mango-boot 0.2.1

// SPDX-License-Identifier: MIT OR Apache-2.0
//
// Copyright (c) 2021-2023 Andre Richter <andre.o.richter@gmail.com>

//! Architectural boot code.
//!
//! # Orientation
//!
//! Since arch modules are imported into generic modules using the path attribute, the path of this
//! file is:
//!
//! crate::cpu::boot::arch_boot

use aarch64_cpu::{asm, registers::*};
use core::arch::global_asm;
use mango_hal::systems::rpi::{memory, memory::Address};
use tock_registers::interfaces::Writeable;

// Assembly counterpart to this file.
global_asm!(
    include_str!("boot.s"),
    CONST_CURRENTEL_EL2 = const 0x8,
    CONST_CORE_ID_MASK = const 0b11
);

//--------------------------------------------------------------------------------------------------
// Private Code
//--------------------------------------------------------------------------------------------------

/// Prepares the transition from EL2 to EL1.
///
/// # Safety
///
/// - The `bss` section is not initialized yet. The code must not use or reference it in any way.
/// - The HW state of EL1 must be prepared in a sound way.
#[inline(always)]
unsafe fn prepare_el2_to_el1_transition(
  virt_boot_core_stack_end_exclusive_addr: u64,
  virt_kernel_entry_addr: u64,
)
{
  // Enable timer counter registers for EL1.
  CNTHCTL_EL2.write(CNTHCTL_EL2::EL1PCEN::SET + CNTHCTL_EL2::EL1PCTEN::SET);

  // No offset for reading the counters.
  CNTVOFF_EL2.set(0);

  // Set EL1 execution state to AArch64.
  HCR_EL2.write(HCR_EL2::RW::EL1IsAarch64);

  // Set up a simulated exception return.
  //
  // First, fake a saved program status where all interrupts were masked and SP_EL1 was used as a
  // stack pointer.
  SPSR_EL2.write(
    SPSR_EL2::D::Masked
      + SPSR_EL2::A::Masked
      + SPSR_EL2::I::Masked
      + SPSR_EL2::F::Masked
      + SPSR_EL2::M::EL1h,
  );

  // Second, let the link register point to kernel_entry().
  ELR_EL2.set(virt_kernel_entry_addr);

  // Set up SP_EL1 (stack pointer), which will be used by EL1 once we "return" to it. Since there
  // are no plans to ever return to EL2, just re-use the same stack.
  SP_EL1.set(virt_boot_core_stack_end_exclusive_addr);
}

//--------------------------------------------------------------------------------------------------
// Public Code
//--------------------------------------------------------------------------------------------------

/// The Rust entry of the `kernel` binary.
///
/// The function is called from the assembly `_start` function.
///
/// # Safety
///
/// - Exception return from EL2 must must continue execution in EL1 with `kernel_entry()`.
#[no_mangle]
pub unsafe extern "C" fn _start_rust(
  phys_kernel_tables_base_addr: u64,
  virt_boot_core_stack_end_exclusive_addr: u64,
  virt_kernel_entry_addr: u64,
) -> !
{
  prepare_el2_to_el1_transition(
    virt_boot_core_stack_end_exclusive_addr,
    virt_kernel_entry_addr,
  );

  // Turn on the MMU for EL1.
  let addr = Address::new(phys_kernel_tables_base_addr as usize);
  memory::mmu::enable_mmu_and_caching(addr).unwrap();

  // Use `eret` to "return" to EL1. Since virtual memory will already be enabled, this results in
  // execution of kernel_entry() in EL1 from its _virtual address_.
  asm::eret()
}