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114 lines
3.6 KiB
Rust
114 lines
3.6 KiB
Rust
// SPDX-License-Identifier: MIT OR Apache-2.0
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//
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// Copyright (c) 2021-2022 Andre Richter <andre.o.richter@gmail.com>
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//! Architectural boot 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::boot::arch_boot
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use crate::{memory, memory::Address};
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use core::{
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arch::global_asm,
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sync::atomic::{compiler_fence, Ordering},
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};
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use cortex_a::{asm, registers::*};
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use tock_registers::interfaces::Writeable;
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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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// Private Code
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//--------------------------------------------------------------------------------------------------
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/// Prepares the transition from EL2 to EL1.
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///
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/// # Safety
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///
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/// - The `bss` section is not initialized yet. The code must not use or reference it in any way.
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/// - The HW state of EL1 must be prepared in a sound way.
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#[inline(always)]
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unsafe fn prepare_el2_to_el1_transition(
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virt_boot_core_stack_end_exclusive_addr: u64,
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virt_kernel_init_addr: u64,
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) {
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// Enable timer counter registers for EL1.
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CNTHCTL_EL2.write(CNTHCTL_EL2::EL1PCEN::SET + CNTHCTL_EL2::EL1PCTEN::SET);
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// No offset for reading the counters.
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CNTVOFF_EL2.set(0);
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// Set EL1 execution state to AArch64.
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HCR_EL2.write(HCR_EL2::RW::EL1IsAarch64);
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// Set up a simulated exception return.
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//
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// First, fake a saved program status where all interrupts were masked and SP_EL1 was used as a
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// stack pointer.
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SPSR_EL2.write(
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SPSR_EL2::D::Masked
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+ SPSR_EL2::A::Masked
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+ SPSR_EL2::I::Masked
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+ SPSR_EL2::F::Masked
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+ SPSR_EL2::M::EL1h,
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);
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// Second, let the link register point to kernel_init().
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ELR_EL2.set(virt_kernel_init_addr);
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// Set up SP_EL1 (stack pointer), which will be used by EL1 once we "return" to it. Since there
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// are no plans to ever return to EL2, just re-use the same stack.
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SP_EL1.set(virt_boot_core_stack_end_exclusive_addr);
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}
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/// Reset the backtrace by setting link register and frame pointer to zero.
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///
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/// # Safety
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///
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/// - This function must only be used immediately before entering EL1.
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#[inline(always)]
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unsafe fn prepare_backtrace_reset() {
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compiler_fence(Ordering::SeqCst);
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FP.set(0);
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LR.set(0);
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}
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//--------------------------------------------------------------------------------------------------
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// Public Code
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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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///
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/// # Safety
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///
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/// - Exception return from EL2 must must continue execution in EL1 with `kernel_init()`.
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#[no_mangle]
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pub unsafe extern "C" fn _start_rust(
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phys_kernel_tables_base_addr: u64,
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virt_boot_core_stack_end_exclusive_addr: u64,
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virt_kernel_init_addr: u64,
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) -> ! {
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prepare_el2_to_el1_transition(
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virt_boot_core_stack_end_exclusive_addr,
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virt_kernel_init_addr,
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);
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// Turn on the MMU for EL1.
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let addr = Address::new(phys_kernel_tables_base_addr as usize);
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memory::mmu::enable_mmu_and_caching(addr).unwrap();
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// Make the function we return to the root of a backtrace.
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prepare_backtrace_reset();
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// Use `eret` to "return" to EL1. Since virtual memory will already be enabled, this results in
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// execution of kernel_init() in EL1 from its _virtual address_.
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asm::eret()
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}
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