kernel/

kernel.rs

// Licensed under the Apache License, Version 2.0 or the MIT License.
// SPDX-License-Identifier: Apache-2.0 OR MIT
// Copyright Tock Contributors 2022.

//! Tock's main kernel loop, scheduler loop, and Scheduler trait.
//!
//! This module also includes utility functions that are commonly used by
//! scheduler policy implementations. Scheduling policy (round robin, priority,
//! etc.) is defined in the `scheduler` subcrate and selected by a board.

use core::cell::Cell;
use core::num::NonZeroU32;

use crate::capabilities;
use crate::config;
use crate::debug;
use crate::deferred_call::DeferredCall;
use crate::errorcode::ErrorCode;
use crate::grant::{AllowRoSize, AllowRwSize, Grant, UpcallSize};
use crate::ipc;
use crate::memop;
use crate::platform::chip::Chip;
use crate::platform::mpu::MPU;
use crate::platform::platform::ContextSwitchCallback;
use crate::platform::platform::KernelResources;
use crate::platform::platform::{ProcessFault, SyscallDriverLookup, SyscallFilter};
use crate::platform::scheduler_timer::SchedulerTimer;
use crate::platform::watchdog::WatchDog;
use crate::process::{self, ProcessId, Task};
use crate::scheduler::{Scheduler, SchedulingDecision};
use crate::syscall::SyscallDriver;
use crate::syscall::{ContextSwitchReason, SyscallReturn};
use crate::syscall::{Syscall, YieldCall};
use crate::syscall_driver::CommandReturn;
use crate::upcall::{Upcall, UpcallId};
use crate::utilities::cells::NumericCellExt;

/// Threshold in microseconds to consider a process's timeslice to be exhausted.
/// That is, Tock will skip re-scheduling a process if its remaining timeslice
/// is less than this threshold.
pub(crate) const MIN_QUANTA_THRESHOLD_US: u32 = 500;

/// Main object for the kernel. Each board will need to create one.
pub struct Kernel {
    /// This holds a pointer to the static array of Process pointers.
    processes: &'static [Option<&'static dyn process::Process>],

    /// A counter which keeps track of how many process identifiers have been
    /// created. This is used to create new unique identifiers for processes.
    process_identifier_max: Cell<usize>,

    /// How many grant regions have been setup. This is incremented on every
    /// call to `create_grant()`. We need to explicitly track this so that when
    /// processes are created they can be allocated pointers for each grant.
    grant_counter: Cell<usize>,

    /// Flag to mark that grants have been finalized. This means that the kernel
    /// cannot support creating new grants because processes have already been
    /// created and the data structures for grants have already been
    /// established.
    grants_finalized: Cell<bool>,
}

/// Represents the different outcomes when trying to allocate a grant region
enum AllocResult {
    NoAllocation,
    NewAllocation,
    SameAllocation,
}

/// Tries to allocate the grant region for specified driver and process.
/// Returns if a new grant was allocated or not
fn try_allocate_grant(driver: &dyn SyscallDriver, process: &dyn process::Process) -> AllocResult {
    let before_count = process.grant_allocated_count().unwrap_or(0);
    match driver.allocate_grant(process.processid()).is_ok() {
        true if before_count == process.grant_allocated_count().unwrap_or(0) => {
            AllocResult::SameAllocation
        }
        true => AllocResult::NewAllocation,
        false => AllocResult::NoAllocation,
    }
}

impl Kernel {
    /// Create the kernel object that knows about the list of processes.
    ///
    /// Crucially, the processes included in the `processes` array MUST be valid
    /// to execute. Any credential checks or validation MUST happen before the
    /// `Process` object is included in this array.
    pub fn new(processes: &'static [Option<&'static dyn process::Process>]) -> Kernel {
        Kernel {
            processes,
            process_identifier_max: Cell::new(0),
            grant_counter: Cell::new(0),
            grants_finalized: Cell::new(false),
        }
    }

    /// Helper function that moves all non-generic portions of process_map_or
    /// into a non-generic function to reduce code bloat from monomorphization.
    pub(crate) fn get_process(&self, processid: ProcessId) -> Option<&dyn process::Process> {
        // We use the index in the [`ProcessId`] so we can do a direct lookup.
        // However, we are not guaranteed that the app still exists at that
        // index in the processes array. To avoid additional overhead, we do the
        // lookup and check here, rather than calling `.index()`.
        match self.processes.get(processid.index) {
            Some(Some(process)) => {
                // Check that the process stored here matches the identifier
                // in the `processid`.
                if process.processid() == processid {
                    Some(*process)
                } else {
                    None
                }
            }
            _ => None,
        }
    }

    /// Run a closure on a specific process if it exists. If the process with a
    /// matching [`ProcessId`] does not exist at the index specified within the
    /// [`ProcessId`], then `default` will be returned.
    ///
    /// A match will not be found if the process was removed (and there is a
    /// `None` in the process array), if the process changed its identifier
    /// (likely after being restarted), or if the process was moved to a
    /// different index in the processes array. Note that a match _will_ be
    /// found if the process still exists in the correct location in the array
    /// but is in any "stopped" state.
    pub(crate) fn process_map_or<F, R>(&self, default: R, processid: ProcessId, closure: F) -> R
    where
        F: FnOnce(&dyn process::Process) -> R,
    {
        match self.get_process(processid) {
            Some(process) => closure(process),
            None => default,
        }
    }

    /// Run a closure on a specific process if it exists. If the process with a
    /// matching `ProcessId` does not exist at the index specified within the
    /// `ProcessId`, then `default` will be returned.
    ///
    /// A match will not be found if the process was removed (and there is a
    /// `None` in the process array), if the process changed its identifier
    /// (likely after being restarted), or if the process was moved to a
    /// different index in the processes array. Note that a match _will_ be
    /// found if the process still exists in the correct location in the array
    /// but is in any "stopped" state.
    ///
    /// This is functionally the same as `process_map_or()`, but this method is
    /// available outside the kernel crate and requires a
    /// `ProcessManagementCapability` to use.
    pub fn process_map_or_external<F, R>(
        &self,
        default: R,
        processid: ProcessId,
        closure: F,
        _capability: &dyn capabilities::ProcessManagementCapability,
    ) -> R
    where
        F: FnOnce(&dyn process::Process) -> R,
    {
        match self.get_process(processid) {
            Some(process) => closure(process),
            None => default,
        }
    }

    /// Run a closure on every valid process. This will iterate the array of
    /// processes and call the closure on every process that exists.
    pub(crate) fn process_each<F>(&self, mut closure: F)
    where
        F: FnMut(&dyn process::Process),
    {
        for process in self.processes.iter() {
            match process {
                Some(p) => {
                    closure(*p);
                }
                None => {}
            }
        }
    }

    /// Returns an iterator over all processes loaded by the kernel.
    pub(crate) fn get_process_iter(
        &self,
    ) -> core::iter::FilterMap<
        core::slice::Iter<Option<&dyn process::Process>>,
        fn(&Option<&'static dyn process::Process>) -> Option<&'static dyn process::Process>,
    > {
        fn keep_some(
            &x: &Option<&'static dyn process::Process>,
        ) -> Option<&'static dyn process::Process> {
            x
        }
        self.processes.iter().filter_map(keep_some)
    }

    /// Run a closure on every valid process. This will iterate the array of
    /// processes and call the closure on every process that exists.
    ///
    /// This is functionally the same as `process_each()`, but this method is
    /// available outside the kernel crate and requires a
    /// `ProcessManagementCapability` to use.
    pub fn process_each_capability<F>(
        &'static self,
        _capability: &dyn capabilities::ProcessManagementCapability,
        mut closure: F,
    ) where
        F: FnMut(&dyn process::Process),
    {
        for process in self.processes.iter() {
            match process {
                Some(p) => {
                    closure(*p);
                }
                None => {}
            }
        }
    }

    /// Run a closure on every process, but only continue if the closure returns
    /// `None`. That is, if the closure returns any non-`None` value, iteration
    /// stops and the value is returned from this function to the called.
    pub(crate) fn process_until<T, F>(&self, closure: F) -> Option<T>
    where
        F: Fn(&dyn process::Process) -> Option<T>,
    {
        for process in self.processes.iter() {
            match process {
                Some(p) => {
                    let ret = closure(*p);
                    if ret.is_some() {
                        return ret;
                    }
                }
                None => {}
            }
        }
        None
    }

    /// Checks if the provided [`ProcessId`] is still valid given the processes
    /// stored in the processes array. Returns `true` if the ProcessId still
    /// refers to a valid process, and `false` if not.
    ///
    /// This is needed for `ProcessId` itself to implement the `.index()`
    /// command to verify that the referenced app is still at the correct index.
    pub(crate) fn processid_is_valid(&self, processid: &ProcessId) -> bool {
        self.processes
            .get(processid.index)
            .is_some_and(|p| p.is_some_and(|process| process.processid().id() == processid.id()))
    }

    /// Create a new grant. This is used in board initialization to setup grants
    /// that capsules use to interact with processes.
    ///
    /// Grants **must** only be created _before_ processes are initialized.
    /// Processes use the number of grants that have been allocated to correctly
    /// initialize the process's memory with a pointer for each grant. If a
    /// grant is created after processes are initialized this will panic.
    ///
    /// Calling this function is restricted to only certain users, and to
    /// enforce this calling this function requires the
    /// `MemoryAllocationCapability` capability.
    pub fn create_grant<
        T: Default,
        Upcalls: UpcallSize,
        AllowROs: AllowRoSize,
        AllowRWs: AllowRwSize,
    >(
        &'static self,
        driver_num: usize,
        _capability: &dyn capabilities::MemoryAllocationCapability,
    ) -> Grant<T, Upcalls, AllowROs, AllowRWs> {
        if self.grants_finalized.get() {
            panic!("Grants finalized. Cannot create a new grant.");
        }

        // Create and return a new grant.
        let grant_index = self.grant_counter.get();
        self.grant_counter.increment();
        Grant::new(self, driver_num, grant_index)
    }

    /// Returns the number of grants that have been setup in the system and
    /// marks the grants as "finalized". This means that no more grants can
    /// be created because data structures have been setup based on the number
    /// of grants when this function is called.
    ///
    /// In practice, this is called when processes are created, and the process
    /// memory is setup based on the number of current grants.
    pub(crate) fn get_grant_count_and_finalize(&self) -> usize {
        self.grants_finalized.set(true);
        self.grant_counter.get()
    }

    /// Returns the number of grants that have been setup in the system and
    /// marks the grants as "finalized". This means that no more grants can
    /// be created because data structures have been setup based on the number
    /// of grants when this function is called.
    ///
    /// In practice, this is called when processes are created, and the process
    /// memory is setup based on the number of current grants.
    ///
    /// This is exposed publicly, but restricted with a capability. The intent
    /// is that external implementations of `Process` need to be able to
    /// retrieve the final number of grants.
    pub fn get_grant_count_and_finalize_external(
        &self,
        _capability: &dyn capabilities::ExternalProcessCapability,
    ) -> usize {
        self.get_grant_count_and_finalize()
    }

    /// Create a new unique identifier for a process and return the identifier.
    ///
    /// Typically we just choose a larger number than we have used for any
    /// process before which ensures that the identifier is unique.
    pub(crate) fn create_process_identifier(&self) -> usize {
        self.process_identifier_max.get_and_increment()
    }

    /// Cause all apps to fault.
    ///
    /// This will call `set_fault_state()` on each app, causing the app to enter
    /// the state as if it had crashed (for example with an MPU violation). If
    /// the process is configured to be restarted it will be.
    ///
    /// Only callers with the `ProcessManagementCapability` can call this
    /// function. This restricts general capsules from being able to call this
    /// function, since capsules should not be able to arbitrarily restart all
    /// apps.
    pub fn hardfault_all_apps<C: capabilities::ProcessManagementCapability>(&self, _c: &C) {
        for p in self.processes.iter() {
            p.map(|process| {
                process.set_fault_state();
            });
        }
    }

    /// Perform one iteration of the core Tock kernel loop.
    ///
    /// This function is responsible for three main operations:
    ///
    /// 1. Check if the kernel itself has any work to be done and if the
    ///    scheduler wants to complete that work now. If so, it allows the
    ///    kernel to run.
    /// 2. Check if any processes have any work to be done, and if so if the
    ///    scheduler wants to allow any processes to run now, and if so which
    ///    one.
    /// 3. After ensuring the scheduler does not want to complete any kernel or
    ///    process work (or there is no work to be done), are there are no
    ///    outstanding interrupts to handle, put the chip to sleep.
    ///
    /// This function has one configuration option: `no_sleep`. If that argument
    /// is set to true, the kernel will never attempt to put the chip to sleep,
    /// and this function can be called again immediately.
    pub fn kernel_loop_operation<KR: KernelResources<C>, C: Chip, const NUM_PROCS: u8>(
        &self,
        resources: &KR,
        chip: &C,
        ipc: Option<&ipc::IPC<NUM_PROCS>>,
        no_sleep: bool,
        _capability: &dyn capabilities::MainLoopCapability,
    ) {
        let scheduler = resources.scheduler();

        resources.watchdog().tickle();
        unsafe {
            // Ask the scheduler if we should do tasks inside of the kernel,
            // such as handle interrupts. A scheduler may want to prioritize
            // processes instead, or there may be no kernel work to do.
            match scheduler.do_kernel_work_now(chip) {
                true => {
                    // Execute kernel work. This includes handling
                    // interrupts and is how code in the chips/ and capsules
                    // crates is able to execute.
                    scheduler.execute_kernel_work(chip);
                }
                false => {
                    // No kernel work ready, so ask scheduler for a process.
                    match scheduler.next() {
                        SchedulingDecision::RunProcess((processid, timeslice_us)) => {
                            self.process_map_or((), processid, |process| {
                                let (reason, time_executed) =
                                    self.do_process(resources, chip, process, ipc, timeslice_us);
                                scheduler.result(reason, time_executed);
                            });
                        }
                        SchedulingDecision::TrySleep => {
                            // For testing, it may be helpful to
                            // disable sleeping the chip in case
                            // the running test does not generate
                            // any interrupts.
                            if !no_sleep {
                                chip.atomic(|| {
                                    // Cannot sleep if interrupts are pending,
                                    // as on most platforms unhandled interrupts
                                    // will wake the device. Also, if the only
                                    // pending interrupt occurred after the
                                    // scheduler decided to put the chip to
                                    // sleep, but before this atomic section
                                    // starts, the interrupt will not be
                                    // serviced and the chip will never wake
                                    // from sleep.
                                    if !chip.has_pending_interrupts() && !DeferredCall::has_tasks()
                                    {
                                        resources.watchdog().suspend();
                                        chip.sleep();
                                        resources.watchdog().resume();
                                    }
                                });
                            }
                        }
                    }
                }
            }
        }
    }

    /// Main loop of the OS.
    ///
    /// Most of the behavior of this loop is controlled by the [`Scheduler`]
    /// implementation in use.
    pub fn kernel_loop<KR: KernelResources<C>, C: Chip, const NUM_PROCS: u8>(
        &self,
        resources: &KR,
        chip: &C,
        ipc: Option<&ipc::IPC<NUM_PROCS>>,
        capability: &dyn capabilities::MainLoopCapability,
    ) -> ! {
        resources.watchdog().setup();
        // Before we begin, verify that deferred calls were soundly setup.
        DeferredCall::verify_setup();
        loop {
            self.kernel_loop_operation(resources, chip, ipc, false, capability);
        }
    }

    /// Transfer control from the kernel to a userspace process.
    ///
    /// This function is called by the main kernel loop to run userspace code.
    /// Notably, system calls from processes are handled in the kernel, *by the
    /// kernel thread* in this function, and the syscall return value is set for
    /// the process immediately. Normally, a process is allowed to continue
    /// running after calling a syscall. However, the scheduler is given an out,
    /// as `do_process()` will check with the scheduler before re-executing the
    /// process to allow it to return from the syscall. If a process yields with
    /// no upcalls pending, exits, exceeds its timeslice, or is interrupted,
    /// then `do_process()` will return.
    ///
    /// Depending on the particular scheduler in use, this function may act in a
    /// few different ways. `scheduler.continue_process()` allows the scheduler
    /// to tell the Kernel whether to continue executing the process, or to
    /// return control to the scheduler as soon as a kernel task becomes ready
    /// (either a bottom half interrupt handler or dynamic deferred call), or to
    /// continue executing the userspace process until it reaches one of the
    /// aforementioned stopping conditions. Some schedulers may not require a
    /// scheduler timer; passing `None` for the timeslice will use a null
    /// scheduler timer even if the chip provides a real scheduler timer.
    /// Schedulers can pass a timeslice (in us) of their choice, though if the
    /// passed timeslice is smaller than `MIN_QUANTA_THRESHOLD_US` the process
    /// will not execute, and this function will return immediately.
    ///
    /// This function returns a tuple indicating the reason the reason this
    /// function has returned to the scheduler, and the amount of time the
    /// process spent executing (or `None` if the process was run
    /// cooperatively). Notably, time spent in this function by the kernel,
    /// executing system calls or merely setting up the switch to/from
    /// userspace, is charged to the process.
    fn do_process<KR: KernelResources<C>, C: Chip, const NUM_PROCS: u8>(
        &self,
        resources: &KR,
        chip: &C,
        process: &dyn process::Process,
        ipc: Option<&crate::ipc::IPC<NUM_PROCS>>,
        timeslice_us: Option<NonZeroU32>,
    ) -> (process::StoppedExecutingReason, Option<u32>) {
        // We must use a dummy scheduler timer if the process should be executed
        // without any timeslice restrictions. Note, a chip may not provide a
        // real scheduler timer implementation even if a timeslice is requested.
        let scheduler_timer: &dyn SchedulerTimer = if timeslice_us.is_none() {
            &() // dummy timer, no preemption
        } else {
            resources.scheduler_timer()
        };

        // Clear the scheduler timer and then start the counter. This starts the
        // process's timeslice. Since the kernel is still executing at this
        // point, the scheduler timer need not have an interrupt enabled after
        // `start()`.
        scheduler_timer.reset();
        if let Some(timeslice) = timeslice_us {
            scheduler_timer.start(timeslice)
        }

        // Need to track why the process is no longer executing so that we can
        // inform the scheduler.
        let mut return_reason = process::StoppedExecutingReason::NoWorkLeft;

        // Since the timeslice counts both the process's execution time and the
        // time spent in the kernel on behalf of the process (setting it up and
        // handling its syscalls), we intend to keep running the process until
        // it has no more work to do. We break out of this loop if the scheduler
        // no longer wants to execute this process or if it exceeds its
        // timeslice.
        loop {
            let stop_running = match scheduler_timer.get_remaining_us() {
                Some(us) => us.get() <= MIN_QUANTA_THRESHOLD_US,
                None => true,
            };
            if stop_running {
                // Process ran out of time while the kernel was executing.
                process.debug_timeslice_expired();
                return_reason = process::StoppedExecutingReason::TimesliceExpired;
                break;
            }

            // Check if the scheduler wishes to continue running this process.
            let continue_process = unsafe {
                resources
                    .scheduler()
                    .continue_process(process.processid(), chip)
            };
            if !continue_process {
                return_reason = process::StoppedExecutingReason::KernelPreemption;
                break;
            }

            // Check if this process is actually ready to run. If not, we don't
            // try to run it. This case can happen if a process faults and is
            // stopped, for example.
            if !process.ready() {
                return_reason = process::StoppedExecutingReason::NoWorkLeft;
                break;
            }

            match process.get_state() {
                process::State::Running => {
                    // Running means that this process expects to be running, so
                    // go ahead and set things up and switch to executing the
                    // process. Arming the scheduler timer instructs it to
                    // generate an interrupt when the timeslice has expired. The
                    // underlying timer is not affected.
                    resources
                        .context_switch_callback()
                        .context_switch_hook(process);
                    process.setup_mpu();
                    chip.mpu().enable_app_mpu();
                    scheduler_timer.arm();
                    let context_switch_reason = process.switch_to();
                    scheduler_timer.disarm();
                    chip.mpu().disable_app_mpu();

                    // Now the process has returned back to the kernel. Check
                    // why and handle the process as appropriate.
                    match context_switch_reason {
                        Some(ContextSwitchReason::Fault) => {
                            // The app faulted, check if the chip wants to
                            // handle the fault.
                            if resources
                                .process_fault()
                                .process_fault_hook(process)
                                .is_err()
                            {
                                // Let process deal with it as appropriate.
                                process.set_fault_state();
                            }
                        }
                        Some(ContextSwitchReason::SyscallFired { syscall }) => {
                            self.handle_syscall(resources, process, syscall);
                        }
                        Some(ContextSwitchReason::Interrupted) => {
                            if scheduler_timer.get_remaining_us().is_none() {
                                // This interrupt was a timeslice expiration.
                                process.debug_timeslice_expired();
                                return_reason = process::StoppedExecutingReason::TimesliceExpired;
                                break;
                            }
                            // Go to the beginning of loop to determine whether
                            // to break to handle the interrupt, continue
                            // executing this process, or switch to another
                            // process.
                            continue;
                        }
                        None => {
                            // Something went wrong when switching to this
                            // process. Indicate this by putting it in a fault
                            // state.
                            process.set_fault_state();
                        }
                    }
                }
                process::State::Yielded => {
                    // If the process is yielded or hasn't been started it is
                    // waiting for a upcall. If there is a task scheduled for
                    // this process go ahead and set the process to execute it.
                    match process.dequeue_task() {
                        None => break,
                        Some(cb) => match cb {
                            Task::ReturnValue(_) => {
                                // Per TRD104, Yield-Wait does not wake the
                                // process for events that generate Null
                                // Upcalls.
                                break;
                            }
                            Task::FunctionCall(ccb) => {
                                if config::CONFIG.trace_syscalls {
                                    debug!(
                                        "[{:?}] function_call @{:#x}({:#x}, {:#x}, {:#x}, {:#x})",
                                        process.processid(),
                                        ccb.pc,
                                        ccb.argument0,
                                        ccb.argument1,
                                        ccb.argument2,
                                        ccb.argument3,
                                    );
                                }
                                process.set_process_function(ccb);
                            }
                            Task::IPC((otherapp, ipc_type)) => {
                                ipc.map_or_else(
                                    || {
                                        panic!("Kernel consistency error: IPC Task with no IPC");
                                    },
                                    |ipc| {
                                        // TODO(alevy): this could error for a variety of reasons.
                                        // Should we communicate the error somehow?
                                        // https://github.com/tock/tock/issues/1993
                                        unsafe {
                                            let _ = ipc.schedule_upcall(
                                                process.processid(),
                                                otherapp,
                                                ipc_type,
                                            );
                                        }
                                    },
                                );
                            }
                        },
                    }
                }
                process::State::YieldedFor(upcall_id) => {
                    // If this process is waiting for a specific upcall, see if
                    // it is ready. If so, dequeue it and return its values to
                    // the process without scheduling the callback.
                    match process.remove_upcall(upcall_id) {
                        None => break,
                        Some(task) => {
                            let (a0, a1, a2) = match task {
                                // There is no callback function registered, we
                                // just return the values provided by the driver
                                Task::ReturnValue(rv) => {
                                    if config::CONFIG.trace_syscalls {
                                        debug!(
                                            "[{:?}] Yield-WaitFor: [NU] ({:#x}, {:#x}, {:#x})",
                                            process.processid(),
                                            rv.argument0,
                                            rv.argument1,
                                            rv.argument2,
                                        );
                                    }
                                    (rv.argument0, rv.argument1, rv.argument2)
                                }
                                // There is a registered callback function, but
                                // since the process used `Yield-WaitFor`, we do
                                // not execute it, we just return its arguments
                                // values to the application.
                                Task::FunctionCall(ccb) => {
                                    if config::CONFIG.trace_syscalls {
                                        debug!(
                                            "[{:?}] Yield-WaitFor [Suppressed function_call @{:#x}] ({:#x}, {:#x}, {:#x}, {:#x})",
                                            process.processid(),
                                            ccb.pc,
                                            ccb.argument0,
                                            ccb.argument1,
                                            ccb.argument2,
                                            ccb.argument3,
                                        );
                                    }
                                    (ccb.argument0, ccb.argument1, ccb.argument2)
                                }
                                Task::IPC(_) => todo!(),
                            };
                            process
                                .set_syscall_return_value(SyscallReturn::YieldWaitFor(a0, a1, a2));
                        }
                    }
                }
                process::State::Faulted | process::State::Terminated => {
                    // We should never be scheduling an unrunnable process.
                    // This is a potential security flaw: panic.
                    panic!("Attempted to schedule an unrunnable process");
                }
                process::State::Stopped(_) => {
                    return_reason = process::StoppedExecutingReason::Stopped;
                    break;
                }
            }
        }

        // Check how much time the process used while it was executing, and
        // return the value so we can provide it to the scheduler.
        let time_executed_us = timeslice_us.map(|timeslice| {
            // Note, we cannot call `.get_remaining_us()` again if it has
            // previously returned `None`, so we _must_ check the return reason
            // first.
            if return_reason == process::StoppedExecutingReason::TimesliceExpired {
                // used the whole timeslice
                timeslice.get()
            } else {
                match scheduler_timer.get_remaining_us() {
                    Some(remaining) => timeslice.get() - remaining.get(),
                    None => timeslice.get(), // used whole timeslice
                }
            }
        });

        // Reset the scheduler timer in case it unconditionally triggers
        // interrupts upon expiration. We do not want it to expire while the
        // chip is sleeping, for example.
        scheduler_timer.reset();

        (return_reason, time_executed_us)
    }

    /// Method to invoke a system call on a particular process. Applies the
    /// kernel system call filtering policy (if any). Handles `Yield` and
    /// `Exit`, dispatches `Memop` to `memop::memop`, and dispatches peripheral
    /// driver system calls to peripheral driver capsules through the platforms
    /// `with_driver` method.
    #[inline]
    fn handle_syscall<KR: KernelResources<C>, C: Chip>(
        &self,
        resources: &KR,
        process: &dyn process::Process,
        syscall: Syscall,
    ) {
        // Hook for process debugging.
        process.debug_syscall_called(syscall);

        // Enforce platform-specific syscall filtering here.
        //
        // Before continuing to handle non-yield syscalls the kernel first
        // checks if the platform wants to block that syscall for the process,
        // and if it does, sets a return value which is returned to the calling
        // process.
        //
        // Filtering a syscall (i.e. blocking the syscall from running) does not
        // cause the process to lose its timeslice. The error will be returned
        // immediately (assuming the process has not already exhausted its
        // timeslice) allowing the process to decide how to handle the error.
        match syscall {
            Syscall::Yield {
                which: _,
                param_a: _,
                param_b: _,
            } => {} // Yield is not filterable.
            Syscall::Exit {
                which: _,
                completion_code: _,
            } => {} // Exit is not filterable.
            Syscall::Memop {
                operand: _,
                arg0: _,
            } => {} // Memop is not filterable.
            _ => {
                // Check all other syscalls for filtering.
                if let Err(response) = resources.syscall_filter().filter_syscall(process, &syscall)
                {
                    process.set_syscall_return_value(SyscallReturn::Failure(response));

                    if config::CONFIG.trace_syscalls {
                        debug!(
                            "[{:?}] Filtered: {:?} was rejected with {:?}",
                            process.processid(),
                            syscall,
                            response
                        );
                    }

                    return;
                }
            }
        }

        // Handle each of the syscalls.
        match syscall {
            Syscall::Memop { operand, arg0 } => {
                let rval = memop::memop(process, operand, arg0);
                if config::CONFIG.trace_syscalls {
                    debug!(
                        "[{:?}] memop({}, {:#x}) = {:?}",
                        process.processid(),
                        operand,
                        arg0,
                        rval
                    );
                }
                process.set_syscall_return_value(rval);
            }
            Syscall::Yield {
                which,
                param_a,
                param_b,
            } => {
                if config::CONFIG.trace_syscalls {
                    debug!("[{:?}] yield. which: {}", process.processid(), which);
                }
                match which.try_into() {
                    Ok(YieldCall::NoWait) => {
                        // If this is a `Yield-WaitFor` AND there are no pending
                        // tasks, then return immediately. Otherwise, go into
                        // the yielded state and execute tasks now or when they
                        // arrive.
                        let has_tasks = process.has_tasks();

                        // Set the "did I trigger upcalls" flag.
                        // If address is invalid does nothing.
                        //
                        // # Safety
                        //
                        // This is fine as long as no references to the
                        // process's memory exist. We do not have a reference,
                        // so we can safely call `set_byte()`.
                        unsafe {
                            let address = param_a as *mut u8;
                            process.set_byte(address, has_tasks as u8);
                        }

                        if has_tasks {
                            process.set_yielded_state();
                        }
                    }

                    Ok(YieldCall::Wait) => {
                        process.set_yielded_state();
                    }

                    Ok(YieldCall::WaitFor) => {
                        let upcall_id = UpcallId {
                            driver_num: param_a,
                            subscribe_num: param_b,
                        };
                        process.set_yielded_for_state(upcall_id);
                    }

                    _ => {
                        // Only 0, 1, and 2 are valid, so this is not a valid
                        // yield system call, Yield does not have a return value
                        // because it can push a function call onto the stack;
                        // just return control to the process.
                    }
                }
            }
            Syscall::Subscribe { driver_number, .. }
            | Syscall::Command { driver_number, .. }
            | Syscall::ReadWriteAllow { driver_number, .. }
            | Syscall::UserspaceReadableAllow { driver_number, .. }
            | Syscall::ReadOnlyAllow { driver_number, .. } => {
                resources
                .syscall_driver_lookup()
                .with_driver(driver_number, |driver| match syscall {
                    Syscall::Subscribe {
                        driver_number,
                        subdriver_number,
                        upcall_ptr,
                        appdata,
                    } => {
                        // A upcall is identified as a tuple of the driver
                        // number and the subdriver number.
                        let upcall_id = UpcallId {
                            driver_num: driver_number,
                            subscribe_num: subdriver_number,
                        };

                        // TODO: when the compiler supports capability types
                        // bring this back as a NonNull
                        // type. https://github.com/tock/tock/issues/4134.
                        //
                        // Previously, we had a NonNull type (that had a niche)
                        // here, and could wrap that in Option to fill the niche
                        // and handle the Null case. CapabilityPtr is filling
                        // the gap left by * const(), which does not have the
                        // niche and allows NULL internally. Having a CHERI
                        // capability type with a niche is (maybe?) predicated
                        // on having better compiler support.
                        // Option<NonNull<()>> is preferable here, and it should
                        // go back to it just as soon as we can express "non
                        // null capability". For now, checking for the null case
                        // is handled internally in each `map_or` call.
                        //
                        //First check if `upcall_ptr` is null. A null
                        //`upcall_ptr` will result in `None` here and
                        //represents the special "unsubscribe" operation.
                        //let ptr = NonNull::new(upcall_ptr);

                        // For convenience create an `Upcall` type now. This is
                        // just a data structure and doesn't do any checking or
                        // conversion.
                        let upcall = Upcall::new(process.processid(), upcall_id, appdata, upcall_ptr);

                        // If `ptr` is not null, we must first verify that the
                        // upcall function pointer is within process accessible
                        // memory. Per TRD104:
                        //
                        // > If the passed upcall is not valid (is outside
                        // > process executable memory...), the kernel...MUST
                        // > immediately return a failure with a error code of
                        // > `INVALID`.
                        let rval1 = upcall_ptr.map_or(None, |upcall_ptr_nonnull| {
                            if !process.is_valid_upcall_function_pointer(upcall_ptr_nonnull.as_ptr()) {
                                Some(ErrorCode::INVAL)
                            } else {
                                None
                            }
                        });

                        // If the upcall is either null or valid, then we
                        // continue handling the upcall.
                        let rval = match rval1 {
                            Some(err) => upcall.into_subscribe_failure(err),
                            None => {
                                match driver {
                                    Some(driver) => {
                                        // At this point we must save the new
                                        // upcall and return the old. The
                                        // upcalls are stored by the core kernel
                                        // in the grant region so we can
                                        // guarantee a correct upcall swap.
                                        // However, we do need help with
                                        // initially allocating the grant if
                                        // this driver has never been used
                                        // before.
                                        //
                                        // To avoid the overhead with checking
                                        // for process liveness and grant
                                        // allocation, we assume the grant is
                                        // initially allocated. If it turns out
                                        // it isn't we ask the capsule to
                                        // allocate the grant.
                                        match crate::grant::subscribe(process, upcall) {
                                            Ok(upcall) => upcall.into_subscribe_success(),
                                            Err((upcall, err @ ErrorCode::NOMEM)) => {
                                                // If we get a memory error, we
                                                // always try to allocate the
                                                // grant since this could be the
                                                // first time the grant is
                                                // getting accessed.
                                                match try_allocate_grant(driver, process) {
                                                    AllocResult::NewAllocation => {
                                                        // Now we try again. It
                                                        // is possible that the
                                                        // capsule did not
                                                        // actually allocate the
                                                        // grant, at which point
                                                        // this will fail again
                                                        // and we return an
                                                        // error to userspace.
                                                        match crate::grant::subscribe(
                                                            process, upcall,
                                                        ) {
                                                            // An Ok() returns
                                                            // the previous
                                                            // upcall, while
                                                            // Err() returns the
                                                            // one that was just
                                                            // passed.
                                                            Ok(upcall) => {
                                                                upcall.into_subscribe_success()
                                                            }
                                                            Err((upcall, err)) => {
                                                                upcall.into_subscribe_failure(err)
                                                            }
                                                        }
                                                    }
                                                    alloc_failure => {
                                                        // We didn't actually
                                                        // create a new alloc,
                                                        // so just error.
                                                        match (
                                                            config::CONFIG.trace_syscalls,
                                                            alloc_failure,
                                                        ) {
                                                            (true, AllocResult::NoAllocation) => {
                                                                debug!("[{:?}] WARN driver #{:x} did not allocate grant",
                                                                           process.processid(), driver_number);
                                                            }
                                                            (true, AllocResult::SameAllocation) => {
                                                                debug!("[{:?}] ERROR driver #{:x} allocated wrong grant counts",
                                                                           process.processid(), driver_number);
                                                            }
                                                            _ => {}
                                                        }
                                                        upcall.into_subscribe_failure(err)
                                                    }
                                                }
                                            }
                                            Err((upcall, err)) => {
                                                upcall.into_subscribe_failure(err)
                                            }
                                        }
                                    }
                                    None => upcall.into_subscribe_failure(ErrorCode::NODEVICE),
                                }
                            }
                        };

                        // Per TRD104, we only clear upcalls if the subscribe
                        // will return success. At this point we know the result
                        // and clear if necessary.
                        if rval.is_success() {
                            // Only one upcall should exist per tuple. To ensure
                            // that there are no pending upcalls with the same
                            // identifier but with the old function pointer, we
                            // clear them now.
                            let _ =process.remove_pending_upcalls(upcall_id);
                        }

                        if config::CONFIG.trace_syscalls {
                            debug!(
                                "[{:?}] subscribe({:#x}, {}, @{:#x}, {:#x}) = {:?}",
                                process.processid(),
                                driver_number,
                                subdriver_number,
                                upcall_ptr,
                                appdata,
                                rval
                            );
                        }

                        process.set_syscall_return_value(rval);
                    }
                    Syscall::Command {
                        driver_number,
                        subdriver_number,
                        arg0,
                        arg1,
                    } => {
                        let cres = match driver {
                            Some(d) => d.command(subdriver_number, arg0, arg1, process.processid()),
                            None => CommandReturn::failure(ErrorCode::NODEVICE),
                        };

                        let res = SyscallReturn::from_command_return(cres);

                        if config::CONFIG.trace_syscalls {
                            debug!(
                                "[{:?}] cmd({:#x}, {}, {:#x}, {:#x}) = {:?}",
                                process.processid(),
                                driver_number,
                                subdriver_number,
                                arg0,
                                arg1,
                                res,
                            );
                        }
                        process.set_syscall_return_value(res);
                    }
                    Syscall::ReadWriteAllow {
                        driver_number,
                        subdriver_number,
                        allow_address,
                        allow_size,
                    } => {
                        let res = match driver {
                            Some(driver) => {
                                // Try to create an appropriate
                                // [`ReadWriteProcessBuffer`]. This method will
                                // ensure that the memory in question is located
                                // in the process-accessible memory space.
                                match process
                                    .build_readwrite_process_buffer(allow_address, allow_size)
                                {
                                    Ok(rw_pbuf) => {
                                        // Creating the
                                        // [`ReadWriteProcessBuffer`] worked,
                                        // try to set in grant.
                                        match crate::grant::allow_rw(
                                            process,
                                            driver_number,
                                            subdriver_number,
                                            rw_pbuf,
                                        ) {
                                            Ok(rw_pbuf) => {
                                                let (ptr, len) = rw_pbuf.consume();
                                                SyscallReturn::AllowReadWriteSuccess(ptr, len)
                                            }
                                            Err((rw_pbuf, err @ ErrorCode::NOMEM)) => {
                                                // If we get a memory error, we
                                                // always try to allocate the
                                                // grant since this could be the
                                                // first time the grant is
                                                // getting accessed.
                                                match try_allocate_grant(driver, process) {
                                                    AllocResult::NewAllocation => {
                                                        // If we actually
                                                        // allocated a new
                                                        // grant, try again and
                                                        // honor the result.
                                                        match crate::grant::allow_rw(
                                                            process,
                                                            driver_number,
                                                            subdriver_number,
                                                            rw_pbuf,
                                                        ) {
                                                            Ok(rw_pbuf) => {
                                                                let (ptr, len) = rw_pbuf.consume();
                                                                SyscallReturn::AllowReadWriteSuccess(
                                                                    ptr, len,
                                                                )
                                                            }
                                                            Err((rw_pbuf, err)) => {
                                                                let (ptr, len) = rw_pbuf.consume();
                                                                SyscallReturn::AllowReadWriteFailure(
                                                                    err, ptr, len,
                                                                )
                                                            }
                                                        }
                                                    }
                                                    alloc_failure => {
                                                        // We didn't actually
                                                        // create a new alloc,
                                                        // so just error.
                                                        match (
                                                            config::CONFIG.trace_syscalls,
                                                            alloc_failure,
                                                        ) {
                                                            (true, AllocResult::NoAllocation) => {
                                                                debug!("[{:?}] WARN driver #{:x} did not allocate grant",
                                                                           process.processid(), driver_number);
                                                            }
                                                            (true, AllocResult::SameAllocation) => {
                                                                debug!("[{:?}] ERROR driver #{:x} allocated wrong grant counts",
                                                                           process.processid(), driver_number);
                                                            }
                                                            _ => {}
                                                        }
                                                        let (ptr, len) = rw_pbuf.consume();
                                                        SyscallReturn::AllowReadWriteFailure(
                                                            err, ptr, len,
                                                        )
                                                    }
                                                }
                                            }
                                            Err((rw_pbuf, err)) => {
                                                let (ptr, len) = rw_pbuf.consume();
                                                SyscallReturn::AllowReadWriteFailure(err, ptr, len)
                                            }
                                        }
                                    }
                                    Err(allow_error) => {
                                        // There was an error creating the
                                        // [`ReadWriteProcessBuffer`]. Report
                                        // back to the process with the original
                                        // parameters.
                                        SyscallReturn::AllowReadWriteFailure(
                                            allow_error,
                                            allow_address,
                                            allow_size,
                                        )
                                    }
                                }
                            }
                            None => SyscallReturn::AllowReadWriteFailure(
                                ErrorCode::NODEVICE,
                                allow_address,
                                allow_size,
                            ),
                        };

                        if config::CONFIG.trace_syscalls {
                            debug!(
                                "[{:?}] read-write allow({:#x}, {}, @{:#x}, {}) = {:?}",
                                process.processid(),
                                driver_number,
                                subdriver_number,
                                allow_address as usize,
                                allow_size,
                                res
                            );
                        }
                        process.set_syscall_return_value(res);
                    }
                    Syscall::UserspaceReadableAllow {
                        driver_number,
                        subdriver_number,
                        allow_address,
                        allow_size,
                    } => {
                        let res = match driver {
                            Some(d) => {
                                // Try to create an appropriate
                                // [`UserspaceReadableProcessBuffer`]. This
                                // method will ensure that the memory in
                                // question is located in the process-accessible
                                // memory space.
                                match process
                                    .build_readwrite_process_buffer(allow_address, allow_size)
                                {
                                    Ok(rw_pbuf) => {
                                        // Creating the
                                        // [`UserspaceReadableProcessBuffer`]
                                        // worked, provide it to the capsule.
                                        match d.allow_userspace_readable(
                                            process.processid(),
                                            subdriver_number,
                                            rw_pbuf,
                                        ) {
                                            Ok(returned_pbuf) => {
                                                // The capsule has accepted the
                                                // allow operation. Pass the
                                                // previous buffer information
                                                // back to the process.
                                                let (ptr, len) = returned_pbuf.consume();
                                                SyscallReturn::UserspaceReadableAllowSuccess(
                                                    ptr, len,
                                                )
                                            }
                                            Err((rejected_pbuf, err)) => {
                                                // The capsule has rejected the
                                                // allow operation. Pass the new
                                                // buffer information back to
                                                // the process.
                                                let (ptr, len) = rejected_pbuf.consume();
                                                SyscallReturn::UserspaceReadableAllowFailure(
                                                    err, ptr, len,
                                                )
                                            }
                                        }
                                    }
                                    Err(allow_error) => {
                                        // There was an error creating the
                                        // [`UserspaceReadableProcessBuffer`].
                                        // Report back to the process.
                                        SyscallReturn::UserspaceReadableAllowFailure(
                                            allow_error,
                                            allow_address,
                                            allow_size,
                                        )
                                    }
                                }
                            }

                            None => SyscallReturn::UserspaceReadableAllowFailure(
                                ErrorCode::NODEVICE,
                                allow_address,
                                allow_size,
                            ),
                        };

                        if config::CONFIG.trace_syscalls {
                            debug!(
                                "[{:?}] userspace readable allow({:#x}, {}, @{:#x}, {}) = {:?}",
                                process.processid(),
                                driver_number,
                                subdriver_number,
                                allow_address as usize,
                                allow_size,
                                res
                            );
                        }
                        process.set_syscall_return_value(res);
                    }
                    Syscall::ReadOnlyAllow {
                        driver_number,
                        subdriver_number,
                        allow_address,
                        allow_size,
                    } => {
                        let res = match driver {
                            Some(driver) => {
                                // Try to create an appropriate
                                // [`ReadOnlyProcessBuffer`]. This method will
                                // ensure that the memory in question is located
                                // in the process-accessible memory space.
                                match process
                                    .build_readonly_process_buffer(allow_address, allow_size)
                                {
                                    Ok(ro_pbuf) => {
                                        // Creating the
                                        // [`ReadOnlyProcessBuffer`] worked, try
                                        // to set in grant.
                                        match crate::grant::allow_ro(
                                            process,
                                            driver_number,
                                            subdriver_number,
                                            ro_pbuf,
                                        ) {
                                            Ok(ro_pbuf) => {
                                                let (ptr, len) = ro_pbuf.consume();
                                                SyscallReturn::AllowReadOnlySuccess(ptr, len)
                                            }
                                            Err((ro_pbuf, err @ ErrorCode::NOMEM)) => {
                                                // If we get a memory error, we
                                                // always try to allocate the
                                                // grant since this could be the
                                                // first time the grant is
                                                // getting accessed.
                                                match try_allocate_grant(driver, process) {
                                                    AllocResult::NewAllocation => {
                                                        // If we actually
                                                        // allocated a new
                                                        // grant, try again and
                                                        // honor the result.
                                                        match crate::grant::allow_ro(
                                                            process,
                                                            driver_number,
                                                            subdriver_number,
                                                            ro_pbuf,
                                                        ) {
                                                            Ok(ro_pbuf) => {
                                                                let (ptr, len) = ro_pbuf.consume();
                                                                SyscallReturn::AllowReadOnlySuccess(
                                                                    ptr, len,
                                                                )
                                                            }
                                                            Err((ro_pbuf, err)) => {
                                                                let (ptr, len) = ro_pbuf.consume();
                                                                SyscallReturn::AllowReadOnlyFailure(
                                                                    err, ptr, len,
                                                                )
                                                            }
                                                        }
                                                    }
                                                    alloc_failure => {
                                                        // We didn't actually
                                                        // create a new alloc,
                                                        // so just error.
                                                        match (
                                                            config::CONFIG.trace_syscalls,
                                                            alloc_failure,
                                                        ) {
                                                            (true, AllocResult::NoAllocation) => {
                                                                debug!("[{:?}] WARN driver #{:x} did not allocate grant",
                                                                           process.processid(), driver_number);
                                                            }
                                                            (true, AllocResult::SameAllocation) => {
                                                                debug!("[{:?}] ERROR driver #{:x} allocated wrong grant counts",
                                                                           process.processid(), driver_number);
                                                            }
                                                            _ => {}
                                                        }
                                                        let (ptr, len) = ro_pbuf.consume();
                                                        SyscallReturn::AllowReadOnlyFailure(
                                                            err, ptr, len,
                                                        )
                                                    }
                                                }
                                            }
                                            Err((ro_pbuf, err)) => {
                                                let (ptr, len) = ro_pbuf.consume();
                                                SyscallReturn::AllowReadOnlyFailure(err, ptr, len)
                                            }
                                        }
                                    }
                                    Err(allow_error) => {
                                        // There was an error creating the
                                        // [`ReadOnlyProcessBuffer`]. Report
                                        // back to the process with the original
                                        // parameters.
                                        SyscallReturn::AllowReadOnlyFailure(
                                            allow_error,
                                            allow_address,
                                            allow_size,
                                        )
                                    }
                                }
                            }
                            None => SyscallReturn::AllowReadOnlyFailure(
                                ErrorCode::NODEVICE,
                                allow_address,
                                allow_size,
                            ),
                        };

                        if config::CONFIG.trace_syscalls {
                            debug!(
                                "[{:?}] read-only allow({:#x}, {}, @{:#x}, {}) = {:?}",
                                process.processid(),
                                driver_number,
                                subdriver_number,
                                allow_address as usize,
                                allow_size,
                                res
                            );
                        }

                        process.set_syscall_return_value(res);
                    }
                    Syscall::Yield { .. }
                    | Syscall::Exit { .. }
                    | Syscall::Memop { .. } => {
                        // These variants must not be reachable due to the outer
                        // match statement:
                        debug_assert!(false, "Kernel system call handling invariant violated!");
                    },
                })
            }
            Syscall::Exit {
                which,
                completion_code,
            } => {
                // exit try restart modifies the ID of the process.
                let old_process_id = process.processid();
                let optional_return_value = match which {
                    // The process called the `exit-terminate` system call.
                    0 => {
                        process.terminate(Some(completion_code as u32));
                        None
                    }
                    // The process called the `exit-restart` system call.
                    1 => {
                        process.try_restart(Some(completion_code as u32));
                        None
                    }
                    // The process called an invalid variant of the Exit
                    // system call class.
                    _ => {
                        let return_value = SyscallReturn::Failure(ErrorCode::NOSUPPORT);
                        process.set_syscall_return_value(return_value);
                        Some(return_value)
                    }
                };
                if config::CONFIG.trace_syscalls {
                    debug!(
                        "[{:?}] exit(which: {}, completion_code: {}) = {:?}",
                        old_process_id, which, completion_code, optional_return_value,
                    );
                }
            }
        }
    }
}