RTC driver

The QEMU aarch64 virt machine has a PL031 real-time clock at 0x9010000. For this exercise, you should write a driver for it.

1. Use it to print the current time to the serial console. You can use the chrono crate for date/time formatting.
2. Use the match register and raw interrupt status to busy-wait until a given time, e.g. 3 seconds in the future. (Call core::hint::spin_loop inside the loop.)
3. Extension if you have time: Enable and handle the interrupt generated by the RTC match. You can use the driver provided in the arm-gic crate to configure the Arm Generic Interrupt Controller.
   • Use the RTC interrupt, which is wired to the GIC as IntId::spi(2).
   • Once the interrupt is enabled, you can put the core to sleep via arm_gic::wfi(), which will cause the core to sleep until it receives an interrupt.

Download the exercise template and look in the rtc directory for the following files.

src/main.rs:

#![no_main]
#![no_std]

mod exceptions;
mod logger;
mod pl011;

use crate::pl011::Uart;
use arm_gic::gicv3::GicV3;
use core::panic::PanicInfo;
use log::{error, info, trace, LevelFilter};
use smccc::psci::system_off;
use smccc::Hvc;

/// Base addresses of the GICv3.
const GICD_BASE_ADDRESS: *mut u64 = 0x800_0000 as _;
const GICR_BASE_ADDRESS: *mut u64 = 0x80A_0000 as _;

/// Base address of the primary PL011 UART.
const PL011_BASE_ADDRESS: *mut u32 = 0x900_0000 as _;

#[no_mangle]
extern "C" fn main(x0: u64, x1: u64, x2: u64, x3: u64) {
    // Safe because `PL011_BASE_ADDRESS` is the base address of a PL011 device,
    // and nothing else accesses that address range.
    let uart = unsafe { Uart::new(PL011_BASE_ADDRESS) };
    logger::init(uart, LevelFilter::Trace).unwrap();

    info!("main({:#x}, {:#x}, {:#x}, {:#x})", x0, x1, x2, x3);

    // Safe because `GICD_BASE_ADDRESS` and `GICR_BASE_ADDRESS` are the base
    // addresses of a GICv3 distributor and redistributor respectively, and
    // nothing else accesses those address ranges.
    let mut gic = unsafe { GicV3::new(GICD_BASE_ADDRESS, GICR_BASE_ADDRESS) };
    gic.setup();

    // TODO: Create instance of RTC driver and print current time.

    // TODO: Wait for 3 seconds.

    system_off::<Hvc>().unwrap();
}

#[panic_handler]
fn panic(info: &PanicInfo) -> ! {
    error!("{info}");
    system_off::<Hvc>().unwrap();
    loop {}
}

src/exceptions.rs (you should only need to change this for the 3rd part of the exercise):

#![allow(unused)]
fn main() {
// Copyright 2023 Google LLC
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//      http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

use arm_gic::gicv3::GicV3;
use log::{error, info, trace};
use smccc::psci::system_off;
use smccc::Hvc;

#[no_mangle]
extern "C" fn sync_exception_current(_elr: u64, _spsr: u64) {
    error!("sync_exception_current");
    system_off::<Hvc>().unwrap();
}

#[no_mangle]
extern "C" fn irq_current(_elr: u64, _spsr: u64) {
    trace!("irq_current");
    let intid =
        GicV3::get_and_acknowledge_interrupt().expect("No pending interrupt");
    info!("IRQ {intid:?}");
}

#[no_mangle]
extern "C" fn fiq_current(_elr: u64, _spsr: u64) {
    error!("fiq_current");
    system_off::<Hvc>().unwrap();
}

#[no_mangle]
extern "C" fn serr_current(_elr: u64, _spsr: u64) {
    error!("serr_current");
    system_off::<Hvc>().unwrap();
}

#[no_mangle]
extern "C" fn sync_lower(_elr: u64, _spsr: u64) {
    error!("sync_lower");
    system_off::<Hvc>().unwrap();
}

#[no_mangle]
extern "C" fn irq_lower(_elr: u64, _spsr: u64) {
    error!("irq_lower");
    system_off::<Hvc>().unwrap();
}

#[no_mangle]
extern "C" fn fiq_lower(_elr: u64, _spsr: u64) {
    error!("fiq_lower");
    system_off::<Hvc>().unwrap();
}

#[no_mangle]
extern "C" fn serr_lower(_elr: u64, _spsr: u64) {
    error!("serr_lower");
    system_off::<Hvc>().unwrap();
}
}

src/logger.rs (you shouldn't need to change this):

#![allow(unused)]
fn main() {
// Copyright 2023 Google LLC
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//      http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

// ANCHOR: main
use crate::pl011::Uart;
use core::fmt::Write;
use log::{LevelFilter, Log, Metadata, Record, SetLoggerError};
use spin::mutex::SpinMutex;

static LOGGER: Logger = Logger { uart: SpinMutex::new(None) };

struct Logger {
    uart: SpinMutex<Option<Uart>>,
}

impl Log for Logger {
    fn enabled(&self, _metadata: &Metadata) -> bool {
        true
    }

    fn log(&self, record: &Record) {
        writeln!(
            self.uart.lock().as_mut().unwrap(),
            "[{}] {}",
            record.level(),
            record.args()
        )
        .unwrap();
    }

    fn flush(&self) {}
}

/// Initialises UART logger.
pub fn init(uart: Uart, max_level: LevelFilter) -> Result<(), SetLoggerError> {
    LOGGER.uart.lock().replace(uart);

    log::set_logger(&LOGGER)?;
    log::set_max_level(max_level);
    Ok(())
}
}

src/pl011.rs (you shouldn't need to change this):

#![allow(unused)]
fn main() {
// Copyright 2023 Google LLC
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//      http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

#![allow(unused)]

use core::fmt::{self, Write};
use core::ptr::{addr_of, addr_of_mut};

// ANCHOR: Flags
use bitflags::bitflags;

bitflags! {
    /// Flags from the UART flag register.
    #[repr(transparent)]
    #[derive(Copy, Clone, Debug, Eq, PartialEq)]
    struct Flags: u16 {
        /// Clear to send.
        const CTS = 1 << 0;
        /// Data set ready.
        const DSR = 1 << 1;
        /// Data carrier detect.
        const DCD = 1 << 2;
        /// UART busy transmitting data.
        const BUSY = 1 << 3;
        /// Receive FIFO is empty.
        const RXFE = 1 << 4;
        /// Transmit FIFO is full.
        const TXFF = 1 << 5;
        /// Receive FIFO is full.
        const RXFF = 1 << 6;
        /// Transmit FIFO is empty.
        const TXFE = 1 << 7;
        /// Ring indicator.
        const RI = 1 << 8;
    }
}
// ANCHOR_END: Flags

bitflags! {
    /// Flags from the UART Receive Status Register / Error Clear Register.
    #[repr(transparent)]
    #[derive(Copy, Clone, Debug, Eq, PartialEq)]
    struct ReceiveStatus: u16 {
        /// Framing error.
        const FE = 1 << 0;
        /// Parity error.
        const PE = 1 << 1;
        /// Break error.
        const BE = 1 << 2;
        /// Overrun error.
        const OE = 1 << 3;
    }
}

// ANCHOR: Registers
#[repr(C, align(4))]
struct Registers {
    dr: u16,
    _reserved0: [u8; 2],
    rsr: ReceiveStatus,
    _reserved1: [u8; 19],
    fr: Flags,
    _reserved2: [u8; 6],
    ilpr: u8,
    _reserved3: [u8; 3],
    ibrd: u16,
    _reserved4: [u8; 2],
    fbrd: u8,
    _reserved5: [u8; 3],
    lcr_h: u8,
    _reserved6: [u8; 3],
    cr: u16,
    _reserved7: [u8; 3],
    ifls: u8,
    _reserved8: [u8; 3],
    imsc: u16,
    _reserved9: [u8; 2],
    ris: u16,
    _reserved10: [u8; 2],
    mis: u16,
    _reserved11: [u8; 2],
    icr: u16,
    _reserved12: [u8; 2],
    dmacr: u8,
    _reserved13: [u8; 3],
}
// ANCHOR_END: Registers

// ANCHOR: Uart
/// Driver for a PL011 UART.
#[derive(Debug)]
pub struct Uart {
    registers: *mut Registers,
}

impl Uart {
    /// Constructs a new instance of the UART driver for a PL011 device at the
    /// given base address.
    ///
    /// # Safety
    ///
    /// The given base address must point to the MMIO control registers of a
    /// PL011 device, which must be mapped into the address space of the process
    /// as device memory and not have any other aliases.
    pub unsafe fn new(base_address: *mut u32) -> Self {
        Self { registers: base_address as *mut Registers }
    }

    /// Writes a single byte to the UART.
    pub fn write_byte(&self, byte: u8) {
        // Wait until there is room in the TX buffer.
        while self.read_flag_register().contains(Flags::TXFF) {}

        // Safe because we know that self.registers points to the control
        // registers of a PL011 device which is appropriately mapped.
        unsafe {
            // Write to the TX buffer.
            addr_of_mut!((*self.registers).dr).write_volatile(byte.into());
        }

        // Wait until the UART is no longer busy.
        while self.read_flag_register().contains(Flags::BUSY) {}
    }

    /// Reads and returns a pending byte, or `None` if nothing has been
    /// received.
    pub fn read_byte(&self) -> Option<u8> {
        if self.read_flag_register().contains(Flags::RXFE) {
            None
        } else {
            let data = unsafe { addr_of!((*self.registers).dr).read_volatile() };
            // TODO: Check for error conditions in bits 8-11.
            Some(data as u8)
        }
    }

    fn read_flag_register(&self) -> Flags {
        // Safe because we know that self.registers points to the control
        // registers of a PL011 device which is appropriately mapped.
        unsafe { addr_of!((*self.registers).fr).read_volatile() }
    }
}
// ANCHOR_END: Uart

impl Write for Uart {
    fn write_str(&mut self, s: &str) -> fmt::Result {
        for c in s.as_bytes() {
            self.write_byte(*c);
        }
        Ok(())
    }
}

// Safe because it just contains a pointer to device memory, which can be
// accessed from any context.
unsafe impl Send for Uart {}
}

Cargo.toml (you shouldn't need to change this):

[workspace]

[package]
name = "rtc"
version = "0.1.0"
edition = "2021"
publish = false

[dependencies]
arm-gic = "0.1.0"
bitflags = "2.4.2"
chrono = { version = "0.4.24", default-features = false }
log = "0.4.17"
smccc = "0.1.1"
spin = "0.9.8"

[build-dependencies]
cc = "1.0.73"

build.rs (you shouldn't need to change this):

// Copyright 2023 Google LLC
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//      http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

use cc::Build;
use std::env;

fn main() {
    #[cfg(target_os = "linux")]
    env::set_var("CROSS_COMPILE", "aarch64-linux-gnu");
    #[cfg(not(target_os = "linux"))]
    env::set_var("CROSS_COMPILE", "aarch64-none-elf");

    Build::new()
        .file("entry.S")
        .file("exceptions.S")
        .file("idmap.S")
        .compile("empty")
}

entry.S (you shouldn't need to change this):

/*
 * Copyright 2023 Google LLC
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     https://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

.macro adr_l, reg:req, sym:req
	adrp \reg, \sym
	add \reg, \reg, :lo12:\sym
.endm

.macro mov_i, reg:req, imm:req
	movz \reg, :abs_g3:\imm
	movk \reg, :abs_g2_nc:\imm
	movk \reg, :abs_g1_nc:\imm
	movk \reg, :abs_g0_nc:\imm
.endm

.set .L_MAIR_DEV_nGnRE,	0x04
.set .L_MAIR_MEM_WBWA,	0xff
.set .Lmairval, .L_MAIR_DEV_nGnRE | (.L_MAIR_MEM_WBWA << 8)

/* 4 KiB granule size for TTBR0_EL1. */
.set .L_TCR_TG0_4KB, 0x0 << 14
/* 4 KiB granule size for TTBR1_EL1. */
.set .L_TCR_TG1_4KB, 0x2 << 30
/* Disable translation table walk for TTBR1_EL1, generating a translation fault instead. */
.set .L_TCR_EPD1, 0x1 << 23
/* Translation table walks for TTBR0_EL1 are inner sharable. */
.set .L_TCR_SH_INNER, 0x3 << 12
/*
 * Translation table walks for TTBR0_EL1 are outer write-back read-allocate write-allocate
 * cacheable.
 */
.set .L_TCR_RGN_OWB, 0x1 << 10
/*
 * Translation table walks for TTBR0_EL1 are inner write-back read-allocate write-allocate
 * cacheable.
 */
.set .L_TCR_RGN_IWB, 0x1 << 8
/* Size offset for TTBR0_EL1 is 2**39 bytes (512 GiB). */
.set .L_TCR_T0SZ_512, 64 - 39
.set .Ltcrval, .L_TCR_TG0_4KB | .L_TCR_TG1_4KB | .L_TCR_EPD1 | .L_TCR_RGN_OWB
.set .Ltcrval, .Ltcrval | .L_TCR_RGN_IWB | .L_TCR_SH_INNER | .L_TCR_T0SZ_512

/* Stage 1 instruction access cacheability is unaffected. */
.set .L_SCTLR_ELx_I, 0x1 << 12
/* SP alignment fault if SP is not aligned to a 16 byte boundary. */
.set .L_SCTLR_ELx_SA, 0x1 << 3
/* Stage 1 data access cacheability is unaffected. */
.set .L_SCTLR_ELx_C, 0x1 << 2
/* EL0 and EL1 stage 1 MMU enabled. */
.set .L_SCTLR_ELx_M, 0x1 << 0
/* Privileged Access Never is unchanged on taking an exception to EL1. */
.set .L_SCTLR_EL1_SPAN, 0x1 << 23
/* SETEND instruction disabled at EL0 in aarch32 mode. */
.set .L_SCTLR_EL1_SED, 0x1 << 8
/* Various IT instructions are disabled at EL0 in aarch32 mode. */
.set .L_SCTLR_EL1_ITD, 0x1 << 7
.set .L_SCTLR_EL1_RES1, (0x1 << 11) | (0x1 << 20) | (0x1 << 22) | (0x1 << 28) | (0x1 << 29)
.set .Lsctlrval, .L_SCTLR_ELx_M | .L_SCTLR_ELx_C | .L_SCTLR_ELx_SA | .L_SCTLR_EL1_ITD | .L_SCTLR_EL1_SED
.set .Lsctlrval, .Lsctlrval | .L_SCTLR_ELx_I | .L_SCTLR_EL1_SPAN | .L_SCTLR_EL1_RES1

/**
 * This is a generic entry point for an image. It carries out the operations required to prepare the
 * loaded image to be run. Specifically, it zeroes the bss section using registers x25 and above,
 * prepares the stack, enables floating point, and sets up the exception vector. It preserves x0-x3
 * for the Rust entry point, as these may contain boot parameters.
 */
.section .init.entry, "ax"
.global entry
entry:
	/* Load and apply the memory management configuration, ready to enable MMU and caches. */
	adrp x30, idmap
	msr ttbr0_el1, x30

	mov_i x30, .Lmairval
	msr mair_el1, x30

	mov_i x30, .Ltcrval
	/* Copy the supported PA range into TCR_EL1.IPS. */
	mrs x29, id_aa64mmfr0_el1
	bfi x30, x29, #32, #4

	msr tcr_el1, x30

	mov_i x30, .Lsctlrval

	/*
	 * Ensure everything before this point has completed, then invalidate any potentially stale
	 * local TLB entries before they start being used.
	 */
	isb
	tlbi vmalle1
	ic iallu
	dsb nsh
	isb

	/*
	 * Configure sctlr_el1 to enable MMU and cache and don't proceed until this has completed.
	 */
	msr sctlr_el1, x30
	isb

	/* Disable trapping floating point access in EL1. */
	mrs x30, cpacr_el1
	orr x30, x30, #(0x3 << 20)
	msr cpacr_el1, x30
	isb

	/* Zero out the bss section. */
	adr_l x29, bss_begin
	adr_l x30, bss_end
0:	cmp x29, x30
	b.hs 1f
	stp xzr, xzr, [x29], #16
	b 0b

1:	/* Prepare the stack. */
	adr_l x30, boot_stack_end
	mov sp, x30

	/* Set up exception vector. */
	adr x30, vector_table_el1
	msr vbar_el1, x30

	/* Call into Rust code. */
	bl main

	/* Loop forever waiting for interrupts. */
2:	wfi
	b 2b

exceptions.S (you shouldn't need to change this):

/*
 * Copyright 2023 Google LLC
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     https://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

/**
 * Saves the volatile registers onto the stack. This currently takes 14
 * instructions, so it can be used in exception handlers with 18 instructions
 * left.
 *
 * On return, x0 and x1 are initialised to elr_el2 and spsr_el2 respectively,
 * which can be used as the first and second arguments of a subsequent call.
 */
.macro save_volatile_to_stack
	/* Reserve stack space and save registers x0-x18, x29 & x30. */
	stp x0, x1, [sp, #-(8 * 24)]!
	stp x2, x3, [sp, #8 * 2]
	stp x4, x5, [sp, #8 * 4]
	stp x6, x7, [sp, #8 * 6]
	stp x8, x9, [sp, #8 * 8]
	stp x10, x11, [sp, #8 * 10]
	stp x12, x13, [sp, #8 * 12]
	stp x14, x15, [sp, #8 * 14]
	stp x16, x17, [sp, #8 * 16]
	str x18, [sp, #8 * 18]
	stp x29, x30, [sp, #8 * 20]

	/*
	 * Save elr_el1 & spsr_el1. This such that we can take nested exception
	 * and still be able to unwind.
	 */
	mrs x0, elr_el1
	mrs x1, spsr_el1
	stp x0, x1, [sp, #8 * 22]
.endm

/**
 * Restores the volatile registers from the stack. This currently takes 14
 * instructions, so it can be used in exception handlers while still leaving 18
 * instructions left; if paired with save_volatile_to_stack, there are 4
 * instructions to spare.
 */
.macro restore_volatile_from_stack
	/* Restore registers x2-x18, x29 & x30. */
	ldp x2, x3, [sp, #8 * 2]
	ldp x4, x5, [sp, #8 * 4]
	ldp x6, x7, [sp, #8 * 6]
	ldp x8, x9, [sp, #8 * 8]
	ldp x10, x11, [sp, #8 * 10]
	ldp x12, x13, [sp, #8 * 12]
	ldp x14, x15, [sp, #8 * 14]
	ldp x16, x17, [sp, #8 * 16]
	ldr x18, [sp, #8 * 18]
	ldp x29, x30, [sp, #8 * 20]

	/* Restore registers elr_el1 & spsr_el1, using x0 & x1 as scratch. */
	ldp x0, x1, [sp, #8 * 22]
	msr elr_el1, x0
	msr spsr_el1, x1

	/* Restore x0 & x1, and release stack space. */
	ldp x0, x1, [sp], #8 * 24
.endm

/**
 * This is a generic handler for exceptions taken at the current EL while using
 * SP0. It behaves similarly to the SPx case by first switching to SPx, doing
 * the work, then switching back to SP0 before returning.
 *
 * Switching to SPx and calling the Rust handler takes 16 instructions. To
 * restore and return we need an additional 16 instructions, so we can implement
 * the whole handler within the allotted 32 instructions.
 */
.macro current_exception_sp0 handler:req
	msr spsel, #1
	save_volatile_to_stack
	bl \handler
	restore_volatile_from_stack
	msr spsel, #0
	eret
.endm

/**
 * This is a generic handler for exceptions taken at the current EL while using
 * SPx. It saves volatile registers, calls the Rust handler, restores volatile
 * registers, then returns.
 *
 * This also works for exceptions taken from EL0, if we don't care about
 * non-volatile registers.
 *
 * Saving state and jumping to the Rust handler takes 15 instructions, and
 * restoring and returning also takes 15 instructions, so we can fit the whole
 * handler in 30 instructions, under the limit of 32.
 */
.macro current_exception_spx handler:req
	save_volatile_to_stack
	bl \handler
	restore_volatile_from_stack
	eret
.endm

.section .text.vector_table_el1, "ax"
.global vector_table_el1
.balign 0x800
vector_table_el1:
sync_cur_sp0:
	current_exception_sp0 sync_exception_current

.balign 0x80
irq_cur_sp0:
	current_exception_sp0 irq_current

.balign 0x80
fiq_cur_sp0:
	current_exception_sp0 fiq_current

.balign 0x80
serr_cur_sp0:
	current_exception_sp0 serr_current

.balign 0x80
sync_cur_spx:
	current_exception_spx sync_exception_current

.balign 0x80
irq_cur_spx:
	current_exception_spx irq_current

.balign 0x80
fiq_cur_spx:
	current_exception_spx fiq_current

.balign 0x80
serr_cur_spx:
	current_exception_spx serr_current

.balign 0x80
sync_lower_64:
	current_exception_spx sync_lower

.balign 0x80
irq_lower_64:
	current_exception_spx irq_lower

.balign 0x80
fiq_lower_64:
	current_exception_spx fiq_lower

.balign 0x80
serr_lower_64:
	current_exception_spx serr_lower

.balign 0x80
sync_lower_32:
	current_exception_spx sync_lower

.balign 0x80
irq_lower_32:
	current_exception_spx irq_lower

.balign 0x80
fiq_lower_32:
	current_exception_spx fiq_lower

.balign 0x80
serr_lower_32:
	current_exception_spx serr_lower

idmap.S (you shouldn't need to change this):

/*
 * Copyright 2023 Google LLC
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     https://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

.set .L_TT_TYPE_BLOCK, 0x1
.set .L_TT_TYPE_PAGE,  0x3
.set .L_TT_TYPE_TABLE, 0x3

/* Access flag. */
.set .L_TT_AF, 0x1 << 10
/* Not global. */
.set .L_TT_NG, 0x1 << 11
.set .L_TT_XN, 0x3 << 53

.set .L_TT_MT_DEV, 0x0 << 2			// MAIR #0 (DEV_nGnRE)
.set .L_TT_MT_MEM, (0x1 << 2) | (0x3 << 8)	// MAIR #1 (MEM_WBWA), inner shareable

.set .L_BLOCK_DEV, .L_TT_TYPE_BLOCK | .L_TT_MT_DEV | .L_TT_AF | .L_TT_XN
.set .L_BLOCK_MEM, .L_TT_TYPE_BLOCK | .L_TT_MT_MEM | .L_TT_AF | .L_TT_NG

.section ".rodata.idmap", "a", %progbits
.global idmap
.align 12
idmap:
	/* level 1 */
	.quad		.L_BLOCK_DEV | 0x0		    // 1 GiB of device mappings
	.quad		.L_BLOCK_MEM | 0x40000000	// 1 GiB of DRAM
	.fill		254, 8, 0x0			// 254 GiB of unmapped VA space
	.quad		.L_BLOCK_DEV | 0x4000000000 // 1 GiB of device mappings
	.fill		255, 8, 0x0			// 255 GiB of remaining VA space

image.ld (you shouldn't need to change this):

/*
 * Copyright 2023 Google LLC
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     https://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

/*
 * Code will start running at this symbol which is placed at the start of the
 * image.
 */
ENTRY(entry)

MEMORY
{
	image : ORIGIN = 0x40080000, LENGTH = 2M
}

SECTIONS
{
	/*
	 * Collect together the code.
	 */
	.init : ALIGN(4096) {
		text_begin = .;
		*(.init.entry)
		*(.init.*)
	} >image
	.text : {
		*(.text.*)
	} >image
	text_end = .;

	/*
	 * Collect together read-only data.
	 */
	.rodata : ALIGN(4096) {
		rodata_begin = .;
		*(.rodata.*)
	} >image
	.got : {
		*(.got)
	} >image
	rodata_end = .;

	/*
	 * Collect together the read-write data including .bss at the end which
	 * will be zero'd by the entry code.
	 */
	.data : ALIGN(4096) {
		data_begin = .;
		*(.data.*)
		/*
		 * The entry point code assumes that .data is a multiple of 32
		 * bytes long.
		 */
		. = ALIGN(32);
		data_end = .;
	} >image

	/* Everything beyond this point will not be included in the binary. */
	bin_end = .;

	/* The entry point code assumes that .bss is 16-byte aligned. */
	.bss : ALIGN(16)  {
		bss_begin = .;
		*(.bss.*)
		*(COMMON)
		. = ALIGN(16);
		bss_end = .;
	} >image

	.stack (NOLOAD) : ALIGN(4096) {
		boot_stack_begin = .;
		. += 40 * 4096;
		. = ALIGN(4096);
		boot_stack_end = .;
	} >image

	. = ALIGN(4K);
	PROVIDE(dma_region = .);

	/*
	 * Remove unused sections from the image.
	 */
	/DISCARD/ : {
		/* The image loads itself so doesn't need these sections. */
		*(.gnu.hash)
		*(.hash)
		*(.interp)
		*(.eh_frame_hdr)
		*(.eh_frame)
		*(.note.gnu.build-id)
	}
}

Makefile (you shouldn't need to change this):

# Copyright 2023 Google LLC
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#      http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.

UNAME := $(shell uname -s)
ifeq ($(UNAME),Linux)
	TARGET = aarch64-linux-gnu
else
	TARGET = aarch64-none-elf
endif
OBJCOPY = $(TARGET)-objcopy

.PHONY: build qemu_minimal qemu qemu_logger

all: rtc.bin

build:
	cargo build

rtc.bin: build
	$(OBJCOPY) -O binary target/aarch64-unknown-none/debug/rtc $@

qemu: rtc.bin
	qemu-system-aarch64 -machine virt,gic-version=3 -cpu max -serial mon:stdio -display none -kernel $< -s

clean:
	cargo clean
	rm -f *.bin

.cargo/config.toml (you shouldn't need to change this):

[build]
target = "aarch64-unknown-none"
rustflags = ["-C", "link-arg=-Timage.ld"]

Run the code in QEMU with make qemu.