//! User-space IPC helpers over the kernel's synchronous IPC syscalls. A client
//! `call`s an endpoint (send + block for reply); the VFS server and drivers are
//! reached this way. The server side (`replyWait`, which returns two values) is
//! added with the first server binary.

const danos = @import("danos");
const sc = @import("syscall.zig");

/// A small-int handle into the calling process's handle table.
pub const Handle = usize;

/// A fixed-size, register-friendly message payload. Server protocols (VFS, driver)
/// layer their own wire format on top of the bytes a call carries.
pub const Message = extern struct {
    tag: u64 = 0,
    a: u64 = 0,
    b: u64 = 0,
    c: u64 = 0,
};

/// Whether a syscall return value is a wrapped -errno (lands in the top page).
inline fn failed(r: usize) bool {
    return r > ~@as(usize, 0) - 4095;
}

/// Create a new endpoint owned by this process; returns its handle.
pub fn createEndpoint() ?Handle {
    const r = sc.syscall0(.create_endpoint);
    return if (failed(r)) null else r;
}

/// Publish endpoint `h` under a well-known service id so other processes find it.
pub fn register(id: danos.ServiceId, h: Handle) bool {
    return !failed(sc.syscall2(.ipc_register, @intFromEnum(id), h));
}

/// Find the endpoint published under `id`, installing a handle to it in this
/// process.
pub fn lookup(id: danos.ServiceId) ?Handle {
    const r = sc.syscall1(.ipc_lookup, @intFromEnum(id));
    return if (failed(r)) null else r;
}

pub const CallError = error{Failed};

/// Send `msg` to endpoint `h` and block until the server replies into `reply`.
/// Returns the reply length.
pub fn call(h: Handle, msg: []const u8, reply: []u8) CallError!usize {
    const r = sc.syscall5(.ipc_call, h, @intFromPtr(msg.ptr), msg.len, @intFromPtr(reply.ptr), reply.len);
    return if (failed(r)) error.Failed else r;
}

/// The result of a `replyWait`: the request length and the sender's badge (a
/// task id, or an IRQ notification if the high bit is set).
pub const Received = struct {
    len: usize,
    badge: u64,
};

/// Server side of IPC_ReplyWait: deliver `reply` to the client last received (if
/// any), then block until the next request arrives in `recv`. Returns its length
/// and the sender badge. This syscall returns two values — the length in rax and
/// the badge in rdx — so it needs a hand-written stub: rdx is a read-write
/// operand (input = reply length, arg #3; output = badge).
pub fn replyWait(h: Handle, reply: []const u8, recv: []u8) Received {
    var rax: usize = undefined;
    var rdx: usize = reply.len; // in: reply_len (arg #3 -> rdx); out: badge
    asm volatile ("syscall"
        : [rax] "={rax}" (rax),
          [rdx] "+{rdx}" (rdx),
        : [n] "{rax}" (@intFromEnum(danos.Syscall.ipc_reply_wait)),
          [a0] "{rdi}" (h),
          [a1] "{rsi}" (@intFromPtr(reply.ptr)),
          [a3] "{r10}" (@intFromPtr(recv.ptr)),
          [a4] "{r8}" (recv.len),
        : .{ .rcx = true, .r11 = true, .memory = true });
    return .{ .len = rax, .badge = rdx };
}