serenity/Kernel/Prekernel/Arch/aarch64/init.cpp

/*
 * Copyright (c) 2021, Nico Weber <thakis@chromium.org>
 * Copyright (c) 2021, Marcin Undak <mcinek@gmail.com>
 *
 * SPDX-License-Identifier: BSD-2-Clause
 */

#include <AK/Types.h>
#include <Kernel/Prekernel/Arch/aarch64/Aarch64_asm_utils.h>
#include <Kernel/Prekernel/Arch/aarch64/Mailbox.h>
#include <Kernel/Prekernel/Arch/aarch64/Timer.h>
#include <Kernel/Prekernel/Arch/aarch64/UART.h>

extern "C" [[noreturn]] void halt();
extern "C" [[noreturn]] void init();
extern "C" [[noreturn]] void os_start();

static void set_up_el1_mode();
static void set_up_el2_mode();
static void set_up_el3_mode();
static void print_current_exception_level(const char* msg);
[[noreturn]] static void jump_to_os_start_from_el2();
[[noreturn]] static void jump_to_os_start_from_el3();

extern "C" [[noreturn]] void init()
{
    auto& uart = Prekernel::UART::the();

    uart.print_str("\r\nWelcome to Serenity OS!\r\n");
    uart.print_str("Imagine this being your ideal operating system.\r\n");
    uart.print_str("Observed deviations from that ideal are shortcomings of your imagination.\r\n\r\n");

    u32 firmware_version = Prekernel::Mailbox::query_firmware_version();
    uart.print_str("Firmware version: ");
    uart.print_num(firmware_version);
    uart.print_str("\r\n");

    print_current_exception_level("CPU started in:");

    set_up_el2_mode();
    set_up_el1_mode();

    auto current_exception_level = get_current_exception_level();
    switch (current_exception_level) {
    case 2:
        jump_to_os_start_from_el2();
        break;
    case 3:
        set_up_el3_mode();
        jump_to_os_start_from_el3();
        break;
    default:
        uart.print_str("FATAL: CPU booted in unsupported exception mode!\r\n");
        halt();
    }
}

extern "C" [[noreturn]] void os_start()
{
    auto& uart = Prekernel::UART::the();

    print_current_exception_level("CPU switched to:");

    auto& timer = Prekernel::Timer::the();
    u64 start_musec = 0;
    for (;;) {
        u64 now_musec;
        while ((now_musec = timer.microseconds_since_boot()) - start_musec < 1'000'000)
            ;
        start_musec = now_musec;
        uart.print_str("Timer: ");
        uart.print_num(now_musec);
        uart.print_str("\r\n");
    }
}

// FIXME: Share this with the Intel Prekernel.
extern size_t __stack_chk_guard;
size_t __stack_chk_guard;
extern "C" [[noreturn]] void __stack_chk_fail();

[[noreturn]] void halt()
{
    for (;;) {
        asm volatile("wfi");
    }
}

void __stack_chk_fail()
{
    halt();
}

[[noreturn]] void __assertion_failed(char const*, char const*, unsigned int, char const*)
{
    halt();
}

static void set_up_el1_mode()
{
    Kernel::Aarch64_SCTLR_EL1 system_control_register_el1 = {};

    // Those bits are reserved on ARMv8.0
    system_control_register_el1.LSMAOE = 1;
    system_control_register_el1.nTLSMD = 1;
    system_control_register_el1.SPAN = 1;
    system_control_register_el1.IESB = 1;

    // Don't trap access to CTR_EL0
    system_control_register_el1.UCT = 1;

    // Don't trap WFE instructions
    system_control_register_el1.nTWE = 1;

    // Don't trap WFI instructions
    system_control_register_el1.nTWI = 1;

    // Don't trap DC ZVA instructions
    system_control_register_el1.DZE = 1;

    // Don't trap access to DAIF (debugging) flags of EFLAGS register
    system_control_register_el1.UMA = 1;

    // Enable stack access alignment check for EL0
    system_control_register_el1.SA0 = 1;

    // Enable stack access alignment check for EL1
    system_control_register_el1.SA = 1;

    // Enable memory access alignment check
    system_control_register_el1.A = 1;

    // Set the register
    asm("msr sctlr_el1, %[value]" ::[value] "r"(system_control_register_el1));
}

static void set_up_el2_mode()
{
    Kernel::Aarch64_HCR_EL2 hypervisor_configuration_register_el2 = {};

    // EL1 to use 64-bit mode
    hypervisor_configuration_register_el2.RW = 1;

    // Set the register
    asm("msr hcr_el2, %[value]" ::[value] "r"(hypervisor_configuration_register_el2));
}

static void set_up_el3_mode()
{
    Kernel::Aarch64_SCR_EL3 secure_configuration_register_el3 = {};

    // Don't trap access to Counter-timer Physical Secure registers
    secure_configuration_register_el3.ST = 1;

    // Lower level to use Aarch64
    secure_configuration_register_el3.RW = 1;

    // Enable Hypervisor instructions at all levels
    secure_configuration_register_el3.HCE = 1;

    // Set the register
    asm("msr scr_el3, %[value]" ::[value] "r"(secure_configuration_register_el3));
}

[[noreturn]] static void jump_to_os_start_from_el2()
{
    // Processor state to set when returned from this function (in new EL1 world)
    Kernel::Aarch64_SPSR_EL2 saved_program_status_register_el2 = {};

    // Mask (disable) all interrupts
    saved_program_status_register_el2.A = 1;
    saved_program_status_register_el2.I = 1;
    saved_program_status_register_el2.F = 1;

    // Indicate EL1 as exception origin mode (so we go back there)
    saved_program_status_register_el2.M = Kernel::Aarch64_SPSR_EL2::Mode::EL1h;

    // Set the register
    asm("msr spsr_el2, %[value]" ::[value] "r"(saved_program_status_register_el2));

    // This will jump into os_start()
    return_from_el2();
}

[[noreturn]] static void jump_to_os_start_from_el3()
{
    // Processor state to set when returned from this function (in new EL1 world)
    Kernel::Aarch64_SPSR_EL3 saved_program_status_register_el3 = {};

    // Mask (disable) all interrupts
    saved_program_status_register_el3.A = 1;
    saved_program_status_register_el3.I = 1;
    saved_program_status_register_el3.F = 1;

    // Indicate EL1 as exception origin mode (so we go back there)
    saved_program_status_register_el3.M = Kernel::Aarch64_SPSR_EL3::Mode::EL1h;

    // Set the register
    asm("msr spsr_el3, %[value]" ::[value] "r"(saved_program_status_register_el3));

    // This will jump into os_start() below
    return_from_el3();
}

static void print_current_exception_level(const char* msg)
{
    auto& uart = Prekernel::UART::the();

    auto exception_level = get_current_exception_level();
    uart.print_str(msg);
    uart.print_str(" EL");
    uart.print_num(exception_level);
    uart.print_str("\r\n");
}