M7: synchronous IPC — endpoints, handle table, IPC_Call/ReplyWait

The microkernel message backbone the VFS server and drivers will ride on.

- src/kernel/ipc_sync.zig: Endpoint (sender FIFO threaded via Task.next +
  a recv WaitQueue for servers + a small notification ring). call() (client
  sends, wakes a server, blocks) and replyWait() (server replies to the held
  caller, then receives the next). Reply routing keys on Task.ipc_client —
  synchronous IPC owes one reply at a time. Payloads copy frame-to-frame
  through the physmap (copyAcross on arch.translate, added in M5); an unmapped
  page fails the copy instead of #PF-ing. MSG_MAX 256.
- Bootstrap naming: an integer name registry (danos.ServiceId, vfs=1) with
  create_endpoint / ipc_register / ipc_lookup — any process finds a server
  without threading a handle through spawn.
- notifyFromIsr(): ISR-safe async wake (badge with the high bit set), the
  hook M10's IRQ-as-message needs. Unused/untested until then.
- Task gains handles[16] (opaque *Endpoint, to avoid a sched<->ipc import
  cycle) + ipc_client/send/reply/status fields; sched gains
  blockCurrentLocked/readyLocked; arch gains setSyscallResult2 (rdx badge).
- Syscalls 6..10 wired in process.zig; exit() now drops the caller's endpoint
  refs. lib/ipc.zig: user-side createEndpoint/register/lookup/call
  (server-side replyWait lands with the first server in M9).
- New `ipc-call` test: two kernel tasks ping-pong 100 calls, every reply
  request+1. Suite 29/29.

lib/ipc.zig

@@ -1,10 +1,16 @@
-//! User-space IPC helpers. The kernel's synchronous IPC syscalls
-//! (create_endpoint, ipc_register/lookup, ipc_call, ipc_reply_wait) arrive in a
-//! later milestone; this reserves the module boundary now so drivers and servers
-//! can be written against `rt.ipc` without restructuring once they light up.
+//! 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.
 
-/// A fixed-size, register-friendly message payload. The wire format for the VFS
-/// and driver protocols is layered on top of this by the servers themselves.
+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,
@@ -12,4 +18,37 @@ pub const Message = extern struct {
     c: u64 = 0,
 };
 
-// call() / replyWait() / createEndpoint() land with the kernel IPC syscalls.
+/// 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;
+}
+
+// replyWait() (server side — returns message length + sender badge) lands with
+// the first server binary (M9), where it can be exercised end to end.