init()`, which calls `panic!()`, which eventually halts core0 as well.
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- The library now uses the [cortex-a] crate, which provides zero-overhead abstractions and wraps
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`unsafe` parts when dealing with the CPU's resources.
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- See it in action in `_arch/__arch_name__/cpu.rs`.
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[bss]: https://en.wikipedia.org/wiki/.bss
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[cortex-a]: https://github.com/rust-embedded/cortex-a
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## Diff to previous
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```diff
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diff -uNr 01_wait_forever/Cargo.toml 02_runtime_init/Cargo.toml
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--- 01_wait_forever/Cargo.toml
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+++ 02_runtime_init/Cargo.toml
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@@ -1,6 +1,6 @@
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[package]
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name = "mingo"
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-version = "0.1.0"
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+version = "0.2.0"
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authors = ["Andre Richter <andre.o.richter@gmail.com>"]
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edition = "2018"
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@@ -21,3 +21,8 @@
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##--------------------------------------------------------------------------------------------------
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[dependencies]
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+
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+# Platform specific dependencies
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+[target.'cfg(target_arch = "aarch64")'.dependencies]
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+cortex-a = { version = "6.x.x" }
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+
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diff -uNr 01_wait_forever/Makefile 02_runtime_init/Makefile
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--- 01_wait_forever/Makefile
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+++ 02_runtime_init/Makefile
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@@ -152,6 +152,8 @@
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$(call colorecho, "\nLaunching objdump")
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@$(DOCKER_TOOLS) $(OBJDUMP_BINARY) --disassemble --demangle \
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--section .text \
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+ --section .rodata \
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+ --section .got \
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$(KERNEL_ELF) | rustfilt
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##------------------------------------------------------------------------------
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diff -uNr 01_wait_forever/src/_arch/aarch64/cpu/boot.rs 02_runtime_init/src/_arch/aarch64/cpu/boot.rs
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--- 01_wait_forever/src/_arch/aarch64/cpu/boot.rs
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+++ 02_runtime_init/src/_arch/aarch64/cpu/boot.rs
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@@ -13,3 +13,15 @@
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// Assembly counterpart to this file.
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global_asm!(include_str!("boot.s"));
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+
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+//--------------------------------------------------------------------------------------------------
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+// Public Code
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+//--------------------------------------------------------------------------------------------------
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+
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+/// The Rust entry of the `kernel` binary.
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+///
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+/// The function is called from the assembly `_start` function.
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+#[no_mangle]
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+pub unsafe fn _start_rust() -> ! {
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+ crate::kernel_init()
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+}
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diff -uNr 01_wait_forever/src/_arch/aarch64/cpu/boot.s 02_runtime_init/src/_arch/aarch64/cpu/boot.s
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--- 01_wait_forever/src/_arch/aarch64/cpu/boot.s
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+++ 02_runtime_init/src/_arch/aarch64/cpu/boot.s
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@@ -3,6 +3,24 @@
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// Copyright (c) 2021 Andre Richter <andre.o.richter@gmail.com>
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//--------------------------------------------------------------------------------------------------
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+// Definitions
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+//--------------------------------------------------------------------------------------------------
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+
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+// Load the address of a symbol into a register, PC-relative.
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+//
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+// The symbol must lie within +/- 4 GiB of the Program Counter.
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+//
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+// # Resources
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+//
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+// - https://sourceware.org/binutils/docs-2.36/as/AArch64_002dRelocations.html
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+.macro ADR_REL register, symbol
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+ adrp \register, \symbol
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+ add \register, \register, #:lo12:\symbol
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+.endm
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+
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+.equ _core_id_mask, 0b11
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+
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+//--------------------------------------------------------------------------------------------------
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// Public Code
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//--------------------------------------------------------------------------------------------------
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.section .text._start
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@@ -11,6 +29,34 @@
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// fn _start()
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//------------------------------------------------------------------------------
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_start:
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+ // Only proceed on the boot core. Park it otherwise.
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+ mrs x1, MPIDR_EL1
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+ and x1, x1, _core_id_mask
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+ ldr x2, BOOT_CORE_ID // provided by bsp/__board_name__/cpu.rs
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+ cmp x1, x2
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+ b.ne .L_parking_loop
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+
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+ // If execution reaches here, it is the boot core.
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+
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+ // Initialize DRAM.
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+ ADR_REL x0, __bss_start
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+ ADR_REL x1, __bss_end_exclusive
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+
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+.L_bss_init_loop:
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+ cmp x0, x1
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+ b.eq .L_prepare_rust
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+ stp xzr, xzr, [x0], #16
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+ b .L_bss_init_loop
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+
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+ // Prepare the jump to Rust code.
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+.L_prepare_rust:
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+ // Set the stack pointer.
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+ ADR_REL x0, __boot_core_stack_end_exclusive
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+ mov sp, x0
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+
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+ // Jump to Rust code.
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+ b _start_rust
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+
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// Infinitely wait for events (aka "park the core").
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.L_parking_loop:
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wfe
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diff -uNr 01_wait_forever/src/_arch/aarch64/cpu.rs 02_runtime_init/src/_arch/aarch64/cpu.rs
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--- 01_wait_forever/src/_arch/aarch64/cpu.rs
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+++ 02_runtime_init/src/_arch/aarch64/cpu.rs
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@@ -0,0 +1,26 @@
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+// SPDX-License-Identifier: MIT OR Apache-2.0
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+//
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+// Copyright (c) 2018-2021 Andre Richter <andre.o.richter@gmail.com>
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+
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+//! Architectural processor code.
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+//!
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+//! # Orientation
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+//!
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+//! Since arch modules are imported into generic modules using the path attribute, the path of this
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+//! file is:
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+//!
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+//! crate::cpu::arch_cpu
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+
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+use cortex_a::asm;
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+
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+//--------------------------------------------------------------------------------------------------
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+// Public Code
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+//--------------------------------------------------------------------------------------------------
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+
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+/// Pause execution on the core.
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+#[inline(always)]
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+pub fn wait_forever() -> ! {
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+ loop {
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+ asm::wfe()
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+ }
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+}
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diff -uNr 01_wait_forever/src/bsp/raspberrypi/cpu.rs 02_runtime_init/src/bsp/raspberrypi/cpu.rs
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--- 01_wait_forever/src/bsp/raspberrypi/cpu.rs
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+++ 02_runtime_init/src/bsp/raspberrypi/cpu.rs
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@@ -0,0 +1,14 @@
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+// SPDX-License-Identifier: MIT OR Apache-2.0
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+//
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+// Copyright (c) 2018-2021 Andre Richter <andre.o.richter@gmail.com>
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+
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+//! BSP Processor code.
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+
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+//--------------------------------------------------------------------------------------------------
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+// Public Definitions
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+//--------------------------------------------------------------------------------------------------
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+
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+/// Used by `arch` code to find the early boot core.
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+#[no_mangle]
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+#[link_section = ".text._start_arguments"]
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+pub static BOOT_CORE_ID: u64 = 0;
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diff -uNr 01_wait_forever/src/bsp/raspberrypi/link.ld 02_runtime_init/src/bsp/raspberrypi/link.ld
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--- 01_wait_forever/src/bsp/raspberrypi/link.ld
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+++ 02_runtime_init/src/bsp/raspberrypi/link.ld
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@@ -11,17 +11,43 @@
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PHDRS
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{
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segment_rx PT_LOAD FLAGS(5); /* 5 == RX */
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+ segment_rw PT_LOAD FLAGS(6); /* 6 == RW */
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}
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SECTIONS
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{
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. = __rpi_load_addr;
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+ /* ^ */
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+ /* | stack */
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+ /* | growth */
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+ /* | direction */
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+ __boot_core_stack_end_exclusive = .; /* | */
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/***********************************************************************************************
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- * Code
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+ * Code + RO Data + Global Offset Table
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***********************************************************************************************/
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.text :
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{
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KEEP(*(.text._start))
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+ *(.text._start_arguments) /* Constants (or statics in Rust speak) read by _start(). */
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+ *(.text._start_rust) /* The Rust entry point */
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+ *(.text*) /* Everything else */
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} :segment_rx
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+
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+ .rodata : ALIGN(8) { *(.rodata*) } :segment_rx
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+ .got : ALIGN(8) { *(.got) } :segment_rx
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+
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+ /***********************************************************************************************
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+ * Data + BSS
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+ ***********************************************************************************************/
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+ .data : { *(.data*) } :segment_rw
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+
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+ /* Section is zeroed in pairs of u64. Align start and end to 16 bytes */
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+ .bss : ALIGN(16)
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+ {
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+ __bss_start = .;
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+ *(.bss*);
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+ . = ALIGN(16);
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+ __bss_end_exclusive = .;
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+ } :NONE
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}
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diff -uNr 01_wait_forever/src/bsp/raspberrypi.rs 02_runtime_init/src/bsp/raspberrypi.rs
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--- 01_wait_forever/src/bsp/raspberrypi.rs
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+++ 02_runtime_init/src/bsp/raspberrypi.rs
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@@ -4,4 +4,4 @@
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//! Top-level BSP file for the Raspberry Pi 3 and 4.
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-// Coming soon.
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+pub mod cpu;
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diff -uNr 01_wait_forever/src/cpu.rs 02_runtime_init/src/cpu.rs
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--- 01_wait_forever/src/cpu.rs
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+++ 02_runtime_init/src/cpu.rs
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@@ -4,4 +4,13 @@
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//! Processor code.
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+#[cfg(target_arch = "aarch64")]
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+#[path = "_arch/aarch64/cpu.rs"]
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+mod arch_cpu;
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+
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mod boot;
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+
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+//--------------------------------------------------------------------------------------------------
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+// Architectural Public Reexports
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+//--------------------------------------------------------------------------------------------------
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+pub use arch_cpu::wait_forever;
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diff -uNr 01_wait_forever/src/main.rs 02_runtime_init/src/main.rs
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--- 01_wait_forever/src/main.rs
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+++ 02_runtime_init/src/main.rs
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@@ -102,8 +102,8 @@
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//!
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//! 1. The kernel's entry point is the function `cpu::boot::arch_boot::_start()`.
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//! - It is implemented in `src/_arch/__arch_name__/cpu/boot.s`.
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+//! 2. Once finished with architectural setup, the arch code calls `kernel_init()`.
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-#![feature(asm)]
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#![feature(global_asm)]
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#![no_main]
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#![no_std]
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@@ -112,4 +112,11 @@
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mod cpu;
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mod panic_wait;
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-// Kernel code coming next tutorial.
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+/// Early init code.
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+///
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+/// # Safety
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+///
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+/// - Only a single core must be active and running this function.
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+unsafe fn kernel_init() -> ! {
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+ panic!()
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+}
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diff -uNr 01_wait_forever/src/panic_wait.rs 02_runtime_init/src/panic_wait.rs
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--- 01_wait_forever/src/panic_wait.rs
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+++ 02_runtime_init/src/panic_wait.rs
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@@ -4,9 +4,10 @@
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//! A panic handler that infinitely waits.
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+use crate::cpu;
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use core::panic::PanicInfo;
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#[panic_handler]
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fn panic(_info: &PanicInfo) -> ! {
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- unimplemented!()
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+ cpu::wait_forever()
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}
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```
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