# Syscalls in Xous

Syscalls enable communication between processes, as well as communication to the kernel.  These are guaranteed to never change, but new syscalls may be added.

Syscalls may take up to seven `usize`-bit arguments, and may return up to seven `usize`-bit output operands, plus a tag indicating success or failure.

## Syscalls on RISC-V

RISC-V specifies eight registers as `argument` registers: `$a0`-`$a7`.  When performing a syscall, the following convention is used:

| Register | Usage (Calling) |
| -------- | -------------- |
| a0       | Arg 1 |
| a1       | Arg 2 |
| a2       | Arg 3 |
| a3       | Arg 4 |
| a4       | Arg 5 |
| a5       | Arg 6 |
| a6       | Arg 7 |
| a7       | Syscall Number |

When returning from the syscall, these registers have the following meaning:

| Register | Usage (Return) |
| -------- | -------------- |
| a0       | Return type tag |
| a1       | Arg 1 |
| a2       | Arg 2 |
| a3       | Arg 3 |
| a4       | Arg 4 |
| a5       | Arg 5 |
| a6       | Arg 6 |
| a7       | Arg 7 |

Note that this means that there is a hard limit on the number of arguments that can be passed.

## List of syscalls

Actual system calls function names are all prefixed with `sys_`.  However, when referred to generically, omit this prefix.  For example, prefer `client_send` over `sys_client_send`.

Calls that may be made from an Interrupt context begin with `sysi_` (or, more generally,
with interrupts disabled).

```rust

use core::num::NonZeroUsize;
type MemoryAddress = NonZeroUsize;
type MemorySize = NonZeroUsize;
type StackPointer = usize;
type MessageId = usize;

enum XousError {
    BadAlignment,
    BadAddress,
    OutOfMemory,
    InvalidString,
    ServerExists,
    ServerNotFound,
    ProcessNotFound,
    ProcessNotChild,
    ProcessTerminated,
    Timeout,
}

struct XousContext {
    stack: StackPointer,
    pid: XousPid,
}

struct XousMemoryMessage {
    id: MessageId,
    in_buf: Option<MemoryAddress>,
    in_buf_size: Option<MemorySize>,
    out_buf: Option<MemoryAddress>,
    out_buf_size: Option<MemorySize>,
}

struct XousScalarMessage {
    id: MessageId,
    arg1: usize,
    arg2: usize,
    arg3: usize,
    arg4: usize,
}

enum XousMessage {
    Memory(XousMemoryMessage),
    Scalar(XousScalarMessage),
}

struct XousMessageReceived {
    sender: XousMessageSender,
    message: XousMessage,
}

/// Allocates kernel structures for a new process, and returns the new PID.
/// This removes `page_count` page tables from the calling process at `origin_address`
/// and places them at `target_address`.
///
/// If the process was created successfully, then the new PID is returned to
/// the calling process.  The child is not automatically scheduled for running.
///
/// # Errors
///
/// * **BadAlignment**: `origin_address` or `target_address` were not page-aligned,
///                   or `address_size` was not a multiple of the page address size.
/// * **OutOfMemory**: The kernel couldn't allocate memory for the new process.
fn sys_process_spawn(origin_address: MemoryAddress,
                     target_address: MemoryAddress,
                     address_size: MemorySize) -> Result<XousPid, XousError>;

/// Pauses execution of the current thread and returns execution to the parent
/// process.  This function may return at any time in the future, including immediately.
fn sys_process_yield();

/// Interrupts the current process and returns control to the parent process.
///
/// # Errors
///
/// * **ProcessNotFound**: The provided PID doesn't exist, or is not running on the given CPU.
fn sysi_process_suspend(pid: XousPid, cpu_id: XousCpuId) -> Result<(), XousError>;

/// Claims an interrupt, and calls the given function
/// in an interrupt context.
fn sys_interrupt_claim<F>(irq: usize, f: F)  -> Result<(), XousError>
    where F: Fn(usize);

/// Resumes a process using the given stack pointer.  A parent could use
/// this function to implement multi-threading inside a child process, or
/// to create a task switcher.
///
/// To resume a process exactly where it left off, set `stack_pointer` to `None`.
/// This would be done in a very simple system that has no threads.
///
/// By default, at most three context switches can be made before the quantum
/// expires.  To enable more, pass `additional_contexts`.
///
/// If no more contexts are available when one is required, then the child
/// automatically relinquishes its quantum.
///
/// # Returns
///
/// When this function returns, it provides a list of the processes and
/// stack pointers that are ready to be run.  Three can fit as return values,
/// and additional context switches will be supplied in the slice of context
/// switches, if one is provided.
///
/// # Examples
///
/// If a process called `yield()`, or if its quantum expired normally, then
/// a single context is returned: The target thread, and its stack pointer.
///
/// If the child process called `client_send()` and ended up blocking due to
/// the server not being ready, then this would return no context switches.
/// This thread or process should not be scheduled to run.
///
/// If the child called `client_send()` and the server was ready, then the
/// server process would be run immediately.  If the child process' quantum
/// expired while the server was running, then this function would return
/// a single context containing the PID of the server, and the stack pointer.
///
/// If the child called `client_send()` and the server was ready, then the
/// server process would be run immediately.  If the server then finishes,
/// execution flow is returned to the child process.  If the quantum then
/// expires, this would return two contexts: the server's PID and its stack
/// pointer when it called `client_reply()`, and the child's PID with its
/// current stack pointer.
///
/// If the server in turn called another server, and both servers ended up
/// returning to the child before the quantum expired, then there would be
/// three contexts on the stack.
///
/// # Errors
///
/// * **ProcessNotFound**: The requested process does not exist
/// * **ProcessNotChild**: The given process was not a child process, and
///                        therefore couldn't be resumed.
/// * **ProcessTerminated**: The process has crashed.
fn sys_process_resume(process_id: XousPid, stack_pointer: Option<usize>,
                      additional_contexts: &Option<&[XousContext]>) ->
                Result<(Option<XousContext>,
                        Option<XousContext>,
                        Option<XousContext>),
                XousError>;

/// Causes a process to terminate immediately.
///
/// It is recommended that this function only be called on processes that
/// have cleaned up after themselves, e.g. shut down any servers and
/// flushed any file descriptors.
///
/// # Errors
///
/// * **ProcessNotFound**: The requested process does not exist
/// * **ProcessNotChild**: The requested process is not our child process
fn sys_process_terminate(process_id: XousPid) -> Result<(), XousError>;

/// Allocates pages of memory, equal to a total of `size
/// bytes.  If a physical address is specified, then this
/// can be used to allocate regions such as memory-mapped I/O.
/// If a virtual address is specified, then the returned
/// pages are located at that address.  Otherwise, they
/// are located at an unspecified offset.
///
/// # Errors
///
/// * **BadAlignment**: Either the physical or virtual addresses aren't page-aligned, or the size isn't a multiple of the page width.
/// * **OutOfMemory**: A contiguous chunk of memory couldn't be found, or the system's memory size has been exceeded.
fn sys_memory_allocate(physical: Option<MemoryAddress>,
                       virtual: Option<MemoryAddress>,
                       size: MemorySize) ->
                       Result<MemoryAddress, XousError>;

/// Equivalent to the Unix `sbrk` call.  Adjusts the
/// heap size to be equal to the specified value.  Heap
/// sizes start out at 0 bytes in new processes.
///
/// # Errors
///
/// * **OutOfMemory**: The region couldn't be extended.
fn sys_heap_resize(size: MemorySize) -> Result<(), XousError>;

///! Message Passing Functions

/// Create a new server with the given name.  This enables other processes to
/// connect to this server to send messages.  Only one server name may exist
/// on a system at a time.
///
/// # Errors
///
/// * **ServerExists**: A server has already registered with that name
/// * **InvalidString**: The name was not a valid UTF-8 string
fn sys_server_create(server_name: usize) -> Result<XousSid, XousError>;

/// Suspend the current process until a message is received.  This thread will
/// block until a message is received.
///
/// # Errors
///
fn sys_server_receive(server_id: XousSid) ->
                    Result<XousMessageReceived, XousError>;

/// Reply to a message received.  The thread will be unblocked, and will be
/// scheduled to run sometime in the future.
///
/// If the message that we're responding to is a Memory message, then it should be
/// passed back directly to the destination without modification -- the actual contents
/// will be passed in the `out` address pointed to by the structure.
///
/// # Errors
///
/// * **ProcessTerminated**: The process we're replying to doesn't exist any more.
/// * **BadAddress**: The message didn't pass back all the memory it should have.
fn sys_server_reply(destination: XousMessageSender, message: XousMessage) ->
                    Result<(), XousError>;

/// Look up a server name and connect to it.
///
/// # Errors
///
/// * **ServerNotFound**: No server is registered with that name.
fn sys_client_connect(server_name: usize) -> Result<XousConnection, XousError>;

/// Send a message to a server.  This thread will block until the message is responded to.
/// If the message type is `Memory`, then the memory addresses pointed to will be
/// unavailable to this process until this function returns.
///
/// # Errors
///
/// * **ServerNotFound**: The server does not exist so the connection is now invalid
/// * **BadAddress**: The client tried to pass a Memory message using an address it doesn't own
/// * **Timeout**: The timeout limit has been reached
fn sys_client_send(server: XousConnection, message: XousMessage) ->
                    Result<XousMessage, XousError>;
```