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@ -10,6 +10,15 @@
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[chainloader]: https://en.wikipedia.org/wiki/Chain_loading
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[position independent code]: https://en.wikipedia.org/wiki/Position-independent_code
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## Note
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Please note that there is a lot of stuff going on in this tutorial that is very hard to grasp by
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only looking at the source code changes.
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Please bear with me until I find the time to write it all down here elaborately. For the time being,
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please see this tutorial as an enabler for a convenience feature that allows booting the following
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tutorials in a quick manner.
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## Install and test it
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Our chainloader is called `MiniLoad` and is inspired by [raspbootin].
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@ -189,7 +198,7 @@ diff -uNr 06_drivers_gpio_uart/src/_arch/aarch64/cpu.rs 07_uart_chainloader/src/
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if bsp::cpu::BOOT_CORE_ID == cpu::smp::core_id() {
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SP.set(bsp::memory::boot_core_stack_end() as u64);
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- runtime_init::runtime_init()
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+ relocate::relocate_self::<u64>()
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+ relocate::relocate_self()
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} else {
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// If not core0, infinitely wait for events.
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wait_forever()
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@ -273,7 +282,7 @@ diff -uNr 06_drivers_gpio_uart/src/bsp/raspberrypi/link.ld 07_uart_chainloader/s
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+
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+ /* Fill up to 8 byte, b/c relocating the binary is done in u64 chunks */
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+ . = ALIGN(8);
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+ __binary_end = .;
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+ __binary_end_inclusive = . - 8;
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+
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/DISCARD/ : { *(.comment*) }
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}
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@ -281,7 +290,16 @@ diff -uNr 06_drivers_gpio_uart/src/bsp/raspberrypi/link.ld 07_uart_chainloader/s
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diff -uNr 06_drivers_gpio_uart/src/bsp/raspberrypi/memory.rs 07_uart_chainloader/src/bsp/raspberrypi/memory.rs
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--- 06_drivers_gpio_uart/src/bsp/raspberrypi/memory.rs
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+++ 07_uart_chainloader/src/bsp/raspberrypi/memory.rs
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@@ -23,10 +23,12 @@
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@@ -12,6 +12,8 @@
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// Symbols from the linker script.
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extern "Rust" {
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+ static __binary_start: UnsafeCell<u64>;
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+ static __binary_end_inclusive: UnsafeCell<u64>;
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static __bss_start: UnsafeCell<u64>;
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static __bss_end_inclusive: UnsafeCell<u64>;
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}
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@@ -23,10 +25,12 @@
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/// The board's memory map.
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#[rustfmt::skip]
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pub(super) mod map {
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@ -297,14 +315,24 @@ diff -uNr 06_drivers_gpio_uart/src/bsp/raspberrypi/memory.rs 07_uart_chainloader
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/// Physical devices.
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#[cfg(feature = "bsp_rpi3")]
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@@ -59,6 +61,12 @@
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@@ -59,6 +63,22 @@
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map::BOOT_CORE_STACK_END
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}
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+/// The address on which the Raspberry firmware loads every binary by default.
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+#[inline(always)]
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+pub fn board_default_load_addr() -> usize {
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+ map::BOARD_DEFAULT_LOAD_ADDRESS
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+pub fn board_default_load_addr() -> *const u64 {
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+ map::BOARD_DEFAULT_LOAD_ADDRESS as _
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+}
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+
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+/// Return the inclusive range spanning the whole binary.
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+///
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+/// # Safety
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+///
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+/// - Values are provided by the linker script and must be trusted as-is.
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+/// - The linker-provided addresses must be u64 aligned.
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+pub fn binary_range_inclusive() -> RangeInclusive<*mut u64> {
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+ unsafe { RangeInclusive::new(__binary_start.get(), __binary_end_inclusive.get()) }
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+}
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+
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/// Return the inclusive range spanning the .bss section.
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@ -402,7 +430,7 @@ diff -uNr 06_drivers_gpio_uart/src/main.rs 07_uart_chainloader/src/main.rs
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+ unsafe {
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+ // Read the kernel byte by byte.
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+ for i in 0..size {
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+ *kernel_addr.offset(i as isize) = console().read_char() as u8;
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+ core::ptr::write_volatile(kernel_addr.offset(i as isize), console().read_char() as u8)
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}
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}
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@ -419,7 +447,7 @@ diff -uNr 06_drivers_gpio_uart/src/main.rs 07_uart_chainloader/src/main.rs
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- println!(" {}. {}", i + 1, driver.compatible());
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- }
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+ // Use black magic to get a function pointer.
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+ let kernel: extern "C" fn() -> ! = unsafe { core::mem::transmute(kernel_addr as *const ()) };
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+ let kernel: fn() -> ! = unsafe { core::mem::transmute(kernel_addr as *const ()) };
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- println!(
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- "[2] Chars written: {}",
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@ -438,7 +466,7 @@ diff -uNr 06_drivers_gpio_uart/src/main.rs 07_uart_chainloader/src/main.rs
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diff -uNr 06_drivers_gpio_uart/src/relocate.rs 07_uart_chainloader/src/relocate.rs
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--- 06_drivers_gpio_uart/src/relocate.rs
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+++ 07_uart_chainloader/src/relocate.rs
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@@ -0,0 +1,52 @@
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@@ -0,0 +1,49 @@
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+// SPDX-License-Identifier: MIT OR Apache-2.0
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+//
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+// Copyright (c) 2018-2020 Andre Richter <andre.o.richter@gmail.com>
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@ -458,104 +486,35 @@ diff -uNr 06_drivers_gpio_uart/src/relocate.rs 07_uart_chainloader/src/relocate.
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+///
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+/// - Only a single core must be active and running this function.
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+/// - Function must not use the `bss` section.
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+pub unsafe fn relocate_self<T>() -> ! {
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+ extern "C" {
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+ static __binary_start: usize;
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+ static __binary_end: usize;
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+ }
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+
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+ let binary_start_addr: usize = &__binary_start as *const _ as _;
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+ let binary_end_addr: usize = &__binary_end as *const _ as _;
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+ let binary_size_in_byte: usize = binary_end_addr - binary_start_addr;
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+
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+ // Get the relocation destination address from the linker symbol.
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+ let mut reloc_dst_addr: *mut T = binary_start_addr as *mut T;
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+pub unsafe fn relocate_self() -> ! {
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+ let range = bsp::memory::binary_range_inclusive();
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+ let mut reloc_destination_addr = *range.start();
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+ let reloc_end_addr_inclusive = *range.end();
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+
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+ // The address of where the previous firmware loaded us.
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+ let mut src_addr: *const T = bsp::memory::board_default_load_addr() as *const _;
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+ let mut src_addr = bsp::memory::board_default_load_addr();
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+
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+ // TODO Make it work for the case src_addr > reloc_addr as well.
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+ let diff = reloc_destination_addr as usize - src_addr as usize;
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+
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+ // Copy the whole binary.
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+ //
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+ // This is essentially a `memcpy()` optimized for throughput by transferring in chunks of T.
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+ let n = binary_size_in_byte / core::mem::size_of::<T>();
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+ for _ in 0..n {
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+ use core::ptr;
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+
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+ ptr::write_volatile::<T>(reloc_dst_addr, ptr::read_volatile::<T>(src_addr));
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+ reloc_dst_addr = reloc_dst_addr.offset(1);
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+ loop {
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+ core::ptr::write_volatile(reloc_destination_addr, core::ptr::read_volatile(src_addr));
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+ reloc_destination_addr = reloc_destination_addr.offset(1);
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+ src_addr = src_addr.offset(1);
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+ }
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+
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+ // Call `runtime_init()` through a trait object, causing the jump to use an absolute address to
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+ // reach the relocated binary. An elaborate explanation can be found in the `runtime_init.rs`
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+ // source comments.
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+ runtime_init::get().runtime_init()
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+}
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diff -uNr 06_drivers_gpio_uart/src/runtime_init.rs 07_uart_chainloader/src/runtime_init.rs
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--- 06_drivers_gpio_uart/src/runtime_init.rs
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+++ 07_uart_chainloader/src/runtime_init.rs
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@@ -7,9 +7,43 @@
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use crate::{bsp, memory};
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//--------------------------------------------------------------------------------------------------
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+// Private Definitions
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+//--------------------------------------------------------------------------------------------------
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+
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+struct Traitor;
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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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+/// We are outsmarting the compiler here by using a trait as a layer of indirection. Because we are
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+/// generating PIC code, a static dispatch to `init()` would generate a relative jump from the
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+/// callee to `init()`. However, when calling `init()`, code just finished copying the binary to the
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+/// actual link-time address, and hence is still running at whatever location the previous loader
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+/// has put it. So we do not want a relative jump, because it would not jump to the relocated code.
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+///
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+/// By indirecting through a trait object, we can make use of the property that vtables store
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+/// absolute addresses. So calling `init()` this way will kick execution to the relocated binary.
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+pub trait RunTimeInit {
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+ /// Equivalent to `crt0` or `c0` code in C/C++ world. Clears the `bss` section, then jumps to
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+ /// kernel 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 runtime_init(&self) -> ! {
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+ zero_bss();
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+
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+ crate::kernel_init()
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+ if reloc_destination_addr > reloc_end_addr_inclusive {
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+ break;
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+ }
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+ }
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+}
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+
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+//--------------------------------------------------------------------------------------------------
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// Private Code
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//--------------------------------------------------------------------------------------------------
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+impl RunTimeInit for Traitor {}
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+ let relocated_runtime_init_addr = runtime_init::runtime_init as *const () as usize + diff;
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+ let relocated_runtime_init: fn() -> ! =
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+ core::mem::transmute(relocated_runtime_init_addr as *const ());
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+
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/// Zero out the .bss section.
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///
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/// # Safety
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@@ -24,14 +58,7 @@
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// Public Code
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//--------------------------------------------------------------------------------------------------
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-/// Equivalent to `crt0` or `c0` code in C/C++ world. Clears the `bss` section, then jumps to kernel
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-/// 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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-pub unsafe fn runtime_init() -> ! {
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- zero_bss();
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-
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- crate::kernel_init()
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+/// Give the callee a `RunTimeInit` trait object.
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+pub fn get() -> &'static dyn RunTimeInit {
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+ &Traitor {}
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}
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+ relocated_runtime_init()
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+}
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```
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Andre Richter
4 years ago